Earth's Interior Structure and Plate Tectonics Explained

How Scientists Know What Lies Inside the Earth

Scientists learn about Earth's interior through direct and indirect methods.

Direct Methods

• Mines: Excavation of materials from the surface.
• Drill holes: Boreholes that reach the subsoil. Both of these show that temperature increases with depth.
• Volcanoes: Where magma rises to the surface, providing samples of material from inside the Earth.

These studies allow scientists to examine material from inside the planet and draw conclusions about its interior.

Indirect Methods

• Earth's density: Shows that the interior is made of denser material.
• Seismic waves: Earthquake waves provide information about the internal temperature, composition, and state of the Earth's layers.
• Magnetic field and meteorites: Further clues about the planet's internal structure.

Seismic waves traveled through the Earth's interior produce a thermal effect.

Types of Seismic Waves

• P waves (primary): The fastest waves; they travel through both liquids and solids and cause compression movement in the same direction as the wave.
• S waves (secondary): Slower waves that travel only through solids and cause movement perpendicular to the wave direction.

Earth's Internal Discontinuities

• Mohorovičić discontinuity (Moho): Separates the crust from the mantle.
• Gutenberg discontinuity: Separates the mantle from the core.

Seismic waves show that the Earth is divided into distinct layers: the crust, mantle, and core.

Internal Temperature of the Earth

Temperature increases about 30 °C per kilometer of depth, known as the geothermal gradient. If this rate were uniform, the temperature at the center would reach _30,000 degrees, which is far too high. Seismic studies show that the gradient decreases with depth, so the temperature in the core is lower than this estimate.

The outer core is liquid, composed mainly of iron and nickel, while the inner core is solid due to the high pressure. The temperature at the boundary between the outer and inner core is about _3,800 °C, and at the center of the inner core it reaches about _5,000 °C.

Terrestrial Magnetism

The Earth's magnetic field acts like a magnet with north and south poles. It is generated by electric currents within the liquid outer core.

Meteorites

Meteorites are rocky or metallic bodies from space that cross Earth's orbit and fall to the surface. They provide information about the internal structure of planets, since they originate from differentiated bodies.

Internal Structure of the Earth by Criteria

Geochemical Units

Crust: Divided into two types:

• Continental crust: Thickness 25 to 75 km, heterogeneous (sedimentary, metamorphic, and plutonic rocks), with a density of 2.4 to 3 g/cm³.
• Oceanic crust: Thickness 5 to 10 km, homogeneous (sedimentary and magmatic rocks).

Mantle: Makes up about _83% of Earth's volume. It is solid, but its upper part is plastic-ductile. It is formed mainly of magmatic and plutonic rocks, such as peridotites.

Core: Makes up about _16% of Earth's volume. It is located at the center of the planet and is composed mainly of iron, nickel, and smaller amounts of other elements. The outer core is liquid, while the inner core is solid.

Dynamic Units

Mechanically, the Earth is divided as follows:

• Lithosphere: Rigid and forms the crust and the uppermost part of the mantle.
• Upper mantle (asthenosphere): Lies beneath the lithosphere. Temperatures and pressures are high enough that rocks behave plastically over long periods, allowing slow convection currents.
• Lower mantle (mesosphere): Plastic, with convection currents.
• Outer core: Liquid, with convection currents that generate the magnetic field.
• Inner core: Solid.

The Lithosphere and Continental Drift

The lithosphere floats on the layers beneath it and is subject to two types of movement:

• Vertical movement: Explained by the theory of isostasy (isostasy = equilibrium, which explains relief).
• Horizontal movement: Explained by plate tectonics and by the earlier theory of continental drift proposed by Alfred Wegener, who suggested that all continents were once joined in a single supercontinent, Pangaea, which later broke apart.

Evidence for Continental Drift

• Paleontological evidence: Fossils of the same reptile species have been found in Africa and America.
• Geographic evidence: The coasts of Africa and South America fit together almost perfectly.
• Geological evidence: Similar mountain ranges, such as the Appalachians and the mountains of Scotland and Scandinavia, appear on both sides of the Atlantic.
• Paleoclimatic evidence: Deposits of glaciers found in places now very distant from the poles.

Wegener proposed that the continents moved through forces caused by the Earth's rotation and the attraction of the Moon and the Sun, which displaced the continental crust.

Plate Tectonic Theory

Studies of the Ocean Floor

Oceanographic surveys revealed mid-ocean ridges. In the Atlantic, a central valley runs along the ridge, called a rift. Transform faults are transverse fractures.

The ocean floor moves through seafloor spreading: oceanic ridges create new oceanic lithosphere from magma rising from within the Earth. This theory explains volcanic and seismic activity. Subduction is the process in which the oceanic lithosphere sinks into the mantle beneath the continents.

Postulates of Plate Tectonics

1. The lithosphere is divided into blocks called plates.
2. The greatest geological activity (earthquakes and volcanoes) occurs at the boundaries of the plates.
3. New oceanic lithosphere is continuously generated at mid-ocean ridges and destroyed in trenches.
4. Plates move carrying the continents with them, and when they interact, they produce deformation.
5. In production areas, the separation of plates generates new oceanic crust, forming ocean basins and mountain ranges. Where plates collide, mountains rise, such as the Himalayas.

Types of Plate Boundaries

• Constructive or divergent: Plates separate, and new oceanic lithosphere forms from magma. This results in ridges and the formation of rifts.
• Destructive or convergent: Subduction zones, where plates collide and one is destroyed. Oceanic trenches, mountain ranges, and island arcs appear.
• Passive: Transform faults, where plates slide laterally past each other. Lithosphere is neither created nor destroyed; these boundaries mainly produce earthquakes and shallow volcanoes.