Eno Geologic Principles Igneous Rocks - Introduction Igneous rocks, born from molten rock called magma, dominate the western portions of the Eno River. All igneous rocks originate deep in the Earth as magma that rises toward the Earth’s surface at temperatures ranging from 1,800 to 2,200 degrees Fahrenheit (°F) (1,000 to 1,200 degrees Celsius (°C)). Igneous rocks are separated into two main categories: intrusive and volcanic rocks. Intrusive rocks form when magma solidifies within the Earth’s crust. Volcanic rocks form when magma solidifies on top of the Earth’s crust on land or in water. Intrusive Igneous Rocks Magma that cools and solidifies before reaching the surface of the Earth forms intrusive igneous rocks. Intrusive rocks are also commonly known as plutonic rocks. The word plutonic comes from Pluto, the Roman god of the underworld. Plutonic rocks form bodies known as plutons. Intrusive rocks cool slowly since they are insulated by the surrounding earth. This slow cooling allows the chemical elements within the magma to organize themselves into individual crystals that are visible with the naked eye.

Figure 5 shows an example of a circa 630 million-year-old intrusive rock from the Eno River area. East of Roxboro Road, in the Triassic basin, a relatively young intrusive rock called diabase is present. The diabase is around 200 million years old and is present as tabular-shaped bodies that either intruded the surrounding sedimentary rocks parallel to the existing layering as sills or cut across the existing layering as dikes (fig. 6). Figure 5 - Example of a 630 million year old igneous intrusive rock of the Carolina terrane. The rock is a granodiorite (granite-like rock) Figure 6 – Sample of diabase from the Eno River area. Photograph shows typical typical rust-colored weathering rind. Volcanic Igneous Rocks (AKA - Extrusive Igneous Rocks) Volcanic rocks, also born from molten magma, form when hot magma erupts out of a volcanic vent or when magma explodes from volcanoes. The word volcanic comes from Vulcan, the Roman god of fire.

Geologists studying ancient volcanic regions, like the Eno River area, use modern volcanoes and their deposits as a guide to help them unravel the geologic history. Modern volcanic terranes are dominated by four basic volcanic deposits: a) lava flows, b) pyroclastic flows, c) tephra, and d) lahars ( fig. 7 ). The Eno River area has evidence of all four types of these deposits. Lava flows, pyroclastic flows and tephra deposits are also known as extrusive volcanic rocks. Extrusive refers to the magma being extruded onto the surface of the Earth. As magma is extruded on the surface, it cools very quickly and individual chemical elements do not have time to grow into minerals visible to the naked eye and produce a fine-grained or aphanitic rock. This is in contrast to the intrusive rocks, that cool slowly, and typically have mineral grains that are visible with the naked eye. Lava flows When hot magma is erupted onto the surface of the Earth, it is called lava. The word lava is used to describe both moving melted rock, and cooled and hardened rock that originated as lava.

Lava flows are streams of molten rock that pour or ooze from an erupting volcanic vent. Lava comes in many types defined by the range of minerals in the rock. Millions of years ago, lava flowed in the Eno River area. One type of rock common in the Eno River area formed from solidified lava is called dacitic lava (fig. 8a). When molten, dacitic lava is viscous (thick and sticky) and does not flow very far from the volcanic vent. It forms steep-sided mounds of lava called domes. Lava domes grow by the extrusion of many individual flows over time. The lava domes within the crater of Mt. St. Helens in Washington State are examples of recent dome formation (fig 8b). Figure 8a – Example of ancient dacitic lava from the Eno River area. Figure 8b – Modern day example of a dacitic lava dome in the crater of Mt. St. Helens in Washington State. Note people for scale indicated by red circle. Photograph from USGS. Another type of rock in the Eno River area of solidified lava is basaltic lava (fig. 8c).

Basaltic lava, or basalt, is typical of the Hawaiian Islands (fig. 8d). Basaltic lava differs from dacitic lava (dacite) in that it is less viscous (able to flow easier and faster) and tends to spread out from the vent forming thin sheets of lava. This results in thinner geologic deposits when compared to the larger dome-like deposits of dacitic lava. Figure 8c –Example of basaltic lava exposed in the Eno River area. Figure 8d –Example of basaltic lava erupting from a vent on Kilauea Volcano in Hawaii. Photograph from USGS taken by C. Heliker, September 12, 2003. Pyroclastic flows Pyroclastic flows are formed from explosive volcanic eruptions and are fast-moving clouds of hot gas, rock fragments and ash. The rock fragments can range in size from ash to boulders and can travel across the ground hurricane-force speeds often more than 60 miles per hour (100 km per hour). Pyroclastic flows are violent and will knock down, shatter, bury or carry away nearly all objects in their path.

These flows are also very hot, generally between 400°F and 1,300°F (200°C and 700°C) and will virtually incinerate anything in their path. Pyroclastic flows typically form during the collapse of lava domes or after the collapse of dense eruption clouds. Rocks interpreted to have been deposited via pyroclastic flows are common in the Eno River area interlayered with dacitic lavas (fig. 9). Figure 9a – Example of ancient pyroclastic flow deposits from the Eno River area. Outcrop of welded lithic tuff with flattened pumice (fiamme) from the Eno River area. Figure 9b – Example of ancient pyroclastic flow deposits from the Eno River area. Reddish-colored welded tuff from the Eno River area. Figure 9c – Example of a modern day pyroclastic flow eruption at Mount St. Helens on July 22, 1980 (USGS).

