rock | geology | Britannica.com geology metamorphic rock Rock, in geology, naturally occurring and coherent aggregate of one or more minerals. Such aggregates constitute the basic unit of which the solid Earth is comprised and typically form recognizable and mappable volumes. Rocks are commonly divided into three major classes according to the processes that resulted in their formation. These classes are (1) igneous rocks, which have solidified from molten material called magma; (2) sedimentary rocks, those consisting of fragments derived from preexisting rocks or of materials precipitated from solutions; and (3) metamorphic rocks, which have been derived from either igneous or sedimentary rocks under conditions that caused changes in mineralogical composition , texture, and internal structure. These three classes, in turn, are subdivided into numerous groups and types on the basis of various factors, the most important of which are chemical, mineralogical, and textural attributes. Rocks can be any size. Some are smaller than these grains of sand.

Others, like this large rock … (Left) © Bobanny; (right) © Martin Fowler/Shutterstock.com General considerations Rock types Igneous rocks are those that solidify from magma , a molten mixture of rock-forming minerals and usually volatiles such as gases and steam. Since their constituent minerals are crystallized from molten material, igneous rocks are formed at high temperatures. They originate from processes deep within the Earth—typically at depths of about 50 to 200 kilometres (30 to 120 miles)—in the mid- to lower-crust or in the upper mantle. Igneous rocks are subdivided into two categories: intrusive (emplaced in the crust), and extrusive (extruded onto the surface of the land or ocean bottom), in which case the cooling molten material is called lava . The Earth’s surface and crust are constantly evolving through a process called the rock cycle. Encyclopædia Britannica, Inc. Sedimentary rocks are those that are deposited and lithified (compacted and cemented together) at the Earth’s surface, with the assistance of running water , wind , ice , or living organisms.

Most are deposited from the land surface to the bottoms of lakes, rivers, and oceans. Sedimentary rocks are generally stratified—i.e., they have layering. Layers may be distinguished by differences in colour, particle size, type of cement, or internal arrangement. Similar Topics geochemical cycle Metamorphic rocks are those formed by changes in preexisting rocks under the influence of high temperature, pressure , and chemically active solutions. The changes can be chemical (compositional) and physical (textural) in character. Metamorphic rocks are often formed by processes deep within the Earth that produce new minerals, textures, and crystal structures. The recrystallization that takes place does so essentially in the solid state, rather than by complete remelting, and can be aided by ductile deformation and the presence of interstitial fluids such as water. Metamorphism often produces apparent layering, or banding, because of the segregation of minerals into separate bands. Metamorphic processes can also occur at the Earth’s surface due to meteorite impact events and pyrometamorphism taking place near burning coal seams ignited by lightning strikes. Rock cycle Geologic materials—mineral crystals and their host rock types—are cycled through various forms.

The process depends on temperature, pressure, time, and changes in environmental conditions in the Earth’s crust and at its surface. The rock cycle illustrated in Figure 1 reflects the basic relationships among igneous, metamorphic, and sedimentary rocks. Erosion includes weathering (the physical and chemical breakdown of minerals) and transportation to a site of deposition . Diagenesis is, as previously explained, the process of forming sedimentary rock by compaction and natural cementation of grains, or crystallization from water or solutions, or recrystallization. The conversion of sediment to rock is termed lithification . Abundance of rock types An estimate of the distribution of rock types in large structural units of the terrestrial crust is given in the Table. The relative abundance of main rock types and minerals in the crust is shown in the Table. See Full Size Scientists Ponder Menopause in Killer Whales The texture of a rock is the size, shape, and arrangement of the grains (for sedimentary rocks) or crystals (for igneous and metamorphic rocks). Also of importance are the rock’s extent of homogeneity (i.e., uniformity of composition throughout) and the degree of isotropy .

The latter is the extent to which the bulk structure and composition are the same in all directions in the rock. Rocks have many different textures. Layered sandstone produces a gritty texture, whereas coquina … Encyclopædia Britannica, Inc. Analysis of texture can yield information about the rock’s source material, conditions and environment of deposition (for sedimentary rock) or crystallization and recrystallization (for igneous and metamorphic rock, respectively), and subsequent geologic history and change. Classification by grain or crystal size The common textural terms used for rock types with respect to the size of the grains or crystals, are given in the Table . The particle-size categories are derived from the Udden-Wentworth scale developed for sediment. For igneous and metamorphic rocks, the terms are generally used as modifiers—e.g., medium-grained granite. Aphanitic is a descriptive term for small crystals, and phaneritic for larger ones. Very coarse crystals (those larger than 3 centimetres, or 1.2 inches) are termed pegmatitic.

For sedimentary rocks, the broad categories of sediment size are coarse (greater than 2 millimetres, or 0.08 inch), medium (between 2 and 1/16 millimetres), and fine (under 1/16 millimetre). The latter includes silt and clay , which both have a size indistinguishable by the human eye and are also termed dust. Most shales (the lithified version of clay) contain some silt. Pyroclastic rocks are those formed from clastic (from the Greek word for broken) material ejected from volcanoes. Blocks are fragments broken from solid rock, while bombs are molten when ejected. Porosity The term rock refers to the bulk volume of the material, including the grains or crystals as well as the contained void space. The volumetric portion of bulk rock that is not occupied by grains, crystals, or natural cementing material is termed porosity. That is to say, porosity is the ratio of void volume to the bulk volume (grains plus void space). This void space consists of pore space between grains or crystals, in addition to crack space.

In sedimentary rocks, the amount of pore space depends on the degree of compaction of the sediment (with compaction generally increasing with depth of burial), on the packing arrangement and shape of grains, on the amount of cementation, and on the degree of sorting . Typical cements are siliceous, calcareous or carbonate, or iron-bearing minerals. Connect with Britannica Facebook Twitter YouTube Instagram Pinterest Sorting is the tendency of sedimentary rocks to have grains that are similarly sized—i.e., to have a narrow range of sizes (see Figure 2 ). Poorly sorted sediment displays a wide range of grain sizes and hence has decreased porosity. Well-sorted indicates a grain size distribution that is fairly uniform. Depending on the type of close-packing of the grains, porosity can be substantial. It should be noted that in engineering usage—e.g., geotechnical or civil engineering—the terminology is phrased oppositely and is referred to as grading. A well-graded sediment is a (geologically) poorly sorted one, and a poorly graded sediment is a well-sorted one.

Total porosity encompasses all the void space, including those pores that are interconnected to the surface of the sample as well as those that are sealed off by natural cement or other obstructions. Thus the total porosity (ϕT) is where VolG is the volume of grains (and cement, if any) and VolB is the total bulk volume. Alternatively, one can calculate ϕT from the measured densities of the bulk rock and of the (mono)mineralic constituent. Thus, where ρB is the density of the bulk rock and ρG is the density of the grains (i.e., the mineral , if the composition is monomineralogic and homogeneous). For example, if a sandstone has a ρB of 2.38 grams per cubic centimetre (g/cm3) and is composed of quartz (SiO2) grains having ρG of 2.65 g/cm3, the total porosity is Apparent (effective, or net) porosity is the proportion of void space that excludes the sealed-off pores.

