5. Introduction to rocks
5.1. What is a rock?
A rock is a naturally-occurring solid material usually composed of an aggregate of mineral matter.
Two exceptions are coal and volcanic glass (obsidian). The former is composed in large parts oforganic compounds which cannot be regarded as minerals. The latter is a solid glassy material inwhich the arrangement of atoms displays irregularities and therefore does not match the definitionof a mineral.
Properties of rocks that are commonly used to characterize them are their color, their texture(coarse-grained vs. fine-grained), and their mineralogical and chemical compositions.

5.2. Why study rocks?
We study rocks to learn about the history of the Earth. Rocks provide information about the origin
and evolution of the terrestrial biosphere, lithosphere, atmosphere, and hydrosphere during thepast 4.6 billion years. The study of rocks is not limited to our planet. By analyzing meteorites, rocks
fallen from the sky, we can learn about the origin of the solar system. Samples of Moon rocks andsoil have been collected during the Appolo missions and by the Russian Luna spacecrafts in thelate 60s and early 70s. More recently, NASA rovers analyze Martian rocks to unravel the history of
the red planet and to seek traces of present and past life.
Rocks are important from an economical viewpoint as well because they contain valuable minerals.
The industrial revolution during the first half of the 19th century would not have been possible
without coal. Coal and other rocks rich in organic compounds are associated with the production ofoil and natural gas. Rocks contain minerals that can be exploited if their concentration issufficiently high. These rocks are called ores. An example of iron ore is the banded iron formation(BIF) already mentioned in the previous chapter. Rocks are also quarried for various purposes in
the construction industry (e.g. concrete manufacturing, construction aggregates, building stones).
Therefore, understanding how rocks form and how they are distributed is important for the mining
industry.
The study of rocks can help solving environmental issues. For instance, plans to store carbon
dioxide within certain rock formations with the purpose to alleviate the current rise in global
temperature are now being evaluated by experts. The storage of highly radioactive waste deep
underground is also considered a relatively safe way to dispose of this dangerous material.
Drinking water is a fundamental resource for all countries in the world. The presence or absence ofgroundwater reservoirs and their characteristics is determined by rock properties. The study ofrocks is therefore essential for the localization, characterization, and sustainable exploitation ofgroundwater reservoirs.

225.3. Classification of rocks
Rocks can be classified into three families:(1) Igneous rocks form by solidification of a cooling magma (molten rock). The size of mineral
grains depends on the cooling rate (Fig. 1). Fine-grained igneous rocks are those which form
near or at the surface of the Earth’s crust where magma cools more rapidly (= extrusive
igneous rocks). Volcanoes form where magma reaches the surface (more or less violently). If
the cooling rate is extremely high (e.g. magma in contact with air or water), crystals may noteven have time to form and volcanic glass is produced. Coarse-grained igneous rocks arethose which form within the Earth’s crust (= intrusive igneous rocks). They result from theslow cooling of magma within the crust. Bodies of magma present in the crust are called
magmatic intrusions.

Figure 1: Examples of igneous rocks (hand-size specimens, scale bar = 2 cm, and photomicrographs taken incross-polarized light, scale bar = 1 mm). (A) Intrusive igneous rock (granite) with large mineral grains crystallized ina slowly cooling magma, (B) extrusive igneous rock (basalt) with tiny mineral grains crystallized in a rapidly coolingmagma, and (C) extrusive igneous rock (basalt) with tiny mineral grains mixed with volcanic glass (in black on thephotomicrograph) resulting from a very high cooling rate (note the shiny surface of the sample). Source: Imperial
College Rock Library.

(2) Sedimentary rocks form by accumulation and subsequent lithification of sediments(fragments of preexisting rocks or elements of biological origin, Figs 2A-C) or by precipitationof minerals from an aqueous solution (Fig. 2D). Sedimentary rocks form in depressions of theEarth’s crust where sediments can accumulate. Most originate in the largest depressions, i.e.
the ocean basins. Sediments can be fragments of igneous, sedimentary or metamorphic rocks,
or fragments of individual minerals (e.g. quartz sand). Sediments can also be of biologicalorigin, such as mollusk shells, coral skeletons, bones and plant remains. Sedimentary rocks
formed by precipitation of minerals in saline lakes or embayments subject to evaporation arecalled evaporites.

Figure 2: Examples of sedimentary rocks (scale bar = 1 cm). (A) Conglomerate composed of rounded rockfragments (source: Imperial College Rock Library), (B) limestone composed of fragments of shells made of calcite(CaCO3) (black arrows indicate shells of single-celled organisms called foraminifera), (C) coal (rock derived from anaccumulation of plant debris, source: USGS) , and (D) evaporite composed of layers of anhydrite (CaSO4) (source:Garcia-Veigas et al., 2013).