Figure 9d - Collapse of lava dome generates pyroclastic flow on Unzen Volcano, Japan, on March 23, 1993 (USGS) Tephra Tephra is the general name for fragments of volcanic rock that are blasted into the air by volcanic explosions or carried upward by hot gases in eruption clouds above volcanic vents. Tephra can range in size from less than 1/32 inch (2 millimeters) to more than 3 feet (1 meter) in diameter. Tephra less than 2 millimeters diameter is called volcanic ash. Tephra consists of a wide range of rock particles, including combinations of pumice, glass shards, crystals from different types of minerals, and shattered rocks of all types (igneous, sedimentary and metamorphic). The largest-sized tephra (rocks several meters in diameter) typically is deposited close to the erupting volcanic vent. The fine-grained ash may be blown by the wind hundreds to thousands of miles away. A single eruption can rain down tephra and blanket the landscape close to the volcanic vent with hundreds of feet of ash and volcanic debris.

Farther from the volcanic vent, the tephra that falls to the ground is smaller in size and forms only a thin layer of ash a few centimeters thick. When the tephra is buried and hardens into rock it become a rock named tuff. The tuffs exposed today in the vicinity of the Eno River (fig. 10) record hundreds, possibly thousands, of ancient eruptions. Figure 10a – Example of tuff with white-colored weathering rind from the Eno River area. Scale bar is in centimeters. Figure 10b – Example of lithic tuff with black clasts of dacitic lava in a green ash matrix from the Eno River area. Figure 10c – Example of an ash cloud from an eruption of Soufriere Hills volcano on the Caribbean island of Mountserrat. Photograph by Heather Hanna 2002. Figure 10d – Example of layered ash deposits from 1990 eruption of Redout volcano in Alaska. Photograph from USGS. Lahars Lahar is an Indonesian word used by geologists to describe a mudflow associated with volcanic activity.

Lahars are very powerful and are capable of moving great quantities of debris (as large as house-sized boulders) long distances in a short amount of time. Modern lahars look and behave like flowing wet concrete and destroy virtually everything in their path – including houses, cars, trees, etc. (fig. 11a and 11b). Volcanic eruptions may initiate lahars when snow or glacial ice is rapidly melted and travels down the mountain. Lahars may also be initiated during steam explosions or the collapse of a volcano from earthquakes. Intense rainfall, years after a volcanic eruption, may destabilize volcanic debris and initiate a lahar. Lahars typically follow the topography and flow into, and fill in, valleys on the flanks and base of a volcano. Lahars are one of the most dangerous features of modern volcanoes. An example of an ancient lahar deposit is visible at Few’s Ford in Eno River State Park (Allen and Wilson, 1968 and Rochester, 1978) (fig. 11c). The lahar deposits at Few’s Ford are described as part of the field trip guides of this publication.

Figure 11a - Lahar deposit from 1995 on southwest flank of Unzen volcano, Japan. Photograph by Tom Pierson - USGS. Figure 11b - Lahar deposit from eruption of Soufriere Hills volcano - Town of Plymouth, Mountserrat. Photograph shows war memorial buried in approximately 18 feet of ash and lahar deposits. Photograph by Heather Hanna 2002. Figure 11c - Ancient lahar deposit at Few's Ford Eno River State Park Special Note About the Igneous Rocks of the Eno River Area The majority of the igneous rocks associated with the Carolina terrane have been subjected to low-grade metamorphism; subsequently, the prefix “meta” is commonly added to the rock name. The metamorphosed igneous rocks should technically be called meta-intrusive rocks (e.g., metagranodiorite, metagranite) and metavolcanic rocks (e.g., metatuff and metabasalt). Since many of the rocks exposed along the Eno River retain the original intrusive and volcanic textures present in the rock at formation, geologists often leave off the “meta” prefix.

The low-grade metamorphosed examples of the above igneous rocks from the Eno River area are still excellent examples of igneous rocks for in hand sample they show all the same textures as a pristine un-metamorphosed rock. Geology Dictionary - Magma, Mudstone, Mylonite - GEOLOGY.COM Maar A maar is a small, shallow volcanic crater with steep sides that forms in an explosive volcanic eruption that is triggered when rapidly ascending magma contacts groundwater at a shallow depth and a steam explosion occurs. Maars are usually surrounded by tephra deposits that are thickest near the maar and thin with distance. Most maars are only a few hundred to several thousand feet across and are at most a few hundred feet deep. After cinder cones, they are the second most common volcanic landform. Mafic A term used to describe an igneous rock that has a large percentage of dark-colored minerals such as amphibole, pyroxene, and olivine . Also used in reference to the magmas from which these rocks crystallize. Mafic rocks are generally rich in magnesium and iron (think MAgnesium and FerrIC). Basalt and gabbro are examples of mafic rocks.

(See felsic to contrast.) Magma Molten rock material that occurs below Earth's surface. When it erupts onto the surface, it is known as "lava." Magma Chamber A subsurface reservoir of magma at a shallow depth in Earth's crust. It can serve as a source of magma and gases for volcanic eruptions if it connects to the surface through a pipe or connects to another magma chamber through a dike or a sill. Some magma chambers never produce a volcano and slowly cool to produce a differentiated mass of igneous rock . Magmatic Water Water that is dissolved in a magma or water that is released from a magma. Some magmas can contain up to several percent dissolved water by weight. The presence of water can lower the melting temperature of a rock . Magnetic Anomaly An increase or decrease in the local magnetic field compared to the normally expected value. The image is a section of the United States Geological Survey's full spectrum magnetic anomaly grid of the United States with improved long wavelengths for studying continental dynamics. Magnetic Declination The horizontal angular difference between Geographic North and Magnetic North. This angular difference varies, depending upon your location on the face of the Earth.

Dictionary of Geological Terms - Only $18.95 All scientific disciplines have an essential vocabulary that students and professionals must understand to learn and communicate effectively. A geology dictionary that is used regularly is one of the most important tools for developing professional competence. A good dictionary should be on the desk of every geologist and within easy reach. This dictionary is compact and inexpensive at only $18.95. More information . Magnetic Inclination The vertical angular difference between a horizontal plane and the orientation of Earth's magnetic field. Near the magnetic equator the magnetic inclination will be approximately zero. As the pole is approached the magnetic inclination will increase, and at the magnetic pole the magnetic inclination will be 90 degrees. Magnetic North The direction that a compass points. The location where Earth's magnetic field dips vertically into the Earth. Magnetic Reversal A change in the polarity of Earth's magnetic field in which the north magnetic pole becomes the south magnetic pole and vice versa. Also known as geomagnetic reversal or polarity reversal. Earth's magnetic field has reversed many times in the past, and the time intervals between these changes are known as polarity epochs.

