Layers of the Earth: Structure, Composition, and Study Methods
Layers of the Earth
Lithosphere
The lithosphere is the Earth’s rigid outer layer, composed of the crust and uppermost mantle. It extends to a depth of about 100 km in continental areas and 50 km in oceanic areas. There are two types of lithosphere:
- Continental Lithosphere: Made up of the continental crust and upper mantle, reaching a thickness of up to 300 km under mountain ranges.
- Oceanic Lithosphere: Composed of the oceanic crust and upper mantle, with a thickness ranging from less than 20 km in younger areas to 100 km in older parts of the oceans.
The lithosphere is fragmented into lithospheric plates that rest on the asthenosphere.
Asthenosphere
The asthenosphere is a region of the upper mantle located below the lithosphere, approximately between 100 and 240 km below the Earth’s surface. It is characterized by slow convection currents, which are thought to drive continental drift. Basalt flows in the asthenosphere are extruded along oceanic ridges, renewing the ocean floor. At convergent plate boundaries, the denser plate sinks beneath the other in a process called subduction, returning material to the asthenosphere.
There are three types of plates based on their composition:
- Oceanic Plates: Composed entirely of oceanic lithosphere (e.g., Pacific, Cocos, and Nazca plates).
- Continental Plates: Composed entirely of continental lithosphere (e.g., Arabian plate).
- Mixed Plates: Contain both continental and oceanic lithosphere. These are the most common and largest plates (e.g., North American, South American, African, Eurasian, Pacific, Antarctic, and Australian plates).
Smaller fragments of lithosphere, called microplates or litosferoclastos, are also present and are pushed by the larger plates surrounding them.
The rest of the upper mantle beneath the asthenosphere is subjected to high temperatures and pressures, allowing it to flow slowly like an extremely viscous liquid.
Mantle
The mantle is a layer of rock beneath the crust that extends to the core-mantle boundary at a depth of 2900 km. It is composed primarily of peridotite, a rock rich in olivine. The mantle is divided into two parts:
- Upper Mantle: Extends to a depth of 1000 km.
- Lower Mantle: Extends from 1000 km to 2900 km.
At a depth of 670 km, pressure compacts peridotite minerals, increasing their density. As a result, the lower mantle is denser than the upper mantle.
Core
The Earth’s core is composed primarily of iron (85%), nickel (5%), and other elements like silicon, oxygen, and carbon (10%). It is divided into two parts with the same composition but different physical states:
- Outer Core: Extends to a depth of 5100 km and is liquid. Its fluidity is similar to water, and it is characterized by vigorous convection currents that generate Earth’s magnetic field.
- Inner Core: Extends from 5100 km to the Earth’s center at 6371 km and is solid.
Seismic Discontinuities
Seismic discontinuities are boundaries within the Earth where seismic wave velocities change abruptly. Some notable discontinuities include:
- Mohorovičić Discontinuity (Moho): Separates the crust from the mantle, located at a depth of 30-70 km.
- Repetti Discontinuity: Separates the upper mantle from the lower mantle, located at a depth of 670 km.
- Gutenberg Discontinuity: Separates the mantle from the outer core, located at a depth of 2900 km.
- Lehmann Discontinuity: Separates the outer core from the inner core, located at a depth of 5150 km.
Continental Drift and Plate Tectonics
Meteorologist Alfred Wegener proposed the theory of continental drift, suggesting that continents were once united in a supercontinent called Pangaea and have since moved apart. While Wegener provided evidence for his theory, he could not explain the mechanism driving continental movement. His theory was initially met with skepticism.
Methods of Studying the Earth’s Interior
Direct Methods
- Surveys: Drilling into the Earth provides direct samples but is limited by depth due to increasing temperature and pressure.
- Study of Magma: Analyzing the composition of erupted magma provides insights into the mantle’s composition.
Indirect Methods
- Study of Meteorites: Meteorites offer clues about the composition and structure of planetary interiors.
- Gravimetry: Variations in gravitational acceleration (g) reveal density differences within the Earth, suggesting the presence of denser materials in the interior.
- Geothermal Gradient: The rate of temperature increase with depth (geothermal gradient) provides information about the Earth’s internal heat. The gradient is higher in volcanic areas and lower in older, stable regions.
- Magnetometry: Studying the magnetic field recorded in volcanic rocks, particularly those containing iron-rich minerals like magnetite, helps understand the Earth’s magnetic field and its history.
- Seismology: Analyzing the behavior of seismic waves as they travel through the Earth is the most important indirect method. Seismic wave velocities change as they encounter different layers with varying densities and compositions, revealing the Earth’s internal structure. The distribution of earthquakes and volcanoes aligns with plate boundaries, supporting the theory of plate tectonics.
The “Ring of Fire” in the Pacific Ocean is a zone of intense volcanic and seismic activity where oceanic lithosphere subducts beneath continental plates, forming volcanic mountain ranges and islands.
Paleomagnetism: The study of the Earth’s ancient magnetic field preserved in rocks. The magnetic field’s polarity has reversed multiple times throughout Earth’s history. These reversals are recorded in volcanic rocks containing magnetic minerals like magnetite. When lava cools and solidifies, magnetite crystals align with the prevailing magnetic field. Geologists Vine and Matthews discovered symmetrical patterns of magnetic reversals in rocks on either side of mid-ocean ridges, providing strong evidence for seafloor spreading and plate tectonics.