Earth’s Structure and Composition: A Detailed Study
Theme 3: Structure and Composition of the Earth
Methods of Study
Meteorites
Meteorites are solid, metallic, or stony objects that move at high speed through space. Normally, they are consumed when penetrating the atmosphere, but if large enough, they can reach the Earth’s surface. Today, they are thought to be remnants of the material that formed the solar system. Consequently, analogies are sought between these and the Earth. Among their minerals, there are some unknown on Earth. The most common are iron, diamonds, graphite, magnetite, and quartz.
Seismic Waves
Seismic waves are caused by different phenomena, but only those generated by major earthquakes, volcanic eruptions, asteroid impacts, and nuclear explosions can pass completely through the Earth. They are of two types:
- Surface Waves: Cause the most damage during earthquakes.
- Internal Waves: The most useful for studying the Earth’s interior.
An earthquake generates two types of seismic waves: P-waves and S-waves.
- P-waves (Primary Waves): These are the fastest and can pass through solids, liquids, or gases. The materials through which a P-wave passes expand and compress as the wave moves, recovering their initial size and shape when the wave has passed.
- S-waves (Secondary Waves): These are generally slower than primary waves and can only pass through solids. S-waves are transverse waves, flattening the material through which they spread.
The velocities of P and S-waves depend on the density and elasticity of the materials they traverse. Seismic waves move outwards as wave fronts from their area of origin. The behavior and the time it takes for the waves to travel through the interior of the Earth give scientists a lot of information about its structure.
The speed of P and S-waves depends on the density and elasticity of the materials, and both properties are related. If the inside of the Earth were homogeneous, seismic rays of P and S-waves would move in straight lines. As the waves pass through materials of different density and elasticity, they are continually refracted, so their paths are curved.
Seismic rays encounter discontinuities that separate materials of different elasticity or density within the Earth. They are refracted as they pass through the interface, and some of their energy is reflected back to the Earth’s surface.
Structure and Composition of the Earth
Structure
The presence of four abrupt changes in the propagation speed of waves at different depths defines four discontinuities that clearly delineate Earth’s layers:
- Crust
- Upper Mantle
- Lower Mantle
- Outer Core
- Inner Core
There is a level of partially molten materials: the asthenosphere. The absence of S-waves results in a shadow zone between 103º and 143º of the Earth’s sphere. The arrival of some P-waves in the shaded area demonstrates the existence of a solid inner core, as these travel faster than in the outer core.
Composition
We distinguish:
- Crust: It is the thin outer layer of the Earth. We can distinguish:
- Continental Crust: It is very heterogeneous and consists of less dense rock.
- Oceanic Crust: It is under the oceans and is much thinner.
- Mantle: It extends from the base of the crust to a depth of 2,900 km.
- Core: It is the central area of the planet.