3. The Earth system
Our planet is composed of three complex natural systems (Figs. 1 & 2): (1) the climate system,(2) the plate tectonic system, and (3) the geodynamo system. The first is driven by an externalsource of heat (the Sun), whereas the second and the third are driven by an internal source of heat(original heat accumulated during Earth’s formation and heat being generated by the decay ofradioactive elements inside the Earth).
(1) The climate system involves all the components of the Earth whose interactions control Earth’sclimate:
Atmosphere (layer of gas around the Earth)
Cryosphere (surface ice and snow; e.g., ice caps, glaciers)
Hydrosphere (liquid surface waters, including groundwater)
Lithosphere (rigid rocky outer layer of Earth)
Biosphere (all living things on Earth)
Recent global warming calls to our attention the impact of human activities on climate. The
anthropogenic influence on nature has become so significant that it is nowadays adequate todefine yet another component of the climate system: the anthroposphere (sum of all humanactivities influencing the environment).
(2) The plate tectonic system involves all the components of the Earth which control themovements of continents, the formation of mountains and ocean basins, and events such as
volcanic eruptions and earthquakes. These components are the lithosphere, theasthenosphere, and the deep mantle. The principles of plate tectonics are described insection 3.2 (see slides for additional figures and further information on subjects related to platetectonics).
(3) The geodynamo system produces and maintains Earth’s magnetic field and involves
movements of charged particles in the liquid outer core.

Figure 1: Illustrations of the climate system (typhoon, A), the plate tectonic system (lava, B), and the geodynamo
system (compass, C). Sources: A. NOAA, B. Encyclopaedia Britannica, C. Wikipedia.

1Figure 2: Model of the Earth and the three systems: climate, plate tectonics, and the geodynamo.
Reference: John Grotzinger, and Thomas H. Jordan, Understanding Earth 6th edition, Boston, Bedford,, Boston,Bedford, 2010.

3.1. The age and structure of the Earth
The exact age of the Earth cannot be determined directly but a good approximation can be obtainedby measuring the age of meteorites. Meteorites are rocks from space falling on the surface of theEarth. They originate primarily from the collision and fragmentation of asteroids. Asteroids are theleftover of the process of planetary accretion. In other words they did not aggregate with otherasteroids to form planets when the solar system formed. The age of the Solar System (and theEarth) derived from meteorites is 4.6 billion years.
During the first 700 million years or so, the solar system was not yet cleared of the majority of its
asteroids which means there was a higher probability of collision among asteroids and planets, andtherefore a higher number of meteorites falling on Earth. This period of Earth’s history is referred toas the Heavy Bombardment. Heat energy released by the collision between Earth and large
meteorites was high enough to melt Earth’s surface. Other important sources of heat were provided
by gravitational contraction (conversion of gravitational potential energy, i.e. heat released byfriction) and by the decay of radioactive elements contained within Earth’s interior. Consequently
our planet was occasionally in a molten (“soft”) state during which elements could migrate freely,and matter redistributed according to density. Heavier elements (e.g., Fe, Ni) migrated toward the2center of the Earth to form the core, whereas lighter elements (e.g., Al, K, Na, Mg) remained closer
to the surface. The process by which Earth became a layered planet is called gravitational
differentiation.
Earth is composed of three main layers: the crust, the mantle and the core (Fig. 3A). Each layer
has a distinct chemical composition. Temperature and pressure increase toward the center. Only
the outer core is in a liquid state. The inner core is solid because the pressure is extremely highand keeps matter into a solid state despite the very high temperature.

Figure 3: Schematic cross section of the Earth. (A) Earth’s main layers and (B) convection movements in themantle (dark green = colder material sinking, yellow-orange = hotter material rising).
Reference: John Grotzinger, and Thomas H. Jordan, Understanding Earth 6th edition, Boston, Bedford, 2010.

Most of the Earth’s volume consists of the solid mantle. 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- inwhich each of the four oxygen atoms shares one electron with the silicon atom. The core is Earth’s
densest layer and composed mostly of an iron-nickel alloy. The mantle is slightly denser than thecrust and composed of silicates rich in Fe and Mg. The crust is divided into the oceanic andcontinental crust. The former is thinner (up to 7 km thick), heavier, and enriched in Fe and Mg*.
The oceanic crust lies beneath the ocean floor. The latter is thicker (up to 40 km thick), lighter andenriched in Al, K and Na (Fig. 4). The continental crust forms the continents and their continental
shelves (shallow seabed adjacent to the coast). Where does the evidence supporting the model ofa layered Earth come from? Sources of evidence include: (1) the study of meteorites originatingfrom the fragmentation of larger objects which have been subject to gravitational differentiation, (2)the study of seismic waves traveling inside the Earth (see Fundamental of Earth Science II), (3) theanalysis of fragments of rocks from the mantle brought to Earth’s surface by volcanic eruptions, (4) Note that the chemical composition of the oceanic crust is close to that of the mantle because the oceanic crust is derived fromthe partial melting of mantle rock (see chapter on igneous rocks).

