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Reading: Earth's Interior

 

INTRODUCTION

If we could journey to the center of the earth we would have to travel about 6,400 km (4,000 miles). Along the way to earth’s core we would pass layers of rock that can be classified in two different ways, either by their chemistry or their physical behavior.

According to the chemical composition of the rocks, earth’s interior can be differentiated into three layers—crust, mantle, and core.

When considering the rocks of earth’s interior in terms of their physical behavior, six layers can be differentiated from the surface to the core. The characteristics that distinguish these six different layers are based on the relative strength of a given layer in response to stress and whether it is solid or liquid.

TOUR THROUGH THE LAYERS OF EARTH

The chemical compositions and physical behaviors of the rocks of earth’s interior converge in many ways but it is worthwhile to become familiar with the each of the different set of layers individually before exploring how they overlap.

The chemical composition and physical behavior of rock inside the earth relate to each other because the chemical composition of a rock is one of the factors that determines its physical behavior. However, the physical behavior of rock also depends on the pressure and temperature it is subjected to at its depth within the earth. As depth inside the earth increases, the pressure and temperature increase. Some layers in the earth are harder or softer than adjacent layers, even though they have the same composition, because they are at different pressures and temperatures.

Surface to Core, Chemically

Crust

The tour starts at the surface with earth’s crust. Generally speaking, the crust is predominately silicon oxide and aluminum oxide. Continental crust is thicker and less dense than oceanic crust. Earth’s crust varies in thickness from less than 5 km (under mid-ocean spreading ridges) to more than 70 km (beneath the highest mountain range).

Mantle

The next layer down chemically is the mantle. The mantle has an ultramafic composition – it contains more iron, magnesium, less aluminum and somewhat less silicon than the crust. The mantle is roughly 2,900 km thick. In terms of volume, the mantle is the largest of earth’s three chemical layers.

Core

The final stop on the chemical tour is the core, which is mostly iron and nickel. The core is about 3,500 km thick.

The following table summarizes the chemical layers of the earth.

Chemical Layers of Earth
Crust Mantle Core
composition: high Si, Al, & O composition: moderate Si, high Mg & Fe composition: Fe & Ni
thickness: 5 to 70 km thickness: 2,900 km thickness: 3,500 km

Surface to Core, Physically

Lithosphere

Starting at the surface, the first layer is the lithosphere. We humans, and the other creatures that live on earth, occupy the surface of the lithosphere. The lithosphere is entirely solid except where there are zones of magma beneath volcanoes or in places undergoing magma intrusion. The volume of molten rock in the lithosphere is a tiny fraction, less than 0.1%, of the volume of the entire lithosphere.

The lithosphere itself has two parts. The top part is the crust. The bottom part is the lithospheric mantle. The two components of the lithosphere, in combination, form a relatively strong, rigid layer of rock that covers the earth. Earth’s tectonic plates, all of which are in motion relative to each other, make up the lithosphere.

Asthenosphere

Beneath the lithosphere is a relatively weak and ductile layer of the mantle called the asthenosphere. Although the asthenosphere is solid, not liquid, it flows at geological rates, up to several cm (several inches) per year. In other words, the asthenosphere behaves much more plastically than the rigid lithosphere above it does.

The chemical composition of the asthenosphere is about the same as the chemical composition of the overlying lithospheric mantle. Why, then, is the asthenosphere soft and the lithosphere rigid? It is because at the depth of the asthenosphere, temperatures are very close to the melting point of the rock, weakening the rock. In fact, it is thought likely, from indirect evidence, that there is a small percentage of molten rock in the tiny spaces between the minerals of the asthenosphere, which contributes to the soft nature of the rock. However, the solid minerals of the asthenosphere are extensively in contact with each other, forming a material that is solid overall despite the possible presence of a small amount of partial melt.

The asthenosphere is the primary source of most magma. Because the asthenosphere is close to its melting point and may contain everywhere a small proportion of partly molten rock, it does not take much to cause magma to form and separate from the asthenosphere. As explained in the igneous rocks Basics page, melting of the asthenosphere can be caused by addition of fluid, particularly water, or by a decrease in pressure. In subduction zones, slabs of sinking lithosphere release water into the asthenosphere, causing the asthenosphere to melt and produce magma which rises into and through the crust, producing the volcanic arc that is found at every subduction zone. At divergent plate boundaries, the asthenosphere flows upward, or upwells, which reduces the lithostatic pressure enough to cause the rock of the asthenosphere to melt. That is why divergent plate boundaries are volcanic zones. The solidified lavas and intrusions at divergent plate boundaries produce new lithosphere, which fills in and replaces the plate material that spreads away on either side.

Upper Mesosphere

Beneath the asthenosphere is the rest of the mantle, the mesosphere. The mesosphere makes up most of the volume of the mantle. The mesosphere is entirely solid. The temperature and pressure of the rock in the mesosphere keep it from breaking; therefore, no earthquakes originate from the mesosphere.

The upper mesosphere is a transition zone in which the rock rapidly becomes denser with depth in response to the increasing lithostatic pressure.

Lower Mesosphere

The lower mesosphere starts at a depth of 660 km from earth’s surface. At that depth there is an abrupt increase in density. This increase is caused by changes in the crystal structures of the most abundant minerals in the rock. These minerals change from less dense crystal structures above the boundary to more dense crystal structures below the boundary. The lower mesosphere undergoes little density change from its top boundary at 660 km to its base at 2900 km where it meets the outer core.

Outer Core

The bottom of the mesosphere is the boundary with the earth’s core. The core is about twice as dense as the crust, and about 1.5 times as dense as the mantle. The outer core is liquid, as was discovered when it was first observed that S-waves will not pass through it.

Inner Core

The inner core is solid. The inner and outer cores are made of the same iron-rich, metallic composition. The temperature of the inner core is not very much greater than the temperature of the outer core. However, lithostatic pressure keeps increasing with depth and the inner core has the great weight of the rest of the earth pressing in on it. The pressure on the inner core is high enough to keep it in the solid state.

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