Understanding Earth’s Geosphere: Structure and Study Methods
Methods for Studying the Geosphere
The methods used to study the geosphere are classified as follows:
- Direct Methods: Study based on samples obtained by drilling or direct sampling. Information can also be obtained from mines, oil drilling, water wells, and volcanoes.
- Indirect Methods: Based on analyzing and interpreting physical characteristics of our planet, such as Earth’s gravity, seismic waves, and density.
Meteorite Analysis
These celestial bodies are thought to originate from asteroids or remnants of destroyed planets. Their structure is believed to be similar to Earth’s. Meteorites are classified as:
- Siderites or iron meteorites (representing the core)
- Siderolites or stony-iron meteorites (representing the mantle)
- Aerolites or stony meteorites (representing the crust)
Internal Heat Analysis
Temperature increases approximately 3°C for every 100 meters depth within the geosphere. This is known as the geothermal gradient: the rate of temperature increase with depth.
Earth Density Analysis
Density is calculated as Mass / Volume.
Earth Gravity Analysis
The intensity of Earth’s gravitational field varies across the planet’s surface.
Relationship Between Gravity and Density
Generally, where density is higher, gravitational pull (gravity) is stronger.
Seismic Wave Analysis
Much of our knowledge about the geosphere comes from studying seismic waves originating from the hypocenter (earthquake focus). Key seismic waves include:
- Primary Waves (P-waves): These are the fastest seismic waves and can travel through both solid and liquid materials.
- Secondary Waves (S-waves): These are slower than P-waves and cannot travel through liquid materials.
Seismic Discontinuities
A seismic discontinuity is a boundary zone between two layers within the geosphere that have different physical characteristics. When seismic waves pass through a discontinuity, their speed and trajectory change.
There are two main types:
- First-order discontinuities: Characterized by an abrupt change in properties between layers. Examples include the Mohorovičić discontinuity (Moho) (approx. 7-70 km deep) and the Gutenberg discontinuity (approx. 2900 km deep).
- Second-order discontinuities: Characterized by a gradual change in properties between layers. Examples include the Repetti discontinuity (approx. 660-900 km deep) and the Lehmann discontinuity (approx. 5150 km deep).
Earth’s Internal Structure
Chemical Composition Model
This model categorizes layers based on their chemical composition. Three main layers are distinguished: crust, mantle, and core.
- Crust: The outermost, superficial layer, primarily composed of silicates rich in Aluminum (Al), Magnesium (Mg), Potassium (K), Calcium (Ca), etc.
- Mantle: The intermediate layer, composed mainly of iron (Fe) and magnesium (Mg) silicates.
- Core: The innermost layer, primarily formed by iron (Fe) and nickel (Ni).
Physical Properties Model (Mechanical)
This model categorizes layers based on their physical state (solid/liquid) and mechanical behavior. The following layers are distinguished:
- Lithosphere: Rigid, solid outer layer.
- Asthenosphere: Partially molten (about 1-5% melt), plastic layer beneath the lithosphere.
- Mesosphere (Lower Mantle): Solid layer below the asthenosphere.
- Core (Endosphere): Divided into a liquid Outer Core and a solid Inner Core.
Lithosphere and Asthenosphere Details
Lithosphere: This is the rigid, solid outer mechanical layer of the Earth, including the crust and the uppermost part of the mantle. It is fragmented into tectonic plates that float and move over the more ductile asthenosphere.
Asthenosphere: Located beneath the lithosphere, this layer exhibits plasticity (ductility), allowing tectonic plates to move. It includes an upper zone of lower viscosity, sometimes referred to as the low-velocity zone.