It thus measures the pore volume that is effectively interconnected and accessible to the surface of the sample, which is important when considering the storage and movement of subsurface fluids such as petroleum , groundwater, or contaminated fluids. Physical properties Editor Picks: Exploring 10 Types of Basketball Movies Physical properties of rocks are of interest and utility in many fields of work, including geology, petrophysics, geophysics, materials science , geochemistry, and geotechnical engineering. The scale of investigation ranges from the molecular and crystalline up to terrestrial studies of the Earth and other planetary bodies.

Geologists are interested in the radioactive age dating of rocks to reconstruct the origin of mineral deposits; seismologists formulate prospective earthquake predictions using premonitory physical or chemical changes; crystallographers study the synthesis of minerals with special optical or physical properties; exploration geophysicists investigate the variation of physical properties of subsurface rocks to make possible detection of natural resources such as oil and gas, geothermal energy, and ores of metals; geotechnical engineers examine the nature and behaviour of the materials on, in, or of which such structures as buildings, dams, tunnels, bridges, and underground storage vaults are to be constructed; solid-state physicists study the magnetic, electrical, and mechanical properties of materials for electronic devices, computer components, or high-performance ceramics; and petroleum reservoir engineers analyze the response measured on well logs or in the processes of deep drilling at elevated temperature and pressure. Since rocks are aggregates of mineral grains or crystals, their properties are determined in large part by the properties of their various constituent minerals. In a rock these general properties are determined by averaging the relative properties and sometimes orientations of the various grains or crystals.

As a result, some properties that are anisotropic (i.e., differ with direction) on a submicroscopic or crystalline scale are fairly isotropic for a large bulk volume of the rock. Many properties are also dependent on grain or crystal size, shape, and packing arrangement, the amount and distribution of void space, the presence of natural cements in sedimentary rocks, the temperature and pressure, and the type and amount of contained fluids (e.g., water, petroleum, gases). Because many rocks exhibit a considerable range in these factors, the assignment of representative values for a particular property is often done using a statistical variation. Trending Topics Opium Wars Some properties can vary considerably, depending on whether measured in situ (in place in the subsurface) or in the laboratory under simulated conditions. Electrical resistivity , for example, is highly dependent on the fluid content of the rock in situ and the temperature condition at the particular depth. Density Density varies significantly among different rock types because of differences in mineralogy and porosity. Knowledge of the distribution of underground rock densities can assist in interpreting subsurface geologic structure and rock type.

In strict usage, density is defined as the mass of a substance per unit volume; however, in common usage, it is taken to be the weight in air of a unit volume of a sample at a specific temperature. Weight is the force that gravitation exerts on a body (and thus varies with location), whereas mass (a measure of the matter in a body) is a fundamental property and is constant regardless of location. In routine density measurements of rocks, the sample weights are considered to be equivalent to their masses, because the discrepancy between weight and mass would result in less error on the computed density than the experimental errors introduced in the measurement of volume. Thus, density is often determined using weight rather than mass. Density should properly be reported in kilograms per cubic metre (kg/m3), but is still often given in grams per cubic centimetre (g/cm3). Another property closely related to density is specific gravity . It is defined, as noted above, as the ratio of the weight or mass in air of a unit volume of material at a stated temperature to the weight or mass in air of a unit volume of distilled water at the same temperature. Specific gravity is dimensionless (i.e., has no units).

The bulk density of a rock is ρB = WG/VB, where WG is the weight of grains (sedimentary rocks) or crystals (igneous and metamorphic rocks) and natural cements, if any, and VB is the total volume of the grains or crystals plus the void (pore) space. The density can be dry if the pore space is empty, or it can be saturated if the pores are filled with fluid (e.g., water), which is more typical of the subsurface (in situ) situation. If there is pore fluid present, where Wfl is the weight of pore fluid. In terms of total porosity, saturated density is and thus where ρfl is the density of the pore fluid. Density measurements for a given specimen involve the determination of any two of the following quantities: pore volume, bulk volume, or grain volume, along with the weight. A useful way to assess the density of rocks is to make a histogram plot of the statistical range of a set of data.

The representative value and its variation can be expressed as follows: (1) mean, the average value, (2) mode, the most common value (i.e., the peak of the distribution curve), (3) median, the value of the middle sample of the data set (i.e., the value at which half of the samples are below and half are above), and (4) standard deviation, a statistical measure of the spread of the data (plus and minus one standard deviation from the mean value includes about two-thirds of the data). A compilation of dry bulk densities for various rock types found in the upper crust of the Earth is listed in the Table. A histogram plot of these data, giving the percent of the samples as a function of density is shown in Figure 3 . The parameters given include (1) sample division, the range of density in one data column—e.g., 0.036 g/cm3 for Figure 3 , (2) number of samples, and (3) standard deviation. The small inset plot is the percentage of samples (on the vertical axis) that lie within the interval of the “mode - x” to the “mode + x,” where x is the horizontal axis.

Dry bulk densities for various rock types rock type mean (grams per cubic cm) standard deviation mode (grams per cubic cm) median (grams per cubic cm) all rocks 2.22 2.22 Source: After data from H.S. Washington (1917) and R.J. Piersol, L.E. Workman, and M.C. Watson (1940) as compiled by Gary R. Olhoeft and Gordon R. Johnson in Robert S. Carmichael, ed., Handbook of Physical Properties of Rocks, vol. III, CRC Press, Inc. (1984). In Figure 3 , the most common (modal) value of the distribution falls at 2.63 g/cm3, roughly the density of quartz, an abundant rock-forming mineral . Few density values for these upper crustal rocks lie above 3.3 g/cm3. A few fall well below the mode, even occasionally under 1 g/cm3. The reason for this is shown in Figure 4 , which illustrates the density distributions for granite , basalt , and sandstone.

Granite is an intrusive igneous rock with low porosity and a well-defined chemical (mineral) composition; its range of densities is narrow. Basalt is, in most cases, an extrusive igneous rock that can exhibit a large variation in porosity (because entrained gases leave voids called vesicles), and thus some highly porous samples can have low densities. Sandstone is a clastic sedimentary rock that can have a wide range of porosities depending on the degree of sorting, compaction, packing arrangement of grains, and cementation. The bulk density varies accordingly. Other distribution plots of dry bulk densities are given in Figures 5 and 6 , with a sample division of 0.036 g/cm3 for Figures 5 and 6A and of 0.828 percent for Figure 6B . The Table lists typical ranges of dry bulk densities for a variety of other rock types as prepared by the American geologists Gordon R. Johnson and Gary R. Olhoeft. Typical density ranges for some other rock types rock type slate 2.72–2.84 Source: After data from R.A. Daly, G.E. Manger, and S.P. Clark, Jr.

(1966); A.F. Birch (1966); F. Press (1966); and R.N. Schock, B.P. Bonner, and H. Louis (1974) in Robert S. Carmichael, ed., Handbook of Physical Properties of Rocks, vol. III, CRC Press, Inc. (1984). The density of clastic sedimentary rocks increases as the rocks are progressively buried. This is because of the increase of overburden pressure, which causes compaction, and the progressive cementation with age. Both compaction and cementation decrease the porosity. Representative densities for common rock-forming minerals (i.e., ρG) and rocks (i.e., ρB) are listed in the Table. The bulk densities for sedimentary rocks , which typically have variable porosity, are given as ranges of both dry ρB and (water-) saturated ρB. The pore-filling fluid is usually briny water, often indicative of the presence of seawater when the rock was being deposited or lithified.

