1. The Earth system
1.1. introduction
The Earth system can be divided into three subsystems: (1) the climate system, (2) the plate
tectonic system, and (3) the geodynamo system. The first is driven by an external source ofheat (the Sun), whereas the second and the third are driven by an internal source of heat －heataccumulated at the beginning of Earth’s history and generated by the decay of radioactive
elements.
(1) The climate system includes all the components of the Earth system whose interactions
control Earth’s climate: the atmosphere (layer of gas surrounding the Earth), the cryosphere(surface ice and snow, e.g. polar ice caps, mountain glaciers, permafrost), the hydrosphere(liquid surface water, including groundwater), the lithosphere (rigid rocky outer layer of Earth),and the biosphere (all living things). Recent global warming calls to our attention the role ofhuman activities on climate change. So much so that it is adequate to define anothercomponent of the climate system: the anthroposphere (sum of all human activities influencing
the environment).
(2) The plate tectonic system involves all the components of the Earth which control the
movement of continents, the formation of mountains and ocean basins, and events such as
volcanic eruptions and earthquakes. These components are the lithosphere, asthenosphere,
and the deep mantle. These terms will be described in the following paragraphs. As we will
see later, characteristics of the lithosphere such as the position of continents, the presence of
mountain chains or the shape of ocean basins influence the climate system.
(3) The geodynamo system produces and maintains Earth’s magnetic field and involves
movements of matter within the Earth’s core.

Figure 1: Model of the Earth system and its three subsystems (i.e. climate, plate tectonics, and geodynamo).

1.2. Earth structure and plate tectonics
The exact age of the Earth cannot be determined directly but a good approximation can beobtained by measuring the age of meteorites. Meteorites are rocks falling from space on thesurface of the Earth. They originate from the collision and fragmentation of larger bodies. Theselarger bodies －asteroids－ are leftovers from the process of planetary accretion. In other words,
they did not aggregate with other asteroids to form planets. The age of the Earth derived frommeteorites is 4.6 billion years.
During the first billion years, the solar system was not yet cleared of the majority of its asteroids.
This meant a higher probability of collision among asteroids and planets, and therefore a higher

1number of meteorites falling on the Earth. This period of Earth’s history is referred to as the Heavy
Bombardment. Heat released by the collision between Earth and large meteorites was enough tomelt Earth’s surface. Another important source of heat was provided by the decay of radioactive
elements contained inside the Earth. Consequently, our planet was occasionally in a molten (“soft”)
state, during which elements could migrate freely and matter redistributed according to its density.
Heavier elements (Fe, Ni) migrated toward the center of the Earth, whereas lighter elements
remained at the surface (Mg, Al, K). This process, by which Earth became a layered planet, iscalled gravitational differentiation.
Earth is composed of 3 layers: the crust, the mantle and the core (Fig. 2A). Each layer has adistinct chemical composition. Temperature and pressure increase toward the center. Only theouter core is in a liquid state. The inner core is solid because the pressure is extremely high and“forces” matter into a solid state despite the very high temperature.

Figure 2: Schematic cross section of the Earth. (A) Earth’s main layers and (B) convection movements in the
mantle (dark green = colder material sinking, yellow-orange = hotter material rising).

Most of the Earth’s volume consists of the solid mantle. Solid does not mean totally rigid in thiscase. Slow, plastic deformations can take place, allowing matter to move. The rock “flow” inside
the mantle is driven by differences of temperature/density. Hotter material rises toward the surface
whereas cooler material sinks (Fig. 2B). This vertical motion is called convection (like convection
taking place in a hot miso soup in which colder, denser soup in contact with the bowl sinks andhotter, lighter soup rises, creating convective cells).
Rocks of the crust and mantle are composed primarily of minerals of the silicate family. The basic
structural unit of silicate minerals is [SiO4]4-, in which each oxygen atom shares one electron withthe silicon atom. The crust is divided into an oceanic and continental crust. The former is thinner(up to 7 km thick), heavier and enriched in Fe and Mg, and lies beneath the ocean floor. The latteris thicker (up to 40 km thick), lighter and enriched in Al, K, and Na, and rises above sea level toform the continents (Fig. 3).

Figure 3: Thickness, composition and density of Earth’s layers. Note the distinction between crust and mantle, andbetween lithosphere and asthenosphere.

The crust and the uppermost part of the mantle form a layer called the lithosphere (Figs. 3). The
lithosphere beneath the oceans (oceanic lithosphere) has an average thickness of 70 km. Beneathcontinents (continental lithosphere), it can be more than 200 km thick. Directly below thelithosphere lies the asthenosphere, a portion of the mantle a few hundred kilometers thick. The
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Note that the mantle is also rich in iron and magnesium. The chemical composition of the oceanic crust is very similar to that of
the mantle because the oceanic crust is derived from the partial melting of mantle rock (see page 3).

