Earth’s Structure, Layers, and Plate Tectonics
Hutton and Geological Time
It seemed that no relief was unchanged. Hutton inferred this by observing river sediments. These processes were also used to infer millions of years of geological time.
The Rock Cycle
Sedimentary rocks are transformed with sediment and geological agents.
- Land surface: Weathering and transformation into sediments.
- Inside the crust: Subjected to high temperatures, they become rock.
Modifications Inside the Earth’s Crust
Changes in materials:
Pressure
The pressure increases rapidly with depth.
Temperature
The temperature increases approximately 3 degrees Celsius per 100 meters of depth.
Compression and Relaxation
These forces compress and stretch the materials of the Earth’s crust.
Changes in Materials
Diagenesis
Transformation of sediments into sedimentary rocks through pressure and temperature.
Metamorphism
Changes in rocks subjected to different pressures and temperatures, resulting in metamorphic rocks.
Magmatism
Fusion of rocks creates magmatic rocks.
Geothermal Gradient
The geothermal gradient is the increasing temperature inside the Earth due to residual heat. It supports the theory that 4000 million years ago, Earth underwent impacts from meteorites, disintegration of radioactive elements, and decanting of denser materials.
Formation of Earth’s Layers
When the Earth was warm, materials were ordered by density:
- Core: Iron
- Mantle: Heavy rock material
- Crust: Very dense rocks
- Atmosphere: Layer of gases
- Hydrosphere and Oceans: Water vapor
The Earth’s Crust
The crust is a thin, solid, rocky layer.
- Continental Crust: 30 to 70 km thick, composed of granite covered by sedimentary rocks and sediments.
- Oceanic Crust: 10 km thick, composed of basalt and gabbro.
The Mantle
The mantle is a rock layer under the crust, 2900 km deep, composed of peridotite.
- Upper Mantle
- Lower Mantle
The Core
The core is composed of metallic elements (85% iron, 5% nickel, 10% non-metallic elements).
- Outer Core
- Inner Core (solid)
Seismic Discontinuities
Seismic discontinuities are surfaces of separation identified and located with seismic studies (separation between layers of the Earth).
Types:
- Mohorovičić Discontinuity: Between the crust and the mantle.
- Repetti Discontinuity: Separates the upper and lower mantle.
- Gutenberg Discontinuity: Separates the mantle and outer core.
- Lehmann Discontinuity: Separates the outer and inner core.
The Lithosphere
The lithosphere is the outer part of the upper mantle, together with the crust. It is fragmented into lithospheric plates. Continental lithosphere is up to 300 km thick in mountainous regions and 100 km thick in continental plains. Oceanic lithosphere is approximately 100 km thick, and as thin as 20 km in the youngest areas of the oceans.
Alfred Wegener and Continental Drift
Alfred Wegener proposed the theory that continents could move. 300 million years ago, they were united in a supercontinent called Pangea.
The Asthenosphere
The asthenosphere is a layer of the inner mantle (Joseph Barrell). It allowed the bottom of basins to sink due to the weight of sediments, and the relay raised to erosion. He called this isostasy.
Expansion of the Ocean Floor
In 1960, it was determined that the seafloor was basaltic (volcanic chain). Harry Hess proposed in 1962 that the ocean floor was expanding. The basalt of the seafloor was older and more distant from the ridge. The Atlantic Ocean was opening, moving Europe and Africa eastward and North and South America westward. The speed of expansion is a few millimeters per year. Seismic studies showed that it was not the crust but the lithosphere that was being dragged by convection currents (Arthur Holmes, 1929).
Magnetic Stripes of the Ocean Floor
In the 20th century, it was discovered that the Earth’s magnetic field is unstable, and its polarity is reversed from time to time. The reversals of the magnetic field are recorded in volcanic rocks (magnetite). When lava cools, magnetite crystals align in a north-south direction. Fred Vine and Matthews published the results of measurements of remnant magnetism, cornering the fixist theories.
Benioff Zone
TP: The Benioff Zone is characterized by earthquakes whose focus depth is located near the coastline, and which increase in depth inland. Mountain ranges are the result of the collision of plates.
Lithospheric Plates
(Crust + Upper Mantle)
- Oceanic Lithosphere: Metamorphic basaltic rocks, dense, displaced by convection currents.
- Continental Lithosphere: Granitic, volcanic, metamorphic, and sedimentary rocks, less dense, displaced by convection currents.
Types of Plates
- Oceanic Plates: Composed of oceanic lithosphere.
- Continental Plates: Composed of continental lithosphere.
- Mixed Plates: Composed of both, almost the total terrestrial surface.
- Microplates: Small fragments that move.
Plate Boundary Movements
- Divergent Boundaries: Creates constructive edges (new oceanic lithosphere), intense volcanism, seafloor spreading.
- Convergent Boundaries:
- Destructive edges (destroying oceanic lithosphere, intense seismicity, volcanism).
- Collision edges (collision across continents, folding of rocks, seismicity).
- Transform Boundaries: Passive edges, seismicity.