It should be noted that the bulk density is less than the grain density of the constituent mineral (or mineral assemblage), depending on the porosity. For example, sandstone (characteristically quartzose) has a typical dry bulk density of 2.0–2.6 g/cm3, with a porosity that can vary from low to more than 30 percent. The density of quartz itself is 2.65 g/cm3. If porosity were zero, the bulk density would equal the grain density. Saturated bulk density is higher than dry bulk density, owing to the added presence of pore-filling fluid. The Table also lists representative values for density of seawater, oil, and methane gas at a subsurface condition—pressure of 200 bars (one bar = 0.987 atmosphere , or 29.53 inches of mercury) and a temperature of about 80° C (176° F). Mechanical properties Stress and strain When a stress σ (force per unit area) is applied to a material such as rock, the material experiences a change in dimension, volume, or shape. This change, or deformation , is called strain (ε).

Stresses can be axial—e.g., directional tension or simple compression—or shear (tangential), or all-sided (e.g., hydrostatic compression). The terms stress and pressure are sometimes used interchangeably, but often stress refers to directional stress or shear stress and pressure (P) refers to hydrostatic compression. For small stresses, the strain is elastic (recoverable when the stress is removed and linearly proportional to the applied stress). For larger stresses and other conditions, the strain can be inelastic, or permanent. Elastic constants In elastic deformation , there are various constants that relate the magnitude of the strain response to the applied stress. These elastic constants include the following: (1) Young’s modulus (E) is the ratio of the applied stress to the fractional extension (or shortening) of the sample length parallel to the tension (or compression). The strain is the linear change in dimension divided by the original length. (2) Shear modulus (μ) is the ratio of the applied stress to the distortion (rotation) of a plane originally perpendicular to the applied shear stress; it is also termed the modulus of rigidity.

(3) Bulk modulus (k) is the ratio of the confining pressure to the fractional reduction of volume in response to the applied hydrostatic pressure. The volume strain is the change in volume of the sample divided by the original volume. Bulk modulus is also termed the modulus of incompressibility. (4) Poisson’s ratio (σp) is the ratio of lateral strain (perpendicular to an applied stress) to the longitudinal strain (parallel to applied stress). For elastic and isotropic materials, the elastic constants are interrelated. For example, and limestone Thermal properties Heat flow (or flux), q, in the Earth’s crust or in rock as a building material, is the product of the temperature gradient (change in temperature per unit distance) and the material’s thermal conductivity (k, the heat flow across a surface per unit area per unit time when a temperature difference exists in unit length perpendicular to the surface). Thus, The units of the terms in this equation are given below, expressed first in the centimetre-gram-second (cgs) system and then in the International System of Units (SI) system, with the conversion factor from the first to the second given between them.

Thermal conductivity Thermal conductivity can be determined in the laboratory or in situ, as in a borehole or deep well, by turning on a heating element and measuring the rise in temperature with time. It depends on several factors: (1) chemical composition of the rock (i.e., mineral content), (2) fluid content (type and degree of saturation of the pore space); the presence of water increases the thermal conductivity (i.e., enhances the flow of heat), (3) pressure (a high pressure increases the thermal conductivity by closing cracks which inhibit heat flow), (4) temperature, and (5) isotropy and homogeneity of the rock. Typical values of thermal conductivities of rock materials are given in the Table. For crystalline silicate rocks—the dominant rocks of the “basement” crustal rocks—the lower values are typical of ones rich in magnesium and iron (e.g., basalt and gabbro) and the higher values are typical of those rich in silica (quartz) and alumina (e.g., granite). These values result because the thermal conductivity of quartz is relatively high, while that for feldspars is low.

Typical values of thermal conductivity (in 0.001 calories per centimetre per second per degree Celsius) material 0.33 1.56 Source: Modified from compilation by William Van Schmus in Robert S. Carmichael, ed., Handbook of Physical Properties of Rocks, vol. III, CRC Press, Inc. (1984). Electrical properties The electrical nature of a material is characterized by its conductivity (or, inversely, its resistivity ) and its dielectric constant, and coefficients that indicate the rates of change of these with temperature, frequency at which measurement is made, and so on. For rocks with a range of chemical composition as well as variable physical properties of porosity and fluid content, the values of electrical properties can vary widely. Resistance (R) is defined as being one ohm when a potential difference (voltage; V) across a specimen of one volt magnitude produces a current (i) of one ampere; that is, V = Ri. The electrical resistivity (ρ) is an intrinsic property of the material. In other words, it is inherent and not dependent on sample size or current path.

It is related to resistance by R = ρL/A where L is the length of specimen, A is the cross-sectional area of specimen, and units of ρ are ohm-centimetre; 1 ohm-centimetre equals 0.01 ohm-metre. The conductivity (σ) is equal to 1/ρ ohm -1 · centimetre-1 (or termed mhos/cm). In SI units, it is given in mhos/metre, or siemens/metre. Some representative values of electrical resistivity for rocks and other materials are listed in the Table. Materials that are generally considered as “good” conductors have a resistivity of 10-5–10 ohm-centimetre (10-7–10-1 ohm-metre) and a conductivity of 10–107 mhos/metre. Those that are classified as intermediate conductors have a resistivity of 100–109 ohm-centimetre (1–107 ohm-metre) and a conductivity of 10-7–1 mhos/metre.

“Poor” conductors, also known as insulators, have a resistivity of 1010–1017 ohm-centimetre (108–1015 ohm-metre) and a conductivity of 10-15–10-8. Seawater is a much better conductor (i.e., it has lower resistivity) than fresh water owing to its higher content of dissolved salts; dry rock is very resistive. In the subsurface, pores are typically filled to some degree by fluids. The resistivity of materials has a wide range—copper is, for example, different from quartz by 22 orders of magnitude. Typical resistivities quartz (18 °C) (1014)–(1016) For high-frequency alternating currents, the electrical response of a rock is governed in part by the dielectric constant , ε. This is the capacity of the rock to store electric charge; it is a measure of polarizability in an electric field. In cgs units, the dielectric constant is 1.0 in a vacuum.

In SI units, it is given in farads per metre or in terms of the ratio of specific capacity of the material to specific capacity of vacuum (which is 8.85 × 10-12 farads per metre). The dielectric constant is a function of temperature, and of frequency, for those frequencies well above 100 hertz (cycles per second). Electrical conduction occurs in rocks by (1) fluid conduction—i.e., electrolytic conduction by ionic transfer in briny pore water—and (2) metallic and semiconductor (e.g., some sulfide ores) electron conduction. If the rock has any porosity and contained fluid, the fluid typically dominates the conductivity response. The rock conductivity depends on the conductivity of the fluid (and its chemical composition), degree of fluid saturation, porosity and permeability , and temperature. If rocks lose water, as with compaction of clastic sedimentary rocks at depth, their resistivity typically increases. Magnetic properties The magnetic properties of rocks arise from the magnetic properties of the constituent mineral grains and crystals. Typically, only a small fraction of the rock consists of magnetic minerals.