2physical properties of the two layers are very different. The lithosphere is rigid and brittle. The
asthenosphere is weak and ductile.
The lithosphere is broken into plates called tectonic plates (Figs. 4 & 5). Tectonic plates move
relative to one another and slide over the asthenosphere. The motion of plates is driven by
convective movements inside the mantle. Three major types of plate boundary can be identified:(1) divergent, (2) convergent, and (3) transform-fault.

Figure 4: The main tectonic plates. A, B and C are the 3 types of plate boundary: (A) divergent boundary, (B)convergent boundary, and (C) transform-fault boundary (note that only major transform faults are indicated on thisfigure).

Figure 5: Global topographic map. Note the correspondence between the location of plate boundaries and Earth’s
topography. The deepest places on Earth consist of oceanic trenches along subduction zones and submerged
mountain chains correspond to mid-ocean ridges (see text for explanations). Source: NOAA.

(1) Divergent boundaries: two plates are pulled apart where hot mantle material rises and a large
valley (rift) forms in between. Mantle rock rising toward the surface partially melts. Some of
the molten rock (magma) solidifies before reaching the surface; some reaches the surface
forming a volcanic chain in the middle of the valley. This process is responsible for theformation of new oceanic lithosphere along plate boundaries called mid-ocean ridges (e.g.
the Mid-Atlantic Ridge, Figs. 5 & 6A). The divergence of oceanic plates at mid-ocean ridges
and the production of new oceanic lithosphere is a process referred to as sea-floor spreading(average spreading rate of mid-ocean ridges = 50 mm/yr).
The other type of divergent boundary is the continental rift where continental lithosphere is
being pulled apart (e.g. the East African Rift, Fig. 6B), eventually leading to the formation of anew ocean basin.
(2) Convergent boundaries: since the Earth’s surface does not increase in time, the continuous
production of oceanic lithosphere at mid-ocean ridges means that it must be destroyed
somewhere else. This happens at subduction zones (Figs. 6C). At subduction zones, twoplates converge (two oceanic plates or one oceanic and one continental plate). The heavier
oceanic plate slips under the other one, and sinks deeper into the mantle where it is“recycled”. The subducting oceanic plate sinks because it is colder and denser than thesurrounding mantle. Subduction zones are characterized by a deep oceanic trench on the
subducting plate side (e.g. the Mariana Trench) and by a mountain chain on the overridingIn this setting, the partial melting of mantle rock is due to the decrease in pressure during its ascension toward the Earth’s
surface (decompression melting). The melting temperature of a rock is lowered when pressure decreases.
In case of a convergence between two oceanic plates, the older, cooler, hence denser plate subducts beneath the other one.

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plate side (e.g. the Andes, Fig. 5). Subduction zones are also associated with volcanic
activity (e.g. Mount Fuji).
The other type of convergent boundary involves the collision between two continents(continental collision, Fig. 6D). In this case, the converging plates are both continental,
hence light compared to the mantle, and no subduction can take place. Instead, a large
mountain chain builds up where the two continents collide (e.g. the Himalayas, Fig. 5).
(3) Transform faults: along transform-fault boundaries, two plates slip past one another (Figs.
6E-F). They most commonly offset mid-ocean ridges but they can also be found on land (e.g.
San Andreas Fault).
Earthquakes occurs along all plate boundaries, whereas the majority of volcanoes are locatedalong subduction zones and mid-ocean ridges.

Figure 6: Schematic cross section of tectonic plate boundaries. (*) Oceanic transform fault viewed from above (TF= transform fault, MOR = mid-ocean ridge). Note the motion of plates in opposite directions between the twosegments of mid-ocean ridge, which triggers earthquakes.

1.3. Minerals: building blocks of rocks
Rocks are usually composed of an aggregate of different minerals although they can be composedof a single mineral too. Minerals are solid inorganic substances with atoms arranged in a regular,
repeating pattern (crystal lattice). They have a specific chemical formula (e.g. NaCl, CaCO3; Fig.
7). The external shape of mineral crystals reflects the internal order of atoms.

Figure 7: Crystal structure of halite (table salt). The green cube represents the repeating structural unit of thecrystal lattice of halite (unit cell). Note the cubic shape of crystals of halite. Source (photograph): Understandingth
Earth 6 edition.

The structure of a mineral (the way atoms are stacked) depends on its chemical composition as
well as the conditions of temperature and pressure during crystallization (Fig. 8).

Figure 8: Two minerals with the same chemical composition (carbon) but with different crystal structures. (A)Diamond formed at high temperature and pressure, and (B) graphite formed at much lower temperature andthpressure. Source: modified from Understanding Earth 6 edition.

Rock melting at subduction zones is facilitated by the presence of H2O contained in sedimentary rock pores and clay minerals.
The effect of water is to lower the melting point of rocks because water helps break chemical bonds (water-induced melting).