Lithification means solidification of a soft sediment (another word is induration). This process occurs primarily by compaction(from the overlying sediment load) and precipitation of minerals in the space between sediment grains (cementation).

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(3) Metamorphic rocks form by transformation of the chemical and/or mineralogicalcomposition and/or texture of a preexisting rock in a solid state due to changing conditions oftemperature and/or pressure or due to interactions with hydrothermal fluids (Fig. 3). This
process of rock transformation is called metamorphism. Rocks caught in subduction zones or
between two colliding continents are subject to tremendous changes in temperature andpressure and can undergo various degrees of metamorphism. In addition, wherever rocks arein contact with magma, these rocks, if not melted, are transformed (metamorphosed) －in thiscase “cooked”－ by increased temperature (contact metamorphism). In addition, water
heated in the vicinity of magma circulates in the crust (hydrothermal circulation) and reacts
with rocks, changing their chemical and mineralogical compositions (metasomatism). For
instance, seawater which penetrates the oceanic crust through fractures near mid-ocean
ridges is heated and leaches metals and sulfur from surrounding rocks. These elements
precipitate as metal sulfides and oxides at hot springs, forming large chimneys rising from thesea floor (black smokers).

Figure 3: Examples of metamorphic rocks (hand-size specimens, scale bar = 2 cm, and photomicrographs taken incross-polarized light, scale bar = 1 mm). (A) gneiss showing an alternation of darker and lighter layers with different
mineralogical compositions (layers are perpendicular to the direction of pressure －indicated by the arrows－
affecting the rock during metamorphism), (B) schist with elongated minerals oriented at right angle to the directionof pressure (foliation), and (C) marble (metamorphosed sedimentary rock, usually limestone. Source: Imperial
College Rock Library.

The distribution of the different families of rocks is closely related to plate tectonics (Fig. 4). The
formation of igneous rocks is linked to magma production and therefore to regions of the crust
where rocks begin to melt. Rock melting takes place at mid-ocean ridges and subduction zones, at
hotspots, and at the root of large mountain chains resulting from continental collision. Most
sedimentary rocks are produced in ocean basins whose formation is controlled by plate tectonics(i.e. the opening of ocean basins through seafloor spreading). The distribution of metamorphic
rocks too is closely linked to plate tectonics. Large variations of temperature and pressure affectrocks along convergent boundaries. Contact metamorphism occurs wherever rocks are in contact
with magma. Metasomatism related to hydrothermal circulation occurs in the vicinity of magmatic
intrusions associated with plate boundaries.

Figure 4: Idealized cross section of the Earth’s crust showing the regions of the crust where most igneous,sedimentary, and metamorphic rocks are produced. O-C SUBD. = ocean-continent subduction, HS = hot spot, O-OSUBD. = ocean-ocean subduction, MOR = mid-ocean ridges, CONT. COLL. = continental collision. Note that thehot spot location is not related to plate boundaries. Hot spots result from hot mantle material rising from great
depths and producing volcanic activity at the surface (e.g. Hawaii).
The texture of metamorphic rocks is determined by the size, shape and orientation of minerals.

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Processes leading to the transformation of a rock of one family to a rock of another family can bedescribed as a cycle called the rock cycle (Fig. 5). The rock cycle illustrates how each rockfamilies can evolve from one another. The transformation of one rock family to another depends onplate tectonics and climate.
The control of plate tectonics on rock formation is clear when one looks at where the different typesof rocks are being produced (see Fig. 5 and related text). Climate controls the production rate of
sediments because the erosion of mountains depends on the amount of precipitation and otherclimatic factors (e.g. temperature). Hence, climate controls the production of sedimentary rocks.
Plate tectonics also controls the production rate of sediments because plate tectonics controls theformation of mountains from which sediments are derived. Moreover, the long term evolution ofEarth’s climate (over millions of years) is influenced by plate tectonics: (1) global volcanism alongplate boundaries influences the concentration of CO2 in the atmosphere; (2) the formation of large
mountain chains along convergent plate boundaries (e.g. Himalayas) influences the concentrationof CO2 in the atmosphere through the process of silicate weathering (see chapter on sedimentaryrocks).

Figure 5: The rock cycle. Each arrow represents a transformation process from one rock family to another. Eachtransformation process is represented by a specific color. Note that metamorphic rocks themselves can also be
metamorphosed.

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