It is this small portion of grains that determines the magnetic properties and magnetization of the rock as a whole, with two results: (1) the magnetic properties of a given rock may vary widely within a given rock body or structure, depending on chemical inhomogeneities, depositional or crystallization conditions, and what happens to the rock after formation; and (2) rocks that share the same lithology (type and name) need not necessarily share the same magnetic characteristics. Lithologic classifications are usually based on the abundance of dominant silicate minerals, but the magnetization is determined by the minor fraction of such magnetic mineral grains as iron oxides. The major rock-forming magnetic minerals are iron oxides and sulfides. Although the magnetic properties of rocks sharing the same classification may vary from rock to rock, general magnetic properties do nonetheless usually depend on rock type and overall composition. The magnetic properties of a particular rock can be quite well understood provided one has specific information about the magnetic properties of crystalline materials and minerals, as well as about how those properties are affected by such factors as temperature, pressure, chemical composition, and the size of the grains.

Understanding is further enhanced by information about how the properties of typical rocks are dependent on the geologic environment and how they vary with different conditions. Applications of the study of rock magnetization An understanding of rock magnetization is important in at least three different areas: prospecting , geology, and materials science. In magnetic prospecting, one is interested in mapping the depth, size, type, and inferred composition of buried rocks. The prospecting, which may be done from ground surface, ship, or aircraft, provides an important first step in exploring buried geologic structures and may, for example, help identify favourable locations for oil, natural gas , and economic mineral deposits. Rock magnetization has traditionally played an important role in geology. Paleomagnetic work seeks to determine the remanent magnetization (see below Types of remanent magnetization ) and thereby ascertain the character of the Earth’s field when certain rocks were formed. The results of such research have important ramifications in stratigraphic correlation, age dating, and reconstructing past movements of the Earth’s crust.

Indeed, magnetic surveys of the oceanic crust provided for the first time the quantitative evidence needed to cogently demonstrate that segments of the crust had undergone large-scale lateral displacements over geologic time , thereby corroborating the concepts of continental drift and seafloor spreading, both of which are fundamental to the theory of plate tectonics (see plate tectonics ). The understanding of magnetization is increasingly important in materials science as well. The design and manufacture of efficient memory cores, magnetic tapes, and permanent magnets increasingly rely on the ability to create materials having desired magnetic properties. Basic types of magnetization There are six basic types of magnetization: (1) diamagnetism , (2) paramagnetism, (3) ferromagnetism, (4) antiferromagnetism, (5) ferrimagnetism, and (6) superparamagnetism. Diamagnetism arises from the orbiting electrons surrounding each atomic nucleus . When an external magnetic field is applied, the orbits are shifted in such a way that the atoms set up their own magnetic field in opposition to the applied field. In other words, the induced diamagnetic field opposes the external field.

Diamagnetism is present in all materials, is weak, and exists only in the presence of an applied field. The propensity of a substance for being magnetized in an external field is called its susceptibility (k) and it is defined as J/H, where J is the magnetization (intensity) per unit volume and H is the strength of the applied field. Since the induced field always opposes the applied field, the sign of diamagnetic susceptibility is negative. The susceptibility of a diamagnetic substance is on the order of -10-6 electromagnetic units per cubic centimetre (emu/cm3). It is sometimes denoted κ for susceptibility per unit mass of material. Paramagnetism results from the electron spin of unpaired electrons. An electron has a magnetic dipole moment —which is to say that it behaves like a tiny bar magnet—and so when a group of electrons is placed in a magnetic field, the dipole moments tend to line up with the field. The effect augments the net magnetization in the direction of the applied field. Like diamagnetism, paramagnetism is weak and exists only in the presence of an applied field, but since the effect enhances the applied field, the sign of the paramagnetic susceptibility is always positive.

The susceptibility of a paramagnetic substance is on the order of 10-4 to 10-6 emu/cm3. Ferromagnetism also exists because of the magnetic properties of the electron. Unlike paramagnetism, however, ferromagnetism can occur even if no external field is applied. The magnetic dipole moments of the atoms spontaneously line up with one another because it is energetically favourable for them to do so. A remanent magnetization can be retained. Complete alignment of the dipole moments would take place only at a temperature of absolute zero (0 kelvin [K], or -273.15° C). Above absolute zero, thermal motions begin to disorder the magnetic moments. At a temperature called the Curie temperature , which varies from material to material, the thermally induced disorder overcomes the alignment, and the ferromagnetic properties of the substance disappear. The susceptibility of ferromagnetic materials is large and positive. It is on the order of 10 to 104 emu/cm3. Only a few materials—iron, cobalt, and nickel—are ferromagnetic in the strict sense of the word and have a strong residual magnetization.

In general usage, particularly in engineering, the term ferromagnetic is frequently applied to any material that is appreciably magnetic. Antiferromagnetism occurs when the dipole moments of the atoms in a material assume an antiparallel arrangement in the absence of an applied field. The result is that the sample has no net magnetization. The strength of the susceptibility is comparable to that of paramagnetic materials. Above a temperature called the Néel temperature , thermal motions destroy the antiparallel arrangement, and the material then becomes paramagnetic. Spin-canted (anti)ferromagnetism is a special condition which occurs when antiparallel magnetic moments are deflected from the antiferromagnetic plane, resulting in a weak net magnetism. Hematite (α-Fe2 O 3) is such a material. Ferrimagnetism is an antiparallel alignment of atomic dipole moments which does yield an appreciable net magnetization resulting from unequal moments of the magnetic sublattices. Remanent magnetization is detectable (see below). Above the Curie temperature the substance becomes paramagnetic. Magnetite (Fe3O4), which is the most magnetic common mineral, is a ferrimagnetic substance.

Superparamagnetism occurs in materials having grains so small (about 100 angstroms) that any cooperative alignment of dipole moments is overcome by thermal energy. Types of remanent magnetization Rocks and minerals may retain magnetization after the removal of an externally applied field, thereby becoming permanent weak magnets. This property is known as remanent magnetization and is manifested in different forms, depending on the magnetic properties of the rocks and minerals and their geologic origin and history. Delineated below are the kinds of remanent magnetization frequently observed. CRM ( chemical, or crystallization, remanent magnetization) can be induced after a crystal is formed and undergoes one of a number of physicochemical changes, such as oxidation or reduction, a phase change, dehydration, recrystallization, or precipitation of natural cements. The induction , which is particularly important in some (red) sediments and metamorphic rocks, typically takes place at constant temperature in the Earth’s magnetic field . DRM (depositional, or detrital, remanent magnetization ) is formed in clastic sediments when fine particles are deposited on the floor of a body of water.

Marine sediments, lake sediments, and some clays can acquire DRM. The Earth’s magnetic field aligns the grains, yielding a preferred direction of magnetization. IRM ( isothermal remanent magnetization) results from the application of a magnetic field at a constant (isothermal) temperature, often room temperature. NRM ( natural remanent magnetization ) is the magnetization detected in a geologic in situ condition. The NRM of a substance may, of course, be a combination of any of the other remanent magnetizations described here. PRM (pressure remanent, or piezoremanent, magnetization) arises when a material undergoes mechanical deformation while in a magnetic field. The process of deformation may result from hydrostatic pressure, shock impact (as produced by a meteorite striking the Earth’s surface), or directed tectonic stress. There are magnetization changes with stress in the elastic range, but the most pronounced effects occur with plastic deformation when the structure of the magnetic minerals is irreversibly changed. TRM ( thermoremanent magnetization ) occurs when a substance is cooled, in the presence of a magnetic field, from above its Curie temperature to below that temperature.