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Three basic processes are responsible for the formation of minerals:(1) Minerals form in saturated fluids when dissolved ions have reached their solubility threshold(e.g. precipitation of salts as water evaporates in a saline lake, Fig. 9A).
(2) Minerals form by solidification (liquid-solid transition) or deposition (gas-solid transition) (e.g.
crystallization of a cooling magma, Fig. 9B)(3) Minerals form by biological processes (biomineralization, e.g. corals, mollusk shells; Fig. 9C)

Figure 9: Illustration of the three processes of mineral formation. (A) Salts deposited at the bottom of a saline lake
in southeastern Tunisia (scale bar = 2.5 km, source: Nasa Earth Observatory website), (B) rock composed of anaggregate of minerals crystallized during the slow cooling of a magma within the Earth’s crust (granite, scale bar =1 cm), and (C) coral skeleton composed of calcium carbonate (the specimen is 11 cm across).

1.4. Rocks and the rock cycle
The study of rocks is important because their mineral and chemical compositions and fossil
contents can be used to reconstruct Earth’s history and understand how life evolved. Rocks
contain groundwater used for agriculture, public consumption and industrial purposes. They also
contain mineral resources, such as gas, oil, coal and ore minerals, which are important from an
economic and technological viewpoint. Their study is also crucial to solve environmental problems,such as the storage of radioactive substances and carbon dioxide, and the underground diffusionof pollutants.
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 (Figs. 10). Fine-grained igneous rocks are those which form
near or at the surface of the Earth’s crust when magma cools down rapidly (extrusive igneousrocks). Volcanoes form where magma reaches the surface (more or less violently). If thecooling rate is extremely high (e.g. magma in contact with air or water), crystals may not evenhave time to form, leading to the formation of a rock called volcanic glass. Coarse-grained
igneous rocks are those which form deep within the Earth’s crust (intrusive igneous rocks).
They result from the slow cooling of magma within the lithosphere (magmatic intrusion).

Figure 10: 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 crystals formed in a slowly
cooling magma, (B) extrusive igneous rock (basalt) with tiny crystals formed in a rapidly cooling magma, and (C)extrusive igneous rock (basalt) with tiny crystals mixed with volcanic glass (in black on the photomicrograph)
resulting from a very high cooling rate (note the shiny surface of the sample). Source: Imperial College Rock
Library.

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(2) Sedimentary rocks form by accumulation and subsequent lithification of sediments(fragments of preexisting rocks or elements of biological origin, Figs 11A-C) or by precipitationof minerals from an aqueous solution (Fig. 11D). Sedimentary rocks forms in depressions of
the Earth’s crust where sediments can accumulate. Most originates in the largest depressions:the ocean basins. Sediments can be fragments of igneous, sedimentary or metamorphic rocks,
or fragments of minerals (Fig. 11A). Sediments can also be of biological origin, such as mollusc
shells, coral skeletons, bones and plant remains (Fig. 11 B-C). Other sedimentary rocks form
by precipitation of minerals in evaporating lakes or embayments and are called evaporites(Fig. 11D).

Figure 11: 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).

(3) Metamorphic rocks form by transformation of the chemical and/or mineralogical composition
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. 12). 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 causing rock metamorphism (regional metamorphism). In addition, wherever rocks
are in contact with magma, these rocks, if not melted, are transformed (metamorphosed) －in
this case “cooked”－ by increasing temperature (contact metamorphism). Water heated in
the vicinity of a magma circulates in the crust (hydrothermal circulation) and reacts withsurrounding 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 at underwater hot springs, forming large chimneys rising from thesea floor (black smokers).

Figure 12: Examples of metamorphic rocks (hand-size specimens, scale bar = 2 cm, and photomicrographs taken
in cross-polarized light, scale bar = 1 mm). (A) gneiss showing an alternation of darker and lighter layers withdifferent mineralogical compositions (layers are perpendicular to the direction of pressure －indicated by thearrows－ affecting the rock during metamorphism), (B) schist with elongated minerals oriented at right angle to thedirection of pressure (foliation), and (C) marble (metamorphosed sedimentary rock, usually limestone. Source:
Imperial College Rock Library.

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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.


The texture is determined by the size, shape and orientation of minerals.

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The distribution of the different families of rocks is closely related to plate tectonics (Fig. 13). 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
and at the root of large mountain chains related to continental collision. Most sedimentary rocks
are produced in ocean basins whose formation is controlled by plate tectonics (i.e. the opening ofocean basins through sea-floor spreading). The distribution of metamorphic rocks too is of course
closely linked to plate tectonics. Large variations of temperature and pressure affect rocks inconvergence zones. Metasomatism related to hydrothermal circulation occurs in the vicinity ofmagmatic intrusions associated with plate boundaries.

Figure 13: 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-O
SUBD. = ocean-ocean subduction, MOR = mid-ocean ridges, CONT. COLL. = continental collision. Note that the
hot 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).

Processes leading to the transformation of a rock of one family to a rock of another family can be
described as a cycle called the rock cycle (Fig. 14). The rock cycle illustrates how each rockfamilies can evolve from one another. The transformation of one rock family to another is controlled
by plate 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. 13 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 control the production rate of sediments because plate tectonics controls theformation of mountains from which sediments are derived. Moreover, plate tectonics influences theclimate system through processes which will be discussed later in this course.

Figure 14: 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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