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# Earth's Inner Layers

## The crust, mantle and core are defined by composition; the lithosphere and asthenosphere from mechanical properties.

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Earth's Composition

Learning Objectives:

• Explain 2 ways scientists define the Earth's layers.
• Identify the five physical layers of the Earth.

Figure 1: Cross section of the Earth.

### Compositional Layers of the Earth

#### The Crust

License: CC BY-NC 3.0

Figure 2 [Figure1]

The crust is the outermost layer of the Earth.  It can be 5 to 100 km thick depending on the location and surface features.  As you can see in Figure 2 above, the areas where continents are located are much thicker than areas where there is ocean.   Oceanic crust is typically more dense than continental crust due to the composition of the rock.

Continental crust usually ranges from 20-70 km thick.  Oceanic crust usually ranges from 4-9 km thick.  Most of the crust is composed of rock that is formed after molten rock has cooled.  Despite these thicknesses, humans have never drilled all the way through the crust.  The closest anyone has gotten was 1,415 m (1.4 km) into oceanic crust.

#### The Mantle

As you saw in Figure 2 above, the mantle is below the crust.  The approximate thickness of the mantle is 2,900 km.  Even though we have never observed the mantle directly, scientists have been able to infer the composition of based on observations of volcanoes.  It is in these locations that molten rock from the mantle flows to the surface. Scientists believe that the magma is composed of mostly iron and magnesium.

#### The Core

The core is the innermost layer of the Earth.  It is about 7,000km thick and goes all the way to the center of the earth.  This layer is the most difficult to study since it cannot be reached.  Scientists believe that the core is composed of mostly iron and nickel.  They have used the data from how seismic waves (from earthquakes) travel through the core to determine this information.

### Layers by Physical Properties

Even though we've talked about the three compositional layers of the earth, scientists recognize a total of five physical layers as pictured below (Figure 3).

Figure 3: A cross section of Earth showing the following layers: (1) crust (2) mantle (3a) outer core (3b) inner core (4) lithosphere (5) asthenosphere (6) outer core (7) inner core.

The crust and upper part of the mantle comprises the lithosphere ( label 4 above).  Because the average temperature of the lithosphere is about 0ºC, the material is brittle and solid.  It is easily cracked or broken.  The lithosphere is not able to flow or move easily, which is often what causes breaking to occur.  The places where the lithosphere has broken, make up the boundaries of plates.  These plates float atop the asthenosphere.

The asthenosphere (as THE nuh sfihr) makes up part of the upper mantle as well, and has an average temperature of 500ºC.  While the lithosphere is solid and can't flow, the asthenosphere (label 5 above) is solid but can flow.  Can you think of a solid that can flow? Consider toothpaste, silly putty, or hot plastic that hasn't melted.  The plastic is really easy to bend, and the silly putty will take the shape of its container over time.  Because of this property, the asthenosphere is classified differently than the lithosphere.

The lower mantle is also known as the mesophere.  The average temperature of this layer is about 2000ºC.  It is the bottom portion of the mantle that isn't included in the lithosphere or asthenosphere.  The temperature of the mesosphere is significantly higher than that of the asthenosphere.  Despite these higher temperatures, the mesophere is still solid because of the intense pressure from the weight of the earth.

The core is made up of the inner and the outer core.  The outer core of the Earth is liquid iron and nickel.  Scientists believe that this liquid outer core is responsible for Earth's magnetic field. The average temperature of the outer core is 5000ºC, which is definitely hot enough to melt iron and nickel.

The inner core is mostly made of iron. The extremely high temperatures (7000ºC) of the inner core would normally melt the metal, but it is under extreme pressure which prevents the molecules from moving much, causing it to remain solidified.

This animation shows the layers by composition and by mechanical properties: http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_layers.html.

### Vocabulary

• asthenosphere: Layer below the lithosphere, made of a portion of the upper mantle; the asthenosphere can flow.
• core: Innermost, densest layer of a celestial body.
• crust: Rocky outer layer of the Earth's surface.
• lithosphere: Layer of solid, brittle rock that makes up the Earth's surface.
• mantle: Middle layer of the Earth; made of hot rock.
• Mesosphere: the lower portion of the mantle, between the asthenosphere and the outer core

### Summary

• By composition, Earth is divided into core, mantle, and crust.
• By physical properties, the crust and upper mantle are divided into lithosphere and asthenosphere with the mesosphere and outer core and inner core.

### Practice

Use the resource below to answer the questions that follow.

1. What is the core?
2. Explain the core's structure.
3. What is the mantle?
4. What is the crust?
5. Why does the Earth have layers?

### Review

1. What are the the layers of Earth based on composition. Where are they located?
2. What is the composition of the different layers?
3. How do the lithosphere and asthenosphere differ from each other?
4. Create a concept map using the following terms:  crust, mantle, core, lithosphere, asthenosphere, mesosphere, outer core, inner core.

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