This form of magnetization is generally the most important, because it is stable and widespread, occurring in igneous and sedimentary rocks. TRM also can occur when dealing exclusively with temperatures below the Curie temperature. In PTRM ( partial thermoremanent magnetization) a sample is cooled from a temperature below the Curie point to yet a lower temperature. VRM ( viscous remanent magnetization) results from thermal agitation. It is acquired slowly over time at low temperatures and in the Earth’s magnetic field. The effect is weak and unstable but is present in most rocks. Hysteresis and magnetic susceptibility The concept of hysteresis is fundamental when describing and comparing the magnetic properties of rocks. Hysteresis is the variation of magnetization with applied field and illustrates the ability of a material to retain its magnetization, even after an applied field is removed. Figure 9 illustrates this phenomenon in the form of a plot of magnetization (J) versus applied field (Hex). Js is the saturation (or “spontaneous”) magnetization when all the magnetic moments are aligned in their configuration of maximum order. It is temperature-dependent, reaching zero at the Curie temperature.

Jr,sat is the remanent magnetization that remains when a saturating (large) applied field is removed, and Jr is the residual magnetization left by some process apart from IRM saturation, as, for example, TRM. Hc is the coercive field (or force) that is required to reduce Jr,sat to zero, and Hc,r is the field required to reduce Jr to zero. Magnetic susceptibility is a parameter of considerable diagnostic and interpretational use in the study of rocks. This is true whether an investigation is being conducted in the laboratory or magnetic fields over a terrain are being studied to deduce the structure and lithologic character of buried rock bodies. Susceptibility for a rock type can vary widely, depending on magnetic mineralogy, grain size and shape, and the relative magnitude of remanent magnetization present, in addition to the induced magnetization from the Earth’s weak field. The latter is given as Jinduced = kHex, where k is the (true) magnetic susceptibility and Hex is the external (i.e., the Earth’s) magnetic field.

If there is an additional remanent magnetization with its ratio (Qn) to induced magnetization being given by then the total magnetization is where kapp, the “apparent” magnetic susceptibility, is k(1 + Qn). Magnetic minerals and magnetic properties of rocks The major rock-forming magnetic minerals are the following iron oxides: the titanomagnetite series, xFe2TiO4 · (1 - x)Fe3O4, where Fe3O4 is magnetite, the most magnetic mineral; the ilmenohematite series, yFeTiO3 · (1 - y)Fe2O3, where α-Fe2O3 (in its rhombohedral structure) is hematite; maghemite, γ-Fe2O3 (in which some iron atoms are missing in the hematite structure); and limonite (hydrous iron oxides). They also include sulfides—namely, the pyrrhotite series, yFeS · (1 - y)Fe1 - xS. The Table gives some typical values of the apparent susceptibility for various rock types, which usually include some remanent as well as induced magnetization.

Values are higher for mafic igneous rocks, especially as the content of magnetite increases. Approximate "apparent susceptibilities" for rock types rock Igneous Rocks Igneous Rocks Although names are legion, igneous rocks are formed from a small number of mineral types This article is intended to present a scheme for remembering the composition of igneous rocks and their relations in connection with their names. Igneous rocks cannot be given names reflecting a specific chemical or mineralogical composition, since these are too variable, although they are composed of minerals of more or less definite composition. They have, instead, been given arbitrary names drawn from geographic names and other sources, with no inherent meanings or mnemonic value. These names have been assigned in great profusion, sometimes reflecting the characteristics of a particular occurrence or suggesting certain associations, and it is very difficult to keep them sorted out in the mind. Moreover, the names reflect texture and mode of occurrence as well as mere composition, adding to the difficulty of mental identification. This account should not be thought of as giving the actual processes of formation of igneous rocks, but simply a logical framework.

An igneous rock is one that has been formed by the reaction of its constituents in an environment that has allowed them to more or less freely associate in forms that minimize the free energy U - TS, or minimum internal energy and maximum entropy, at moderately high temperatures. This need not take place in the liquid state, and indeed it probably seldom does, but also in the presence of active fluids that carry in certain ingredients and carry out others, usually at elevated temperatures. It is too simple to say that igneous rocks crystallized from a liquid magma, though the result is similar. Because different constituents are available, the igneous rocks formed in the crust are different from those formed in the upper mantle. Besides silica, aluminium and the alkalis (K, Na and Ca) are characteristic of the crust, iron and magnesium of the mantle. Accordingly, igneous rocks are classified as sial or acidic, and sima or basic. The terms acidic and basic are not well-chosen, but are traditional. Silicon (the acidic ingredient) is present in all igneous rocks, but free silica, SiO2, only in the more acidic rocks.

Rocks typical of the mantle are different from the more common basic igneous rocks, and are called ultrabasic. We shall see what this means in terms of mineral constitution. The crust of the earth is 59% silica, SiO2, and 15% alumina, Al2O3, so silicon and aluminum must the predominant constituents of any igneous rock. If we mix silica and alumina and heat, the result is pretty much silica and alumina, which have very different crystal structures, and the free energy cannot be lowered by any crafty arrangements. When we add cations, positively-charged atoms like K+ and Na+, the situation changes. Now aluminum tetrahedra, with a composition AlO2, can be associated with the silica tetrahedra, with compositon SiO2, so long as we add one Na or K for each Al to keep things electrically neutral. (Nothing with net charge gives a low free energy). The silicon and aluminium ions are both small (0.039 nm and 0.057 nm, respectively) and fit equally well within four big oxygens.

This allows the building of a very stable tectosilicate, which in this case is feldspar. Rocks of this general composition are also called felsic, from feldspar and silica. Felsic rocks are more than 65% silica. With potassium, we get orthoclase, KAlSi3O8, and with sodium plagioclase, NaAlSi3O8. Note how similar these crystals are. Sodium is a smaller ion than potassium, so slightly different structures result, and the two cations do not substitute for each other. If there is some water around (which can contribute OH ions) it is also possible to make a sheet silicate, with 5 oxygens to every silicon. If we add Al(OH)2·AlO2 (this is electrically neutral) to the orthoclase components, we get the mica muscovite, KAl3Si3O10(OH)2. This formula where everything is packed together is a mess, but we can analyze it into meaningful parts.

Now when we cool our magma, we find that all the aluminum is tied up in feldspar and mica, the relative amounts of orthoclase and plagioclase depending on abundances of Na and K. The silica that remains is what was not required in making feldspar and mica, so we get a rock that is mainly crystalline feldspar with silica between the feldspar crystals, interspersed with flakes of mica. This is an example of the rock granite. Making such a muscovite granite requires a very pure starting mix, with no Ca, Fe or Mg. If there is a little Fe and Mg, it can be soaked up by including (Mg,Fe)3 instead of the Al2 in the muscovite. Mg++ and Fe++ have about the same size (0.078 nm and 0.083 nm, respectively), and so can substitute for one another freely in the crystal. The difference between ions of the same charge and radius is very small, and mixing them up improves the entropy anyway.

The result is K(Mg,Fe)3AlSi3O10(OH)2, the black mica biotite, which is very much more common than muscovite. Again, there is some order in the apparently complicated chemical formula. Therefore, starting with silica, alumina, some cations, and a little iron and magnesium, we get a rock composed mainly of feldspar (using up the aluminium), a little biotite (using up the iron and magnesium), with what silica is left over as clear, crystalline free silica. This is a typical pink granite, the pink coming from the feldspar. The interchangeablility of Fe++ and Mg++ is mirrored by the interchangeability of Na+ and Ca++. The charges are different, but this simply means that we use twice as many sodiums as calciums. This gives a complete series of plagioclase feldspar, from all-sodium albite to all-calcium anorthite, with oligoclase, andesine, labradorite, and bytownite in between.