Magnetic Stratigraphy The correlation of rock units and the study of Earth's history using magnetic events and magnetic epochs as a time reference. Magnetometer An instrument designed to measure the strength and character of Earth's magnetic field or the magnetic properties of the underlying rocks. There are many types of magnetometers for different scales of measurement and for different methods of surveying. Some are carried by hand or in vehicles. Others are towed behind ships or aircraft. Magnitude A measure of earthquake strength based upon the amount of ground motion experienced and corrected for the distance between the observation point and the epicenter. There are several magnitude scales in use. Malachite Malachite is a copper carbonate mineral that has served as a minor ore of copper. It has a bright green color, often with beautiful banding, swirl, and eye patterns. This makes it an attractive gemstone . Because it is soft and cleaves easily, malachite is best used in items that will not be subjected to abrasion or impact. Mali Garnet Mali is a yellow to yellowish green variety of garnet , named after the African country of Mali. It is a mixture of grossular and andradite that is occasionally seen in jewelry.

It has great dispersion (fire) as seen in this photo. Manganese nodule A rounded concretion, rich in manganese minerals with minor concentrations of cobalt, copper , nickel, and other metals. These nodules are abundant on some parts of the deep ocean floor and have been considered as a potential source of manganese. The photo shows iron-manganese nodules on the seafloor north of the Puerto Rico Trench at a depth of about 5339 meters. The nodules are about two to four centimeters in diameter. Mantle A major subdivision of Earth's internal structure. Located between the base of the crust and the top of the core. It is approximately 1800 miles thick and has a composition that is distinctly different from the crust above and the metallic core below. The mantle makes up about 84% of Earth's volume. Mantle Plume A rising mass of hot mantle material that can create an area of volcanic activity in the center of a lithospheric plate. The illustration shown here is a simplified representation of the mantle plume that produced the Hawaiian Islands. Marble A non-foliated metamorphic rock that is produced from the metamorphism of limestone . It is composed primarily of calcium carbonate.

It is often used as a building stone in prestige architecture. The photo shows part of the West Building of the National Gallery of Art, which has extensive use of marble in both the exterior and interior. Mary Ellen Jasper Mary Ellen is a rock that consists of red jasper and hematite with a submetallic luster. The jasper is a fossil stromatolite, a layered structure built up by sediment-trapping algae that lived on Earth about two billion years ago - long before land plants. Massive A term used in reference to a rock unit that is homogeneous in texture, fabric, and appearance. Mass Wasting (also Mass Movement) A general term used for any downslope movement of rock, soil, snow, or ice under the influence of gravity. Includes: landslides , creep, rock falls, and avalanches. Matrix Opal Matrix opal is a material in which precious opal is in an intimate mixture with the host rock, instead of being confined to seams and patches. The specimen in the photo is from Andamooka, Australia, and is a sedimentary rock with opal filling the voids between sedimentary grains.

Maw Sit Sit Maw sit sit is a rock composed of jadeite , albite, and kosmochlor (a mineral related to jadeite). It is attractive, has a bright chrome green color and accepts a bright polish, and for those reasons it is used as a gemstone . Mcf One thousand cubic feet - a standard production and sales volume for natural gas. (The "M" represents the Roman numeral for one thousand.) MMcf One million cubic feet - a standard production and sales volume for natural gas. (The "M" represents the Roman numeral for one thousand. Two "M's" represent one thousand-thousands.) Meandering Stream A stream that has many bends (meanders). This type of drainage pattern usually develops on a nearly level landscape and where the banks of the stream are easily eroded. Mechanical Weathering A general term applied to a variety of weathering processes that result in the particle size reduction of rock materials with no change in composition. Abrasion, frost action, salt crystal growth, and pressure relief fracturing are examples. Also known as physical weathering. The photo shows granite pebbles and boulders on the flood plain of a wash, where flash floods pick up, carry, and abrade the sediment particles.

In the winter, the forces of freezing and thawing slowly damage the rocks. Medial Moraine A streak of till in the center of a glacier . These are found downslope from the junction of two glaciers and are a merging of their lateral moraine deposits. The image is an aerial photo showing the confluence of the Gilkey and Bucher Glaciers, showing numerous medial moraines. Image from the Juneau Icefield, Tongass National Forest, Alaska. Medical Geology The study of human health related to geology. Examples would include the correlation of disease or vitality with residences over specific types of bedrock, or health problems associated with exposure to specific mineral materials. In the photo, a hydrologist measures the quality of irrigation water at a farm. Melanite is a lustrous, black, opaque variety of garnet that is not often seen in jewelry. It is a titanium garnet of the Andradite group and is sometimes called "titanian andradite." Metamorphism Alteration of the minerals, textures, and composition of a rock that is caused by exposure to severe heat, pressure, and chemical actions.

Metamorphism occurs to rocks at convergent plate boundaries, rocks that have been deeply buried, and rocks that have been contacted by migrating magma or hydrothermal fluids. Meteor A streak of light momentarily visible in the night sky when a meteoroid penetrates Earth's atmosphere. Meteoroids can be travelling at speeds of 20 kilometers per second or more. That great speed causes it to impact air molecules with force that is sufficient to heat it to a glowing temperature and vaporize particles from its surface. A trail of these hot particles is left behind the meteoroid. Because of their high temperature they glow momentarily, producing a streak of light. This streak of light is known as a "shooting star" or a "meteor." Meteoric Water Meteorite A particle of iron or rock that has fallen to Earth's surface from interplanetary space. They sometimes have concave pits in their surface caused by ablation as they fall through the atmosphere. Meteorites are also found on the Moon and other bodies in our solar system. In fact, the NASA Mars Rovers have found many Martian meteorites .

