Understanding Earth’s Interior: Seismic and Geochemical Methods
The Seismic Method
The seismic method is an indirect method of study that looks at the variations in propagation speed of seismic waves as they travel through different materials inside the geosphere.
Seismic Waves
Seismic waves are vibrations generated inside the Earth at the point where an earthquake takes place. They propagate in all directions, and the more rigid the material they pass through, the more quickly they travel.
Types of Seismic Waves
- P Waves: Cause the material to vibrate, alternating between compression and dilation. These waves propagate through solids and liquids.
- S Waves: Cause the material to oscillate perpendicular to the direction of propagation. They are slower and cannot travel through liquids.
- Surface Waves: Generated when P and S waves reach the surface of the geosphere. They are not used in the seismic method.
Seismic Discontinuities
Mohorovicic Discontinuity
Average depth of 20 km. Seismic waves speed up due to an increase in the density of the rock. In some regions, an area of low wave speed is registered underneath it, which indicates the presence of molten rock.
Gutenberg Discontinuity
Depth at 2900 km. P waves quickly decrease their speed, while the S waves stop propagating. This indicates that the lower layer is liquid.
Lehmann Discontinuity
Depth of 5100 km. The P waves quickly speed up again. This has been interpreted as a solid compact state of the materials in the centre of the Earth.
The Geochemical Method
The Crust
The surface layer of the geosphere and the thinnest layer.
Types of Crust
- Continental Crust: Comprises the continents and their submerged continental platforms. Its thickness varies between 30 and 40 km and contains a wide variety of rocks, particularly granite, metamorphic rocks, and sedimentary rocks, which are less dense than the oceanic crust.
- Oceanic Crust: Comprises the rest of the areas covered by the oceans. Its thickness varies between 5 and 15 km. It is made up of rocks that are denser than those in the continental crust, such as basalt and gabbro.
The Mantle
The thickest layer, located between the Mohorovicic and Gutenberg discontinuities. It is made up of very dense rocks, such as Peridotite.
Mantle Zones
- Outer Zone: Between the Mohorovicic discontinuity and a level of 30 to 100 km down. Contains rocks that are rich in olivine.
- Inner Zone: Goes down as far as the Gutenberg discontinuity and contains rocks with a composition that is similar to those in the outer zone, although the pressure and temperature become denser.
The Core
Located between a point 2900 km down and the centre of the planet, around 6371 km down. It is made up of metal (mostly iron and nickel) and it is divided into:
Core Layers
- Outer Core: Located between the Gutenberg discontinuity (2900 km) and the Lehmann discontinuity (5100 km). The S waves do not travel through it. This layer is made of molten metal and it contains iron, metal, and some sulfur.
- Inner Core: From a depth of 5100 km to the centre of the Earth (6371 km), made of solid metal, as the P waves increase their speed here. It contains iron and nickel.