This is a perfect example of the terminological chaos in mineralogy and petrology--none of the arbitrary names gives the slightest clue to its position in the series, and all the names refer to what is essentially the same mineral. Therefore, if our mix contains some calcium, it will probably go into the feldspar. We might also note that OH- and F- ions have the same charge, and about the same size (0.014 nm and 0.0133 nm, respectively), so they readily substitute for one another. This means that micas can contain F as well as OH, and do. All magnesium gives amber mica, phlogopite, while lithium in place of potassium gives pink mica, lepidolite. The names for these granites reflect the composition of the feldspar. The name granite is reserved for rocks in which potassium feldspar, orthoclase, dominates. If potassium and sodium feldspar are comparable in amount, then it is called granodiorite. With mainly sodium feldspar it is quartz-diorite. These names refer to rocks with visible crystals, or coarse texture.

With fine texture, they become rhyolite, rhyodacite and dacite, respectively. The fine-textured rocks were usually erupted as a hot colloidal suspension (sol), the particles of which later coagulated into what appears to be a rock solidified from a liquid, but which was probably never liquid at all. To add to the name confusion, rocks of porphyritic texture, consisting of large crystals in a fine-textured groundmass, are given still other names: quartz porphyry, granodiorite porphyry, and quartz porphyrite, when really the only difference between them is the type of alkali feldspar. Therefore, we have six names for what is really only one rock by composition: quartz-feldspar-mica granite. There are, of course, gelogical reasons for the multiplicity of names. If the proportion of silica in the mix is smaller, there may not be any free silica left when all the aluminium is used up. This gives us three rocks analogous to the granite sequence: syenite, monzonite and diorite.

The fine-grained analogues are trachyte, trachyandesite and andesite, and the porphyritic ones porphyry, monzonite-porphyry and porphyrite. We now have nine names for what is, compositionally, almost the same thing, with varying amounts of silica and a varying K:Na ratio. These names reflect the widespread and multifarious occurrences of this fundamental crustal rock. Porphyry originally referred to a beautiful red Egyptian rock of this texture, but now is used more generally for rocks of any color. The textures of igneous rocks are often classified by terms that imply the mode of emplacement of the rocks. These are extrusive, hypabyssal or intrusive, and plutonic. They merely reflect the texture of the rock as fine, medium or coarse, not specifically its mode of emplacement, although extruded rocks are usually fine-grained or glassy, plutonic rocks coarse-grained, and hypabyssal rocks graded between these limits. It would be best if these terms were not used in connection with rock types, since they may lead to unwarranted conclusions. As describing modes of emplacement, they are, of course, unexceptionable.

If there is not enough silica to take up all the aluminium, feldspathoids may be formed instead of feldspars. These are likewise tectosilicates. One is leucite, KAlO2·2SiO2, and another is nepheline, KAlO2·SiO2, with only two and one silica units, respectively, instead of the three in a feldspar. Like feldspars, these minerals are hard and of medium density, but weather more readily. They are relatively rare, and rocks containing them do not appear to have any special names. Rocks poor in silica are called undersaturated. Calcium has not yet been considered, though it is rather common, especially in rocks with a deeper source. When it occurs, it has two main effects. First, it can substitute for Na in plagioclase, as already mentioned, even making the all-calcium feldspar anorthite. Second, it can facilitate the synthesis of chain silicates in cooperation with Mg and OH. With some Al around, it can preferentially form hornblende, an amphibole.

In fact, diorite and andesite can be composed almost entirely of plagioclase and hornblende, where it seems that the calcium has soaked up all the silica not used in making feldspar, in the form of hornblende. If there was very little aluminium, we wind up with a rock that is all hornblende, hornblendite or amphibolite. Amphiboles have double chains, and an 11:4 O:Si ratio. Hornblende is hard (5-6), rather heavy (3.2), and can be fibrous or translucent. It is dark green to black, and includes Ca, Na, Mg, ferrous and ferric iron, aluminium, OH, F and, of course, Si. If there is no aluminium available, calcium and magnesium may form pyroxenes instead. These are single-chain silicates with a 3:1 O-Si ratio, such as CaSiO3 and MgSiO3. The calcium ion is a bit larger than the magnesium ion, 0.106 versus 0.078 nm, but both can be accomodated in the same crystal.

A 1:1 mix gives diopside, CaMgSi2O6, or the common augite, Ca(Mg,Fe,Al)(Al,Si)2O6, very similar except for substitutions. Pyroxenes are, like amphiboles, dark and heavy minerals. When silica is mixed with aluminium, calcium, magnesium and iron oxides, as in material derived from the mantle, the result is calcium plagioclase, typically anorthite, and pyroxene, typically augite. This is a very familiar rock, basalt. It is not a mantle rock, but one that comes from mixing mantle and crustal rocks in varying amounts. Basalt is the fine-grained extrusive form. The less common coarse-grained form is gabbro, and the frequently-seen hypabyssal or intrusive form is dolerite or diabase. These are very important rocks indeed, associated with volcanic eruptions, and arriving near the surface as fluid lavas.

Mantle rocks are typified by olivine, consisting of the substitutional series between MgSiO3, forsterite, and FeSiO3, fayalite, typically dark and heavy, though pure forsterite with a trace of iron gives the attractive gem mineral peridot. If basaltic rocks contain olivine, their names are prefixed with olivine- to show this, and close association with the mantle is indicated. If there is no aluminium, then there is no feldspar, and the rock is called peridotite, and is ultrabasic. Peridotite can contain pyroxene as well, but no free silica, which would react with the other ingredients, and perhaps tremolite, a Mg-Ca amphibole. Amphiboles are weathering products of pyroxenes. Rocks containing olivine, pyroxene and amphibole are often called mafic, from magnesium and ferrous; they contain from 44% to 52% silica. Rocks with from 52% to 65% silica are called mesosilicic or intermediate in composition.

The rock names given above are mainly those in Arthur Holmes's simplified nomenclature (see Reference, p. 116). There are many other rock names of more special application, which are not always used consistently, nor given an obvious definition. For example, a tholeiite has been defined as a basalt that does not contain olivine or similar mantle-like ingredients. However, one prominent book on marine geology refers to an olivine-tholeiite. To illustrate the important effect that rock texture has on naming, we consider some porphyritic granites. First, however, let's look at graphic granite. A broken surface looks as if it is covered with runic symbols that resemble writing, with parallel alignments. The rock is actually one huge crystal of intergrown silica and feldspar. Both are tectosilicates, made up of silicon and aluminium tetrahedra, so it is not surprising that they can intergrow with the maintenance of crystal order over long distances. This intergrowth can exist on a microscopic scale, when the rock is called microgranite or micropegmatite, since graphic granite is often associated with pegmatites and their huge crystals.