Methane Hydrate Methane hydrate is a crystalline solid that consists of a methane molecule surrounded by a cage of interlocking water molecules (see image at left). Methane hydrate is an "ice" that forms in sediments on the continental slope at a depth where cold temperatures occur. It has the potential to be tapped as a source of methane. There is more fuel value contained within Earth's methane hydrate than in all of the remaining oil and natural gas deposits combined. Mica Mica is the name used for a group of sheet silicate minerals with a generalized chemical composition of (K,Na,Ca)(Mg,Fe,Li,Al)2-3(OH,F)2[(Si,Al)4O10]. These minerals have basal cleavage which is so well developed that specimens have the ability to split into very thin sheets. Shown in the photo are the two most common mica minerals, biotite and muscovite . Microseism A vibration of the Earth that is unrelated to earthquake activity - instead it is caused by wind, moving trees, ocean waves, or human activity. Milling The activities of preparing an ore for market.

These activities can include crushing, grinding, concentration, separation of impurities, and conversion into a transportable state. The image shows a ball mill, a large revolving drum into which ore is placed with large steel balls. The drum is rotated and the balls are tossed around inside, repeatedly impacting the ore and crushing it into a fine powder over time. This small particle size allows impurities to be separated from the target material. Milling Capacity The maximum amount of material that a mill can process in a unit of time. Tons per hour is a typical unit for milling capacity. A naturally occurring, inorganic solid with a definite chemical composition and an ordered internal structure. If it isn't grown, it is probably a mineral that is produced from a mine. Mineral Interest An ownership, lease, concession, or other contractual interest that gives a party the right to explore and extract mineral resources on a property. The photo shows test drilling of a mineral property. Mineral Lease A contract in which a mineral interest owner conveys to another party a right to explore for, develop, and produce mineral resources. The lessee acquires a working interest and the lessor retains a royalty interest of a specified percentage. The photo shows test drilling of a mineral property.

Mineralogy Mineraloid A mineraloid is an amorphous, naturally occurring inorganic solid that does not exhibit crystallinity. It may have the outward appearance of a mineral , but it does not have the “ordered atomic structure” required to meet the definition of a mineral. The photo shows a piece of obsidian , which is a mineraloid because as a glass it does not possess a crystalline structure. Other well-known mineraloids are pumice , opal , and limonite . Mineral Pigments Pigments that are made by crushing minerals to a fine powder. Most of the pigments that have been used across history have been made from minerals. People have been collecting minerals and using them as pigments for about 40,000 years. Hematite is used to produce a wide range of red to reddish-brown pigments. Limonite (a mineraloid) has been used to produce yellow to brown pigments as shown in the photo. Glauconite is used to create green, lazurite for blue, psilomelane for black, cinnabar for red, orpiment for orange, malachite for green, and barite for white.

These are just a few examples of minerals used as pigments. The pigment minerals can be mixed with oil, water, and other liquids for use as a paint. They can also be used to give color to plaster, stucco, cosmetics, chalk, and similar material. Mohs Hardness Scale A collection of minerals ranging from very soft to very hard. Use as a comparison scale during mineral identification. From softest to hardest, the ten minerals are: talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, and diamond 10. Developed by Friedrich Mohs, a German mineralogist, in the early 1800s. Moldavite is a glassy material that is thought to have formed when a large asteroid impacted a location in eastern Europe about 15 million years ago. The target rock and impactor melted and solidified into an olive green glass. Molecule A group of two or more atoms held together by chemical bonds. The group of atoms does not have an electrical charge and is the smallest possible unit of that substance that is possible.

The image shows a sodium atom and a chlorine atom combined to produce a salt molecule. Monocline An area of increased dip in otherwise gently dipping strata. The image shows a monocline over a reverse fault. Mookaite A colorful gem material mined in Australia. It forms from the deposition and lithification of a sediment composed of the silica tests of radiolarians. The resulting rock is known as a radiolarite. It is a popular gem material for making cabochons and beads. Moonstone Moonstone is a name given to translucent orthoclase feldspar that exhibits adularescence (a white-to-bluish light that floats just under the surface of the stone when it is turned under a source of light). It is a very popular gemstone . Moraine A mound, ridge, or ground covering of unstratified and unsorted till, deposited by ice action or by melting away of a glacier . Many different types of moraines are distinguished: terminal, ground, lateral, ablation, medial, push, and recessional. The photo shows a push moraine formed by the Harriman Glacier of Alaska.

Morganite, also called "pink beryl," is the pink- to salmon-colored gemstone member of the beryl mineral group. Moss Agate Moss agate is a transparent-to-translucent chalcedony that contains mineral inclusions which are shaped like moss, trees, leaves, or other vegetation. Mother of Pearl Mother of pearl, also known as "MOP," is the thin inner nacreous layer of a mollusk shell. It can be white, cream, or gray in color with a beautiful iridescent play-of-color. It is cut and shaped for use in jewelry, buttons, musical instruments, and more. Mountain A general term used in reference to an area that is at a conspicuously higher elevation than surrounding lands. Mountains are larger than hills and are significant enough in relief that they are given names by local residents. Shown in the photo is Mount Everest, the mountain with the highest altitude (8,850 meters / 29,035 feet). Mud Cracks A network of polygonal shrinkage cracks that open in mud as the water within is slowly evaporated.

