In the previous ar we learned that products in the at an early stage Earth to be sorted v the process of differentiation, with denser products like iron and also nickel sinking come the center, and also lighter products (oxygen, silicon, magnesium) remaining close to the surface. Together a result, the planet is written of layers of different composition and increasing thickness as you move from the surface ar to the facility (Figure 3.2.1).

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Figure 3.2.1 interior structure of planet (By Kelvinsong (Own work) , via Wikimedia Commons).

The classic view based on chemical composition establish four distinct layers:

The inner core lies in ~ the center of the Earth, and is around 1200 kilometres thick. That is composed generally of iron alloys and also nickel, with about 10% consisted of of oxygen, sulfur or hydrogen. The temperature in the inner core is about 6000 oC (10,800 oF), i beg your pardon is approximately the temperature that the surface of the sun (section 3.1 defines the sources of this extreme heat). Regardless of the high temperature that must melt this metals, the extreme pressure (from literally the weight of the world) keeps the inner core in the hard phase. The solid metals additionally make the inner core an extremely dense, at about 17 g/cm3, providing the inner core about one-third of the Earth’s complete mass.

The outer core sits outside of the inside core. It has actually the exact same composition together the inside core, however it exists together a fluid, rather than a solid. The temperature is 4000-6000 oC, and the steels remain in the fluid state because the push is not as an excellent as in the inside core. That is the motion of the liquid iron in the outer core that creates Earth’s magnetic ar (see ar 4.2). The outer core is 2300 km thick, and also has a thickness of 12 g/cm3.

The mantle extends native the outer core to just under Earth’s surface. It is 2900 km thick, and also contains around 80% the the Earth’s volume. The mantle consists of iron and magnesium silicates and also magnesium oxides, so it is an ext similar come the rocks that Earth’s surface ar than to the products in the core. The mantle has a thickness of 4.5 g/cm3, and also temperatures in the selection of 1000-1500 oC. The uppermost class of the mantle is much more rigid, when the deeper regions are fluid, and it is the motion of fluid materials in the mantle the is responsible for plate tectonics (see section 4.3). Magma that rises to the surface through volcanoes originates in the mantle.

The outermost great is the crust, which creates the solid, rocky surface ar of the Earth. The late averages 15-20 km thick, yet in some places, such as under mountains, the crust have the right to reach thicknesses of approximately 100 km. There room two main types of crust; continental crust and also oceanic crust that differ in a number of ways. Continent crust is thicker 보다 oceanic crust, averaging 20-70 kilometres thick, compared to 5-10 kilometres for oceanic crust. Continental crust is less thick than oceanic crust (2.7 g/cm3 vs. 3 g/cm3), and also it is much older. The earliest rocks in continent crust are about 4.4 billion year old, when the oldest oceanic crust only goes back around 180 million years. Finally, the two species of crust differ in their composition. Continent crust is made mostly of granite. This is because underground or surface ar magmas can cool slowly, which allows time for crystal structures to kind before the rocks solidify, which leader to granite. Oceanic late is mainly composed of basalts. Basalts also type from cooling magmas, but they cool in the presence of water, which renders them cool lot faster and does not permit time for crystals to form.

Based on physical characteristics, we can additionally divide the outermost great of planet into the lithosphere and asthenosphere. The lithosphere consists of the crust and also the cool, rigid, outer 80-100 km of the mantle. The crust and also outer mantle moves together as a unit, so they are linked together into the lithosphere. The asthenosphere lies listed below the lithosphere, from about 100-200 km to around 670 kilometres deep. It contains the much more “plastic” softer an ar of the mantle, where fluid movements can occur. The hard lithosphere is for this reason floating on the fluid asthenosphere.


To aid explain exactly how the lithosphere is floating ~ above the asthenosphere, we need to examine the principle of isostasy. Isostasy refers to the method a solid will certainly float top top a fluid. The relationship in between the crust and also the mantle is illustrated in figure 3.2.2. ~ above the ideal is an instance of a non-isostatic relationship between a raft and solid concrete. It’s possible to fill the raft up with lots of people, and it still won’t sink right into the concrete. ~ above the left, the connection is one isostatic one between two different rafts and also a swim pool complete of peanut butter. With only one person on board, the raft floats high in the peanut butter, however with three people, it sink dangerously low. We’re using peanut butter here, rather than water, because its viscosity much more closely to represent the relationship in between the crust and the mantle. Although the has about the same thickness as water, peanut butter is much much more viscous (stiff), and also so although the three-person raft will certainly sink right into the peanut butter, the will carry out so fairly slowly.

Figure 3.2.2 Demonstrating isostasy (Steven Earle, “Physical Geology”).

The connection of earth crust come the mantle is similar to the connection of the rafts to the peanut butter. The raft through one person on it floats comfortably high. Even with three civilization on that the raft is less dense than the peanut butter, so the floats, but it floats uncomfortably low for those three people. The crust, v an average thickness of around 2.6 grams per cubic centimeter (g/cm3), is less dense than the mantle (average density of roughly 3.4 g/cm3 near the surface, but much more than that at depth), and so the is floating top top the “plastic” mantle. When an ext weight is included to the crust, v the procedure of hill building, it progressively sinks deeper right into the mantle and the mantle material that was there is pushed aside (Figure 3.2.3, left). As soon as that load is gotten rid of by erosion over 10s of numerous years, the crust rebounds and the mantle rock flows back (Figure 3.2.3, right).

Figure 3.2.3 Isostatic rebound as soon as mass is eliminated from the late (Steven Earle, “Physical Geology”).

The crust and mantle answers in a similar means to glaciation. Special accumulations that glacial ice add weight to the crust, and also as the mantle in ~ is squeezed come the sides, the late subsides. When the ice ultimately melts, the crust and mantle will slowly rebound, but full rebound will most likely take an ext than 10,000 years. Large parts the Canada are still rebounding together a an outcome of the ns of glacial ice over the previous 12,000 years, and also as shown in number 3.2.4, other parts the the human being are also experiencing isostatic rebound. The highest rate the uplift is in in ~ a big area come the west that Hudson Bay, i beg your pardon is where the Laurentide ice cream Sheet to be the thickest (over 3,000 m). Ice finally left this region around 8,000 year ago, and also the crust is at this time rebounding in ~ a rate of almost 2 cm/year.

Figure 3.2.4 global rates of isostatic mediate (Steven Earle, “Physical Geology”).

Since continental crust is thicker than oceanic crust, it will float higher and expand deeper right into the mantle 보다 oceanic crust. Late is thickest where there space mountains, therefore the Moho will certainly be depths under mountains than under the oceanic crust. Due to the fact that oceanic crust is additionally denser 보다 continental crust, that floats reduced on the mantle. Since the oceanic late lies reduced than the continent crust, and since water operation downhill to with the shortest point, this explains why water has built up over the oceanic late to kind the oceans.

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Figure 3.2.5 Thinner, denser oceanic tardy floats reduced on the mantle 보다 thicker, less thick continental crust (Steven Earle, “Physical Geology”).

*”Physical Geology” by Steven Earle used under a CC-BY 4.0 worldwide license. Download this publication for totally free at