The suffix -phyre hints at porphyritic texture, appearing in granophyre, one of the rocks of interest to us. Granophyre has quartz and feldspar crystals, perhaps not very large but still evident to the eye, in a fine-grained groundmass of microgranite. Granophyre is a characteristic result of metamorphic recrystallization in the presence of active fluids, completely changing the texture of a rock while the composition changes very little. Doris Reynolds found all stages in the transformation of very old granodiorite (granite with roughly equal amounts of potassium and soda feldspar) into newer granophyre at Slieve Gullion in Northern Ireland. The old Torridonian sandstone of the Northwest Highlands has also been found transformed into granophyre. The periphery of many granitic batholith-like bodies has also been altered to granophyre, as in the Wichita Mountains of Oklahoma. In composition, granophyre is just a typical granite, but it is the texture that is remarkable. We should also glance at granite porphyry and quartz porphyry to distinguish them from granophyre.

Granite porphyry has the same phenocrysts of quartz and feldspar as granophyre, but the groundmass is just ordinary fine-grained granite. In fact, it often grades into granophyre. Quartz porphyry can be just a medium-grained intermediate between rhyolite and granite, hardly different from granite porphyry, but it is sometimes defined as an extrusive rock, in which large chunks of granite, perhaps in the form of quartz or feldspar crystals, have been erupted with a hot suspension of similar material, which has welded and cooled into a rhyolite-like groundmass. This shows how care must be taken to define a rock carefully before using the name, so that the compositional and textural significance of the name is clear to the reader. Arthur Holmes, Principles of Physical Geology, 2nd ed. (New York: Nelson, 1965). Chapters IV and V. C. S. Hurlbut, Jr., Dana's Manual of Mineralogy, 16th ed. (New York: John Wiley & Sons, 1952). Composed by J. B.

Calvert Created 24 February 2003 What are igneous rocks - Geology - Global Warming Causes What are igneous rocks Last Updated on Sat, 10 Aug 2013 | Geology N4TdHa Igneous rocks are the rocks that form when molten material cools and crystallizes. Real-World Reading Link At any given point in time, igneous rocks are forming somewhere on Earth. The location and the conditions that are present determine the types of igneous rocks that form. Igneous Rock Formation If you live near an active volcano, you can literally watch igneous rocks form. A hot, molten mass of rock can solidify into solid rock overnight. As you read in Chapter 4, magma is molten rock below Earth's surface. Lava is magma that flows out onto Earth's surface. Igneous rocks form when lava or magma cools and minerals crystallize. In the laboratory, most rocks must be heated to temperatures of 800°C to 1200°C before they melt. In nature, these temperatures are present in the upper mantle and lower crust. Where does this heat come from?

Scientists theorize that the remaining energy from Earth's molten formation and the heat generated from the decay of radioactive elements are the sources of Earth's thermal energy. Composition of magma The type of igneous rock that forms depends on the composition of the magma. Magma is often a slushy mix of molten rock, dissolved gases, and mineral crystals. The common elements present in magma are the same major elements that are in Earth's crust: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), potassium (K), and sodium (Na). Of all the compounds present in magma, silica is the most abundant and has the greatest effect on magma characteristics. As summarized in Table 5.1, magma is classified as basaltic, andesitic, or rhyolitic, based on the amount of silica it contains. Silica content affects melting temperature and impacts how quickly magma flows. Interactive Table To explore 1 Types of Magma more about magma composition, visit glencoe.com. Group more than 66% Yellowstone National Park Once magma is free of the overlying pressure of the rock layers around it, dissolved gases are able to escape into the atmosphere.

Thus, the chemical composition of lava is slightly different from the chemical composition of the magma from which it developed. Magma formation Magma can be formed either by melting of Earth's crust or by melting within the mantle. The four main factors involved in the formation of magma are temperature, pressure, water content, and the mineral content of the crust or mantle. Temperature generally increases with depth in Earth's crust. This temperature increase, known as the geothermal gradient, is plotted in Figure 5.1. Oil-well drillers and miners, such as those shown in Figure 5.2, have firsthand experience with the geothermal gradient. Temperatures encountered when drilling deep oil wells can exceed 200°C. Pressure also increases with depth. This is a result of the weight of overlying rock. Laboratory experiments show that as pressure on a rock increases, its melting point also increases. Thus, a rock that melts at 1100°C at Earth's surface will melt at 1400°C at a depth of 100 km. The third factor that affects the formation of magma is water content. Rocks and minerals often contain small percentages of water, which changes the melting point of the rocks.

As water content increases, the melting point decreases. ^P Reading Check List the main factors involved in magma formation. Mineral content In order to better understand how the types of elements and compounds present give magma its overall character, it is helpful to discuss this fourth factor in more detail. Different minerals have different melting points. For example, rocks such as basalt, which are formed of olivine, calcium feldspar, and pyroxene (pi RAHK seen), melt at higher temperatures than rocks such as granite, which contain quartz and potassium feldspar. Granite has a melting point that is lower than basalt's melting point because granite contains more water and minerals that melt at lower temperatures. In general, rocks that are rich in iron and magnesium melt at higher temperatures than rocks that contain higher levels of silicon. Earth's Geothermal Gradient Earth's Geothermal Gradient Figure 5.1 The average geothermal gradient in the crust is about 25°C/km, but scientists think that it drops sharply in the mantle to as low as 1°C/km. Figure 5.2 The temperature of Earth's upper crust increases with depth by about 30°C for each 1 km.

At a depth of 3 km, this drill bit will encounter rock that is close to the temperature of boiling water. Section 1 • What are igneous rocks? 113 Lowell Georgia/CORBIS Figure 5.3 As the temperature increases in an area, minerals begin to melt. Determine What can you suggest about the melting temperature of quartz based on this diagram? Partial melting Suppose you froze melted candle wax and water in an ice cube tray. If you took the tray out of the freezer and left it at room temperature, the ice would melt, but the candle wax would not. This is because the two substances have different melting points. Rocks melt in a similar way because the minerals they contain have different melting points. Not all parts of a rock melt at the same temperature. This explains why magma is often a slushy mix of crystals and molten rock. The process whereby some minerals melt at relatively low temperatures while other minerals remain solid is called partial melting. Partial melting is illustrated in Figure 5.3. As each group of minerals melts, different elements are added to the magma mixture thereby changing its composition.

If temperatures are not high enough to melt the entire rock, the resulting magma will have a different composition than that of the original rock. This is one way in which different types of igneous rocks form. ^p Reading Check Summarize the formation of magma that has a different chemical composition from the original rock. Bowen's Reaction Series In the early 1900s, Canadian geologist N. L. Bowen demonstrated that as magma cools and crystallizes, minerals form in predictable patterns in a process now known as the Bowen's reaction series. Figure 5.4 illustrates the relationship between cooling magma and the formation of minerals that make up igneous rock. Bowen discovered two main patterns, or branches, of crystallization. The right-hand branch is characterized by a continuous, gradual change of mineral compositions in the feldspar group. An abrupt change of mineral type in the iron-magnesium groups characterizes the left-hand branch. Figure 5.3 As the temperature increases in an area, minerals begin to melt. Determine What can you suggest about the melting temperature of quartz based on this diagram?