They can harden, and if buried, can be lithified as a preserved sediment surface that is evidence of submersion followed by subaerial exposure. They can indicate a sedimentary environment of a lake shore, river bank, or low-energy beach. Also known as desiccation cracks. Mudflow A downslope movement of wet soil and rock debris that is mostly composed of clay-size particles and water. At the base of the slope, the flow spreads out over the runout area in the form of a lobe. The internal particle motion is that of a flow rather than of a rotating or translating mass. Many mudflows move at a rate of a few feet per year or less, but some reach speeds in excess of 60 miles per hour. Movement is usually triggered during times of heavy rainfall or rapid snow melt. Mud Logger A worker at a drilling site who does the work of "mud logging." Mud Logging During the drilling of a well for oil, natural gas, water, gold , coal , iron ore , or almost any resource, a record of the rocks penetrated by the drill is often desired.

This information is valuable for knowing the type of overburden that must be mined through, the roof rock quality of an underground mine, correlation of rock units between wells or outcrops, sampling of the target rock unit, and many other purposes. Samples of drill cuttings are collected from the drilling mud and labeled with depth and other information as the well is being drilled. The cuttings are then cleaned and dried, then examined by geologists known as "mud loggers" who prepare written and photographic records of the rocks penetrated by the well. The cuttings are often archived for future reference. Shown in the photo are samples of wet mud collected from the well before cleaning and drying. For this project, samples were collected every ten feet and placed in a sample tray. This photo shows a small portion of that tray. Mudstone A sedimentary rock composed of clay-size particles but lacking the stratified structure that is characteristic of a shale . The image shows a hole in mudstone on Mars that was drilled by NASA's Curiosity Rover in May 2013. The hole is about 0.6 inch across. Mud Volcano A mud volcano is a vent in Earth's surface from which liquid mud erupts.

They occur in at least three geologic situations: 1) in volcanic areas where hydrothermal activity produces steam and gas pressure that mobilizes mud and forces it to the surface; 2) where tectonic activity forces fluidized sediments to the surface; and, 3) where fluidized sediments pressurized by hydrocarbons are driven to the surface. Multiple Completion Well A well equipped to produce oil and/or gas from more than one subsurface rock unit. This is done by isolating the producing zones within the well with plugs. The production casing will be perforated between the plugs. These perforations will allow fluids that flow from the formation into the well bore to enter the production casing and be evacuated to the surface. M.Y. Geology - Yosemite National Park (U.S. National Park Service) Geology Geology Geology Yosemite is a glaciated landscape, and the scenery that resulted from the interaction of the glaciers and the underlying rocks was the basis for its preservation as a national park.

Iconic landmarks such as Yosemite Valley, Hetch Hetchy, Yosemite Falls, Vernal and Nevada Falls, Bridalveil Fall, Half Dome, the Clark Range, and the Cathedral Range are known throughout the world by the photographs of countless photographers, both amateur and professional. Landforms that are the result of glaciation include U-shaped canyons, jagged peaks, rounded domes, waterfalls, and moraines. Glacially-polished granite is further evidence of glaciation, and is common in Yosemite National Park. The Making of the Landscape Topographically, the Sierra Nevada is an asymmetric mountain range with a long, gentle west slope and a short, steep east escarpment that culminates in the crest of the Sierra Nevada. It is 50 to 80 miles wide and extends in altitude from near sea level along its west edge to more than 13,000 feet along the crest in the Yosemite area, and more than 14,000 feet along the crest in the Sequoia-Kings Canyon area. The highest peak in the Sierra Nevada, and in the continental United States, can be found to the south: Mount Whitney in Sequoia National Park.

Geologically, the Sierra Nevada is a huge block of the Earth's crust that has broken free on the east along a bounding fault system and has been uplifted and tilted westward. This combination of uplift and tilt, which is the underlying geologic process that created the present range, is still going on today. Jointed granite, Olmsted Point. Massive granite dominates the Yosemite area and much of the Sierra Nevada as well. Mount Hoffmann and most of the terrain visible from it are composed of granite, formed deep within the Earth by solidification of formerly molten rock material and subsequently exposed by erosion of the overlying rocks. Because of its massiveness and durability, granite is shaped into bold forms: the cliffs of Yosemite and Hetch Hetchy Valleys, many of the higher peaks in the park, and the striking sheeted domes that can form only in massive, unlayered rock. Although granite dominates nearly the entire length of the Sierra, the granite is not monolithic. Instead, it is a composite of hundreds of smaller bodies of granitic rock that, as magma (molten material), intruded one another over a timespan of more than 100 million years.

This multiplicity of intrusions is one of the reasons why there are so many varieties of granitic rock in Yosemite and the rest of the Sierra. The differences are not always apparent to the casual observer, but they are reflected in sometimes subtle differences in appearance and in differences in response to weathering and erosion acting on the rocks. Layered metamorphic rocks in the foothills at the west edge of the park and along the eastern margin in the summit area are remnants of ancient sedimentary and volcanic rocks that were deformed and metamorphosed in part by the invading granitic intrusions. Other metamorphic rocks that once formed the roof beneath which the granitic rocks solidified were long ago eroded away to expose the granitic core of the range, and only small isolated remnants are left. Because Yosemite is centered on this deeply dissected body of granite, metamorphic rocks are sparse; they occupy less than 5 percent of the area of the park. Former Lyell Glacier, West Lobe Evolution of the landscape is as much a part of the geologic story as the rocks themselves, and Yosemite is a place where the dynamism of geologic processes is well displayed.

By the end of Cretaceous time, about 65 million years ago, after the granitic core of the range had been exposed, the area had a low relief in comparison with the mountains of today. Then, about 25 million years ago, this lowland area began to be uplifted and tilted toward the southwest, a construction that would eventually lead to the present Sierra Nevada. As the rate and degree of southwest tilt increased, the gradients of streams flowing southwestward to California's Central Valley also increased, and the faster flowing streams cut deeper and deeper canyons into the mountain block. About 10 million years ago, from the Tuolumne River northward, these canyons were inundated and buried by volcanic lava flows and mudflows, and the streams were forced to begin their downcutting anew, in many places shifting laterally to find a new route to the Central Valley. The streams were equal to the task, however, and the present river courses and drainage patterns throughout the Sierra became well established.