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deep drilling (see project of drilling the oceanic crust down to the mantle by the Japanese drilling
vessel Chikyu), and (5) high-pressure lab experiments.

Figure 4: Thickness, composition and density of Earth’s layers. Note the distinction between crust and mantle andbetween the lithosphere and the asthenosphere.
Reference: John Grotzinger, and Thomas H. Jordan, Understanding Earth 6th edition, Boston, Bedford, 2010.

3.2. A dynamic Earth: the theory of plate tectonics
The crust and the uppermost part of the mantle form a rigid layer called the lithosphere (Figs. 4).
The lithosphere beneath the oceans (i.e., oceanic lithosphere) is about 70 km thick. The
lithosphere that forms the continents (i.e., continental lithosphere) is about 150 km thick. Directly
below the lithosphere lies the asthenosphere (or asthenospheric mantle), a layer of the uppermantle reaching depths of up to ~200 km. The physical properties of the lithosphere andasthenosphere are very different. The lithosphere is rigid and brittle. The asthenosphere is weakand ductile. Slow, plastic deformations can take place in the asthenospheric mantle, allowing matter“to move around”. The rock “flow” inside the mantle is driven by differences in density. Hotter
material rises toward the surface whereas cooler material sinks (Fig. 3B). This vertical motion iscalled convection (like convection taking place in a hot miso soup in which denser, colder soup incontact with the bowl sinks and lighter, hotter soup rises until the temperature/density inside thesoup becomes uniform).
The lithosphere is broken into plates called tectonic plates (Figs. 5 & 6). Tectonic plates move
relative to one another and slide over the asthenosphere. The energy that fuels the process of plate
tectonics originates from Earth’s internal heat. Three major types of plate boundaries can be4

identified: (1) divergent, (2) convergent, and (3) transform-fault plate boundaries.

Figure 5: 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 byyellow arrows on this figure).
Reference: John Grotzinger, and Thomas H. Jordan, Understanding Earth 6th edition, Boston, Bedford, 2010.

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Figure 6: Global relief map of Earth. Note the correspondence between the location of plate boundaries and Earth’s
topography. Source: NOAA.

(1) Divergent boundaries
Along divergent boundaries two plates are pulled apart and a large valley forms in between. This isa zone where mantle rock rises toward the Earth’s surface and partially melts*. Some of the molten
rock (magma) solidifies before reaching the surface. Some magma reaches the surface and forms
a volcanic chain in the middle of the valley. This process is responsible for the formation of new
oceanic crust along plate boundaries called mid-ocean ridges (e.g., the Mid-Atlantic Ridge, Figs.
6 & 7A). The divergence of oceanic plates at mid-ocean ridges and the production of new oceanic
crust is a process referred to as seafloor spreading (average spreading rate of mid-ocean ridges= 50 mm/yr). The other type of divergent boundary is the continental rift where continental crustis being pulled apart (e.g., the East African Rift, Fig. 7B) eventually leading to the formation of anew ocean basin.
(2) Convergent boundaries
Since the surface area of the Earth’s crust does not increase over time, the continuous productionof oceanic lithosphere at mid-ocean ridges means that oceanic lithosphere must be destroyed
somewhere else. This happens at subduction zones (Figs. 7C). At subduction zones, two plates
converge (two oceanic plates or one oceanic and one continental plate). The heavier oceanic plateslips 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 the surrounding asthenospheric 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 plate side (e.g. the Andes, Fig. 6).
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. 7D).
In this case, the converging plates are both continental, hence light compared to the mantle, andno subduction takes place. Instead, a large mountain chain builds up where the two continents
collide (e.g., the Himalayas, Fig. 6).
(3) Transform faults
The last type of plate boundary is called a transform fault (Figs. 7E-F). Along transform-fault
boundaries, two plates slip past one another. They most commonly offset mid-ocean ridges but they
can also be found on land (e.g., San Andreas Fault).

 In this context, 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.
 Rock melting at subduction zones is facilitated by the presence of H O contained in sedimentary rock pores and minerals. The
2effect of water is to lower the melting point of rocks because water molecules helps break chemical bonds.

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Earthquakes occur along all plate boundaries, whereas the majority of volcanoes are locatedalong subduction zones and mid-ocean ridges.

Figure 7: 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.
Reference: John Grotzinger, and Thomas H. Jordan, Understanding Earth 6th edition, Boston, Bedford, 2010.

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