Earth's Interior Structure: Seismic Waves and Tectonic Theories

The Geosphere and Earth's Structure

Studying the Geosphere and Its Relief

Direct Study Methods

• Observing rocks on the surface
• Boring (drilling)
• Satellite photographs, radar, and sonar

Geosphere's Relief Features

The Geosphere's relief includes:

• Continents: Flat areas, mountain ranges, coastlines, cliffs, and beaches.
• Seabeds: Flat areas with small elevations, such as ridges and trenches.

Seismic Methods and Earth's Interior

The Seismic Method studies the variations in seismic wave speed as they travel through different materials within the geosphere.

Understanding Seismic Waves

Seismic waves are vibrations in the Earth that propagate in all directions. The more rigid the material they pass through, the faster they travel.

Types of Seismic Waves

• P-waves (Primary waves): Materials vibrate like an accordion. They travel through solids and liquids.
• S-waves (Secondary waves): Materials move perpendicular to the wave direction. They are slower than P-waves and travel only through solids.
• Surface waves: Generated when P and S waves reach the surface of the geosphere.

Seismic Discontinuities

When an earthquake occurs, seismic waves propagate through the geosphere in all directions. They are detected by seismographs and recorded in seismograms to analyze the trajectory and speed of the waves, revealing discontinuities.

The Geochemical Model of Earth's Layers

The Crust (Continental and Oceanic)

The Crust is the outer, thinnest layer of the Earth, composed of a great variety of rocks.

• Continental Crust: Primarily metamorphic and sedimentary rocks, typically 30–40 km thick.
• Oceanic Crust: Primarily gabbro, typically 5–15 km thick.

The Mantle (Outer and Inner Zones)

The Mantle is the thickest layer of the geosphere.

• Outer Zone: Composed of rocks containing olivine.
• Inner Zone: Contains rocks similar to the outer zone and extends down to the Gutenberg Discontinuity.

The Core (Outer and Inner Core)

The Core extends from 2,900 km below the surface to the center of the planet and is primarily made of metal.

• Outer Core: Extends from the Gutenberg Discontinuity to the Lehmann Discontinuity. S-waves do not travel through this liquid layer.
• Inner Core: Extends from 5,100 km to the center. P-waves increase their speed here, indicating it is composed of solid metal.

Plate Tectonics: Continental Drift Theory

Alfred Wegener's Hypothesis

Continental Drift was the first non-fixist theory of tectonic processes, proposed by Alfred Wegener. He hypothesized that in the past, there was a supercontinent called Pangaea, which broke down into smaller continents that are still in motion due to tectonic plates.

Evidence Supporting Continental Drift

Wegener provided several lines of evidence:

• The perfect fit between the coastlines of Africa and South America.
• Observation of similar geological formations (deformations) separated by oceans.
• Discovery of fossils of the same age and species found on widely separated continents.

Paleomagnetism and Rock Polarity

Paleomagnetism describes how, when volcanic rocks are in lava form, the magnetic minerals they contain crystallize according to the Earth's magnetic field at that moment. When these rocks solidify, they retain this polarity (North-South).

Magnetometers are instruments used for measuring these magnetic phenomena in rocks.