Maclure Basin from Upper Lyell Base Camp As the world grew colder, beginning about 2 or 3 million years ago, the Sierra Nevada had risen high enough for glaciers and a mountain icefield to form periodically along the range crest. When extensive, the icefield covered much of the higher Yosemite area and sent glaciers down many of the valleys. Glacial ice quarried loose and transported vast volumes of rubble, and used it to help scour and modify the landscape. Much of this debris eventually accumulated along the margins of the glaciers and in widely distributed, hummocky piles. The greatest bulk of this debris, however, was flushed out of the Sierra to the Central Valley by streams swollen with meltwater formerly stored in the glaciers as ice and released as the glaciers melted away. Although many of today's general landforms existed before modification by glacial action, some of them surely did not. Can you imagine the Yosemite landscape with no lakes? Virtually all the innumerable natural lakes in the park are the result of glacial activity. But even these lakes are transitory, doomed to be filled with sediment and become meadows; many lakes already have undergone this transformation. Yosemite Valley itself once contained a lake.

The geologic story of Yosemite National Park can be considered in two parts: (1) deposition and deformation of the metamorphic rocks and emplacement of the granitic rocks during the Paleozoic and Mesozoic; and (2) later uplift, erosion, and glaciation of the rocks during the Cenozoic to form today's landscape. Rocks The vast majority of Yosemite is comprised of plutonic igneous rocks. Plutonic rock forms deep underground when molten rock cools and solidifies very slowly, allowing large crystals to form. In contrast, volcanic igneous rocks form at the surface when molten rock cools and solidifies quickly, resulting in small crystals. Granite, granodiorite, tonalite, quartz monzonite, and quartz monzodiorite are all forms of plutonic rock that are found in Yosemite, and are loosely referred to as granitic rocks . Quartz diorite, diorite and gabbro are plutonic rocks found in Yosemite, but are not technically considered to be granitic rocks. Plutonic rocks are primarily comprised of 5 minerals: quartz, potassium feldspar, plagioclase feldspar, biotite, and hornblende.

Plutonic rocks, including granitic rocks, differ primarily in the relative proportions of quartz and feldspar, although texture is also an important consideration. The plutonic rocks were generally formed during the Cretaceous period. Half Dome Granodiorite at Olmsted Point. The U.S. penny is used for scale. The different individual bodies of plutonic rocks in Yosemite were formed from repeated intrusions of magma into older host rocks beneath the surface of the Earth. These intrusions may have taken place over a time period as long as 130 million years. These plutonic rocks, formerly deep within the Earth, are now exposed at the surface, owing to deep erosion and removal of the formerly overlying rocks. Volcanic igneous rocks are erupted onto the Earth's surface and cool and solidify much more quickly than plutonic igneous rocks. There are small amounts of volcanic igneous rocks within Yosemite and large amounts east of the Sierra Nevada Crest. The volcanic rocks inside the park include basalt flows, latite tuff, and latite lava flows. The volcanic rocks outside the park include these same rocks as well as ash-flow tuff, rhyolite, pumice, obsidian, etc.

The Mono Craters, east and southeast of the park, are volcanoes that erupted 3,000 to 550 years before present. The Inyo Craters, southeast of the park, are volcanoes that erupted 40,000 to 3,000 years before present. Cathedral Peak Granodiorite from near Fairview Dome. The white blocky structures are "potassium feldspar phenocrysts" (crystals). Note the U.S. penny just to the right of middle; it is used for scale. Metamorphic rocks are sedimentary or volcanic rocks that have been changed, or "metamorphosed," due to the temperature, pressure, and shearing stress that can result from being buried in the Earth's crust. There are two northwest-trending belts of metamorphic rocks, one on each side of the batholith of plutonic rocks that is the core of the Sierra Nevada range - these belts existed before the creation of the batholith and its subsequent exposure. Metamorphic rocks of volcanic origin are called metavolcanic rocks, and metamorphic rocks of sedimentary origin are called metasedimentary rocks.

(Huber, 1989) Granite: Igneous Rock - Pictures, Definition & More Granite What is Granite? What is Granite Used For? Granite: The specimen above is a typical granite. It is about two inches across. The grain size is coarse enough to allow recognition of the major minerals. The pink grains are orthoclase feldspar, and the clear to smoky grains are quartz or muscovite. The black grains can be biotite or hornblende. Numerous other minerals can be present in granite. What is Granite? Granite is a light-colored igneous rock with grains large enough to be visible with the unaided eye. It forms from the slow crystallization of magma below Earth's surface. Granite is composed mainly of quartz and feldspar with minor amounts of mica, amphiboles, and other minerals. This mineral composition usually gives granite a red, pink, gray, or white color with dark mineral grains visible throughout the rock. Granite in Yosemite Valley: Photograph of Yosemite Valley, California, showing the steep granite cliffs that form the walls of the valley. © iStockphoto / photo75. The Best-Known Igneous Rock Granite is the best-known igneous rock.

Many people recognize granite because it is the most common igneous rock found at Earth's surface and because granite is used to make many objects that we encounter in daily life. These include counter tops, floor tiles, paving stone, curbing, stair treads, building veneer, and cemetery monuments. Granite is used all around us - especially if you live in a city. Related: Explore the many uses of granite! Granite is also well-known from its many world-famous natural exposures. These include: Stone Mountain, Georgia; Yosemite Valley , California; Mount Rushmore, South Dakota; Pike's Peak, Colorado; and White Mountains, New Hampshire. Rock & Mineral Kits: Get a rock, mineral, or fossil kit to learn more about Earth materials. The best way to learn about rocks is to have specimens available for testing and examination. Yosemite Nature Notes - Granite: This video examines some of the granites that create the scenery and climbing pleasures of Yosemite National Park. Yosemite Nature Notes - Granite: This video examines some of the granites that create the scenery and climbing pleasures of Yosemite National Park. Granite: Photograph of a white, fine-grained granite. This specimen is about two inches across.