Figure 5.4 On the left side of Bowen's reaction series, minerals rich in iron and magnesium change abruptly as the temperature of the magma decreases. Compare How does this compare to the feldspars on the right side of the diagram? Magma types * Potassium feldspar Muscovite mica Quartz High temperature Calcium-rich Low temperature Iron-rich minerals The left branch of Bowen's reaction series represents the iron-rich minerals. These minerals undergo abrupt changes as magma cools and crystallizes. For example, olivine is the first mineral to crystallize when magma that is rich in iron and magnesium begins to cool. When the temperature decreases enough for a completely new mineral, pyroxene, to form, the oliv-ine that previously formed reacts with the magma and is converted to pyroxene. As the temperature decreases further, similar reactions produce the minerals amphibole and biotite mica. Feldspars In Bowen's reaction series, the right branch represents the plagioclase feldspars, which undergo a continuous change of composition. As magma cools, the first feldspars to form are rich in calcium.

As cooling continues, these feldspars react with magma, and their calcium-rich compositions change to sodium-rich compositions. In some instances, such as when magma cools rapidly, the calcium-rich cores are unable to react completely with the magma. The result is a zoned crystal, as shown in Figure 5.5. Fractional Crystallization When magma cools, it crystallizes in the reverse order of partial melting. That is, the first minerals that crystallize from magma are the last minerals that melted during partial melting. This process, called fractional crystallization, is similar to partial melting in that the composition of magma can change. In this case, however, early formed crystals are removed from the magma and cannot react with it. As minerals form and their elements are removed from the remaining magma, it becomes concentrated in silica. Compare Igneous Rocks How do igneous rocks differ? Igneous rocks have many different characteristics. Color and crystal size are some of the features that differentiate igneous rocks. Procedure H] B'i 1. Read and complete the lab safety form. 2. Obtain a set of igneous rock samples from your teacher. 3.

Carefully observe the following characteristics of each rock: overall color, crystal size, and, if possible, mineral composition. 4. Design a data table to record your observations. Analysis 1. Classify your samples as either basaltic, andesitic, or rhyolitic. [Hint: The more silica in the rock, the lighter it is in color.] 2. Compare and contrast your samples using the data from the data table. How do they differ? What characteristics do each of the groups share? 3. Speculate in which order the samples crystallized. [Hint: Use Bowen's reaction series as a guide.] i Figure 5.5 When magma cools quickly, a feldspar crystal might not have time to react completely with the magma and might retain a calcium-rich core. The result is a crystal with distinct calcium-rich and sodium-rich zones. LFoldables| Incorporate information from this section into your Foldable. Section 1 • What are igneous rocks?

115 (t)Marli Miller/Visuals Unlimited, (b)Wally Eberhart/Visuals Unlimited Colorado Parks & Wildlife - Geology COLORADO PARKS & WILDLIFE Page Image Page Content ​​​The Park features a 3,405 surface-acre reservoir lying on the South Platte River at the southern edge of South Park. Formed behind Elevenmile Canyon Dam, other drainages emptying into the reservoir include Cross, Prudence, Union, Balm-of-Gilead, Simms and Spring creeks. The South Platte River Valley is mantled with Wisconsin-aged glacial outwash material (Pleistocene alluvium) which covers older formations. The southeastern portion of the park has been covered by Como-age surface deposits. Most of the park is underlain by Precambrian rocks: Silver Plume granite east and south of Howbert Point and Pikes Peak granite along the river west of Howbert. The western and southern edges consist of Thirtynine Mile andesite deposits; igneous and metamorphic rocks of Tertiary origin. Cross Creek also cuts through another Tertiary deposit called Trachyte (also an igneous and metamorphic rock).

Along the southwestern reservoir margin, Wall Mountain Tuff, part of a south-dipping sequence of welded Tuffs that incorporated volcanic ash and pumice, is exposed as steep cliffs. Thirtynine Mile andesite has been deposited over both the granite and Tuff formations on the southern reservoir edge. Pleistocene alluvium is alluvium that was formed during the Pleistocene Epoch. The Pleistocene Epoch occurred between 1.8 million to 10,000 years before the present and covers the time of the world’s recent period of repeated glaciation. This is also the period during which the rise of early humans occurred. Wisconsin-age refers to the final period of glaciation within the current ice age, as well as to being geographically located within central North America. This age began about 110,000 years ago and ended between 15,000 to 10,000 years before the present. Alluvium is simply soil or sediments deposited by a river or other running water and it is typically made up of a variety of materials, including fine particles of silt and clay, and larger particles of sand and gravel.

Flowing water associated with glaciers may also deposit alluvium, but deposits directly from ice are not alluvium. Precambrian rock is rock that was formed during the Precambrian Era. The Precambrian Era spanned a period of time from the formation of the Earth (around 4.5 billion years ago) to the evolutionary point of abundant macroscopic hard-shelled animals, which marked the start of the Cambrian Era, some 542 million years ago. Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Igneous rock is formed by the solidification of cooled magma and it is intrusive if the crystallization or solidification occurred underground. Felsic is a term referring to silicate minerals, magma and rocks that have been enriched with lighter elements like silicon, oxygen, aluminum, sodium and potassium. Granite has a medium to coarse texture and can be pink to dark gray or even black. Outcrops of granite tend to form tors and rounded massifs. Granite is nearly always massive, hard and tough. Pikes Peak granite is a widespread geologic formation found in the front range of Colorado, including around the Pikes Peak region.

Silver Plume granite is slightly pinkish in color and occurs in the central portion of the Front Range, much of it around Rocky Mountain National Park. They were both created by cooled magma but were the result of different geologic events. Andesite is an igneous, volcanic rock, of intermediate composition, with fine to large grained crystal texture. The mineral assemblage of andesite is typically dominated by plagioclase (tectosilicate minerals found within the feldspar family) plus pyroxene and/or hornblende. Alkali feldspar may be present in minor amounts. Andesites are characteristic of subduction zones, where two tectonic plates meet and move towards one another, with one moving beneath the other. Thirtynine Mile Andesite simply refers to the type of andesite formed around the extinct Thirtynine Mile volcano area. The Tertiary period of geologic time covers roughly the time span between the demise of non-avian dinosaurs and the beginning of the most recent ice-age, approx. 65 to 1.8 million years ago.

Andesite is formed extrusively, from hot magma that has flowed out onto the Earth's surface as lava or has exploded violently into the atmosphere to fall back as pyroclasts (fragments of pre-existing rock composed primarily of volcanic material) or Tuff (volcanic ash ejected from vents). The main effect of extrusion is that the magma cools much more quickly in the open air or seawater and there is little time for the growth of crystals. Often a residual portion fails to crystallize at all, instead becoming an interstitial natural glass or obsidian. Trachyte is another igneous, volcanic rock. The mineral assemblage consists of essential alkali or sanidine feldspar and is usually somewhat porous in appearance, and very often has minute irregular steam cavities which make the broken surfaces of specimens of these rocks rough and irregular. It is from this characteristic that trachyte derives its name. Feldspar is the name of a group of rock forming minerals and sanidine refers to the type of feldspar that contains potassium that has been placed under high temperatures, such as volcanic activity. Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption.

The products of volcanic eruption are volcanic gases, lava, steam, and tephra. Magma is blown apart when it interacts violently with volcanic steam and gasses. The solid material produced and thrown into the air by such volcanic eruptions is called tephra, regardless of composition or fragment size. If the resulting pieces of ejected material are small enough (sand-sized or smaller particles), the material is called volcanic ash. These particles are small, slaggy pieces of magma and rock that have been tossed into the air by outbursts of steam and other gasses; magma may have been torn apart as it became vesicular (containing many vesicles) by the expansion of the gasses within it.​

Question: In geology, igneous refers to rock formed by what effect?
Answer: Volcanic