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

You are watching: Oceanic crust is less dense than continental crust.

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

The classic view based upon chemical composition recognizes four distinct layers:

The inner core lies in ~ the center of the Earth, and is about 1200 km thick. It is composed primarily of iron alloys and also nickel, with about 10% comprised of oxygen, sulfur or hydrogen. The temperature in the inner core is about 6000 oC (10,800 oF), i m sorry is about the temperature that the surface ar of the sun (section 3.1 explains the resources of this intense heat). Despite the high temperature that must melt this metals, the extreme pressure (from precise the load of the world) keeps the inner core in the heavy phase. The hard metals likewise make the within core really dense, at around 17 g/cm3, providing the within core around one-third the the Earth’s full mass.

The outer core sits external of the inner core. It has the same composition together the within core, yet it exists as a fluid, fairly than a solid. The temperature is 4000-6000 oC, and the steels remain in the fluid state since the press is not as an excellent as in the within core. It is the motion of the liquid iron in the outer core the creates Earth’s magnetic ar (see section 4.2). The outer core is 2300 km thick, and has a density of 12 g/cm3.

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

The outermost great is the crust, which forms the solid, rocky surface of the Earth. The crust averages 15-20 km thick, yet in some places, such together under mountains, the crust have the right to reach thicknesses of as much as 100 km. There room two main types of crust; continental crust and also oceanic late that differ in a number of ways. Continental crust is thicker 보다 oceanic crust, averaging 20-70 kilometres thick, compared to 5-10 kilometres for oceanic crust. Continental crust is less dense than oceanic late (2.7 g/cm3 vs. 3 g/cm3), and it is much older. The oldest rocks in continent crust are about 4.4 billion year old, if the earliest oceanic crust only goes back about 180 million years. Finally, the two varieties of crust differ in your composition. Continental crust is made greatly of granite. This is due to the fact that underground or surface magmas can cool slowly, which enables time for crystal frameworks to type before the rocks solidify, which leader to granite. Oceanic tardy is greatly composed of basalts. Basalts also type from cooling magmas, however they cool in the visibility of water, which provides them cool much faster and also does not permit time because that crystals come form.

Based on physics characteristics, us can additionally divide the outermost great of planet into the lithosphere and also asthenosphere. The lithosphere is composed of the crust and the cool, rigid, outer 80-100 km of the mantle. The crust and outer mantle moves together as a unit, for this reason they are combined together into the lithosphere. The asthenosphere lies listed below the lithosphere, from around 100-200 km to around 670 kilometres deep. It includes the more “plastic” softer region of the mantle, where liquid movements deserve to occur. The solid lithosphere is therefore floating top top the fluid asthenosphere.


To aid explain how the lithosphere is floating ~ above the asthenosphere, we need to examine the concept of isostasy. Isostasy describes the means a solid will certainly float top top a fluid. The relationship between the crust and the mantle is shown in figure 3.2.2. ~ above the appropriate is an instance of a non-isostatic relationship in between a raft and solid concrete. It’s possible to load the raft increase with many people, and it still i will not ~ sink into the concrete. Top top the left, the relationship is one isostatic one in between two various rafts and a swim pool complete of peanut butter. With just one person on board, the raft floats high in the peanut butter, but with three people, it sinks dangerously low. We’re utilizing peanut butter here, rather than water, because its viscosity more closely represents the relationship between the crust and the mantle. Although that has around the same density as water, peanut butter is much much more viscous (stiff), and so although the three-person raft will sink right into the peanut butter, it will do so quite slowly.

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

The partnership of earth crust to the mantle is similar to the relationship of the rafts to the peanut butter. The raft v one human being on the floats comfortable high. Also with three people on it the raft is less dense than the peanut butter, so it floats, yet it floats uncomfortably short for those 3 people. The crust, with an average density of about 2.6 grams every cubic centimeter (g/cm3), is less thick than the mantle (average thickness of around 3.4 g/cm3 near the surface, but more than the at depth), and so it is floating on the “plastic” mantle. When an ext weight is added to the crust, v the procedure of hill building, it progressively sinks deeper into the mantle and also the mantle material that was there is thrust aside (Figure 3.2.3, left). As soon as that weight is gotten rid of by erosion over tens of numerous years, the tardy rebounds and also the mantle absent flows back (Figure 3.2.3, right).

Figure 3.2.3 Isostatic rebound when mass is removed from the crust (Steven Earle, “Physical Geology”).

The crust and mantle answers in a similar method to glaciation. Special accumulations the glacial ice include weight come the crust, and as the mantle beneath is squeezed come the sides, the tardy subsides. Once the ice ultimately melts, the crust and mantle will slowly rebound, but full fag will likely take much more than 10,000 years. Huge parts the Canada room still rebounding as a result of the lose of glacial ice cream over the previous 12,000 years, and also as presented in figure 3.2.4, various other parts the the human being are additionally experiencing isostatic rebound. The highest rate that uplift is in within a big area to the west the Hudson Bay, i beg your pardon is where the Laurentide ice cream Sheet was the thickest (over 3,000 m). Ice lastly left this region around 8,000 years ago, and the crust is right now rebounding in ~ a rate of virtually 2 cm/year.

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

Since continental crust is thicker than oceanic crust, it will certainly float greater and expand deeper right into the mantle than oceanic crust. Tardy is thickest where there are mountains, therefore the Moho will certainly be depth under mountains than under the oceanic crust. Because oceanic crust is also denser than 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 lowest point, this explains why water has built up over the oceanic crust to kind the oceans.

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

*”Physical Geology” by Steven Earle provided under a CC-BY 4.0 global license. Download this publication for complimentary at