Multiple Definitions of Granite The word "granite" is used in a variety of ways by different people. A simple definition is used in introductory courses; a more precise definition is used by petrologists (geologists who specialize in the study of rocks); and, the definition of granite expands wildly when used by people who sell decorative stone such as countertops, tile, and building veneer. These multiple definitions of granite can lead to communication problems. However, if you know who is using the word and who they are communicating with, you can interpret the word in its proper context. Three common usages of the word "granite" are explained below. Granite close up: Magnified view of the white, fine-grained granite from the photograph above. The area shown in this image is about 1/4 inch across. A) Introductory Course Definition Granite is a coarse-grained, light-colored igneous rock composed mainly of feldspars and quartz with minor amounts of mica and amphibole minerals. This simple definition enables students to easily identify the rock based upon a visual inspection.

B) Petrologist's Definition Granite is a plutonic rock in which quartz makes up between 10 and 50 percent of the felsic components and alkali feldspar accounts for 65 to 90 percent of the total feldspar content. Applying this definition requires the mineral identification and quantification abilities of a competent geologist. Many rocks identified as "granite" using the introductory course definition will not be called "granite" by the petrologist - they might instead be alkali granites, granodiorites, pegmatites , or aplites. A petrologist might call these "granitoid rocks" rather than granites. There are other definitions of granite based upon mineral composition. The chart below illustrates the range of granite compositions. From the chart you can see that orthoclase feldspar , quartz , plagioclase feldspar , micas, and amphiboles can each have a range of abundances. Pegmatite: Photograph of a granite with very large crystals of orthoclase feldspar. Granites with such large crystals are known as "pegmatites." This rock is about two inches across.

Granite composition chart: This chart illustrates the generalized mineral composition of igneous rocks. Granites and rhyolites (compositionally equivalent to granite but of a fine grain size) are composed mainly of orthoclase feldspar, quartz, plagioclase feldspar, mica, and amphibole. "Granite": All of the rocks above would be called "granite" in the commercial stone industry. Clockwise from top left they are: granite , gneiss , pegmatite , and labradorite . Click on any of their names above for an enlarged view. Each of the images above represents a slab of polished rock about eight inches across. C) Commercial Definition The word "granite" is used by people who sell and purchase cut stone for structural and decorative use. These "granites" are used to make countertops, floor tiles, curbing, building veneer, monuments, and many other products. In the commercial stone industry, a "granite" is a rock with visible grains that is harder than a marble .

Under this definition, gabbro , basalt , pegmatite , schist , gneiss , syenite, monzonite, anorthosite, granodiorite, diabase, diorite , and many other rocks will be called "granite." The collection of images here illustrates the range of rocks that might be called "granite." Granite counter tops: Granite counter tops in a new kitchen. Image © iStockphoto / Bernardo Grijalva. Mount Rushmore: Mount Rushmore in the Black Hills, South Dakota is a sculpture of United States presidents George Washington, Thomas Jefferson, Theodore Roosevelt, and Abraham Lincoln sculpted from a granite outcrop. Image © iStockphoto / Jonathan Larsen. Uses of Granite Granite is the rock most often quarried as a "dimension stone" (a natural rock material that has been cut into blocks or slabs of specific length, width, and thickness). Granite is hard enough to resist most abrasion, strong enough to bear significant weight, inert enough to resist weathering, and it accepts a brilliant polish. These characteristics make it a very desirable and useful dimension stone.

Most of the granite dimension stone produced in the United States comes from high-quality deposits in five states: Massachusetts, Georgia, New Hampshire, South Dakota, and Idaho. Granite has been used for thousands of years in both interior and exterior applications. Rough-cut and polished granite is used in buildings, bridges, paving, monuments, and many other exterior projects. Indoors, polished granite slabs and tiles are used in countertops, tile floors, stair treads, and many other practical and decorative features. Related: Azurite Granite? A Gemstone? High price often reduces the popularity of a construction material, and granite often costs significantly more than man-made materials in most projects. However, granite is frequently selected because it is a prestige material, used in projects to produce impressions of elegance, durability, and lasting quality. Granite is also used as a crushed stone or aggregate. In this form it is used as a base material at construction sites, as an aggregate in road construction, railroad ballast, foundations, and anywhere that a crushed stone is useful as fill. Granitic rocks: This triangular diagram is a classification method for granitic rocks. It is based upon the relative abundance of feldspars (K-Na-Ca) and quartz.

Mafic elements are not considered. It is modified after a classification chart prepared by the International Union of Geological Sciences. Image and modification by the United States Geological Survey. Granite in the Continental Crust Most introductory geology textbooks report that granite is the most abundant rock in the continental crust. At the surface, granite is exposed in the cores of many mountain ranges within large areas known as "batholiths," and in the core areas of continents known as "shields." The large mineral crystals in granite are evidence that it cooled slowly from molten rock material. That slow cooling had to have occurred beneath Earth's surface and required a long period of time to occur. If they are today exposed at the surface, the only way that could happen is if the granite rocks were uplifted and the overlying sedimentary rocks were eroded. In areas where Earth's surface is covered with sedimentary rocks, granites, metamorphosed granites, or closely related rocks are usually present beneath the sedimentary cover. These deep granites are known as "basement rocks." Earth's crust | geology | Britannica.com A cross section of Earth’s outer layers, from the crust through the lower mantle. Encyclopædia Britannica, Inc.

Crustal abundances of elements of atomic numbers 1 to 93. Encyclopædia Britannica, Inc. Figure 22: The topographic expressions of eroded anticlines and synclines. Encyclopædia Britannica, Inc. Figure 21: Three basic fault types: (top) normal fault, (middle) reverse fault, and (bottom) strike-slip fault. Encyclopædia Britannica, Inc. Figure 20: The forms of three types of folds. Encyclopædia Britannica, Inc. Figure 23: Two transform faults offsetting a mid-oceanic ridge. Encyclopædia Britannica, Inc. Volcanic activity and the Earth’s tectonic plates Stratovolcanoes tend to form at subduction zones, or convergent plate margins, where an oceanic plate slides beneath a continental plate and contributes to the rise of magma to the surface. At rift zones, or divergent margins, shield volcanoes tend to form as two oceanic plates pull slowly apart and magma effuses upward through the gap. Volcanoes are not generally found at strike-slip zones, where two plates slide laterally past each other.

“Hot spot” volcanoes may form where plumes of lava rise from deep within the mantle to the Earth’s crust far from any plate margins. Encyclopædia Britannica, Inc. Earth is composed of three layers: the crust, the mantle, and the core. Created and produced by QA International. © QA International, 2010. All rights reserved. Geologic materials cycle through various forms. Sediments composed of weathered rock lithify to form sedimentary rock, which then becomes metamorphic rock under the pressure of Earth’s crust. When tectonic forces thrust sedimentary and metamorphic rocks into the hot mantle, they may melt and be ejected as magma, which cools to form igneous, or magmatic, rock. Created and produced by QA International. © QA International, 2010. All rights reserved. John P. Rafferty, biological and earth science editor of Encyclopædia Britannica, discussing the connection between Earth’s tectonic plates and earthquake development. Encyclopædia Britannica, Inc.

At the margins of Earth’s plates, where two plates pull apart or one plate dives beneath another, magma (molten underground rock) frequently rises to the surface through volcanic vents. Encyclopædia Britannica, Inc. Igneous Rocks Igneous rock is in the making as lava pours down a well-developed lava channel near a series of erupting vents on the northeast rift zone of Mauna Loa Volcano. Igneous rocks The term igneous comes from the Latin ignis, meaning "fire". Igneous is used to describe rocks that crystallize out of hot molten material in the Earth called magma. When magma pushes up through Earth's crust to the surface, it is called lava. Both magma and lava cool and harden to form igneous rocks. Intrusive vs. Extrusive Igneous rocks can be classified into two main categories: intrusive and extrusive. A trick to help kids remember intrusive and extrusive is to think of intrusive -- inside and extrusive -- exit. Granite. Intrusive rocks come from magma. They cool slowly deep in Earth's crust. When magma cools underground, the crust acts like a blanket, insulating it, keeping it warm longer.

Because the magma cools slowly, crystals of different minerals have time to grow. The molecules in the magma have time to arrange themselves into crystal formations before the magma hardens. Intrusive rocks have large crystals that can be seen with the naked eye. A common example of an intrusive igneous rock is granite. Pahoehoe Lava Field, Hawaii. Extrusive igneous rocks come from lava. Lava, at the surface, is exposed to air and water which causes the molten rock to cool rapidly. Solidifying rocks at the surface cool too quickly for large crystals to form. Molecules in the lava do not have time to arrange themselves to form large crystals. Extrusive rocks have crystals that are too small to see without magnification. A common example of an extrusive igneous rock is basalt. Some extrusive rocks, such as obsidian and pumice, cool so rapidly that they completely lack crystal structure and are considered a volcanic glass. Pumice is just like obsidian except it is tiny shards of glass. Texture Within the two main categories of intrusive and extrusive, rock can be classified even further using texture and chemical composition. The word "texture" has nothing to do with how the rock feels.

Texture, in geology, is used to describe how the rock looks. The most noticeable textural feature of igneous rocks is grain size. Grain size refers to the size of the individual mineral crystals. As mentioned above, intrusive igneous rocks, such as granite have large, individual crystals visible to the naked eye. The textural term used to describe a rock with large crystals is coarse-grained. In contrast, fine-grained rocks, such as basalt, are igneous rocks that have crystals too fine to see with the naked eye. Under magnification they are still very small but easily identifiable with a few optic tests. Quickly-cooled lavas can contain trapped bubbles of gas, which are called vesicles. The resulting texture is described as vesicular. Chemical Composition Chemical composition of igneous rock can often be estimated just from looking at the rock. Geologists look at the proportions of light-colored and dark-colored minerals in an igneous rock to estimate the chemical makeup of rock. Light-colored or felsic, minerals have more silica in them. Silica is one of the most abundant elements on Earth and is the chief component of quartz.

Felsic minerals are most often colorless, white, gray or pink but can be any number of colors. The dark, or mafic, minerals are richer in iron and magnesium. Mafic minerals are chiefly black, brown, dark gray and sometimes green. The mineral proportions of the rock are what allows geologists to classify rocks chemically. Depending on the proportion of light minerals to dark minerals, igneous rocks can be broken into four main types: felsic, intermediate, mafic and ultramafic. The following list gives more information about igneous chemical categories. Felsic rocks are high in silica (65% +). They are usually light-colored. Some examples are: Rhyolite (extrusive) and granite (intrusive). Intermediate rocks have lower silica content (55-65%). They are darker than felsic rocks but lighter than mafic rocks. Some examples are: Andesite/dacite (extrusive) and diorite/granodiorite (intrusive). Mafic rocks have low silica content (45-55%). They are usually dark-colored and contain iron and magnesium.

Some examples are: Basalt (extrusive) and gabbro (intrusive). Basalt is the rock that is produced at spreading ridges and makes up the sea floor. Ultramafic rocks have extremely low silica content (less than 45%) and contain large amounts of iron and magnesium. They are usually dark-colored, but high olivine content can lend green shades to the rock. Other rare colors can be found. An example of ultramafic rock is Peridotite (intrusive).

Question: Geology - what word is used to describe rocks formed from volcanoes or from magma within the earth's crust ?
Answer: IGNEOUS