Earth's Layers, Seismic Waves, and the Wilson Cycle

Earth's Internal Structure and Plate Tectonics

Seismic Wave Behavior at Different Depths

P-waves (primary waves) and S-waves (secondary waves) are seismic waves that travel through the Earth's interior. At a depth of 100 km, P-waves have a speed of 6 km/s. This speed increases to 8 km/s as the material becomes more rigid. The speed then decreases at the Mohorovičić discontinuity. After 300 km, the speed begins to rise again until it reaches a point where it increases slowly because the material is less rigid. The Repetti discontinuity separates the lower mantle from the upper mantle. Continuing on, P-waves reach a point where their speed plunges at a depth of over 3,000 km. This is where S-waves diminish completely, as the material is more liquid. The Gutenberg discontinuity separates the core from the mantle. After this, the P-wave speed increases sharply again. At 5,150 km, the Lehmann discontinuity separates the outer core from the inner core.

Earth's Layers

• Geochemical Composition: Crust, Mantle, Core
• Dynamic Behavior: Lithosphere, Asthenosphere, Mesosphere, Endosphere

The Wilson Cycle

The Wilson Cycle describes the cyclical opening and closing of ocean basins.

Stages of the Wilson Cycle

1. Rifting: The asthenosphere is heated, melts, and the material tends to rise. This presses on the lithosphere and breaks it apart, forming a rift valley.
2. Ocean Formation: Magma comes into contact with the surface at the rift, flooding it with an ocean (the opening of an ocean). The magma rises, pushes, and cools, forming a mid-ocean ridge crest.
3. Ocean Expansion: The new sea widens as the magma solidifies and pushes the previous material (the extension of an ocean).
4. Subduction and Mountain Formation: Continents are eroded, and waste material accumulates on the seabed near the shore. Eventually, the weight of this material causes it to sink (subduction), getting beneath the continents. The subducted material is heated and tends to rise, forming volcanoes and mountain ranges.

Plate Boundaries

There are three types of plate boundaries:

• Constructive: New oceanic lithosphere is formed.
• Destructive: Oceanic lithosphere is removed.
• Passive: Lateral movement of plates.

Major Tectonic Plates

Some of the major tectonic plates include the Eurasian, African, Pacific, Antarctic, and North American plates. The lithosphere is broken into these plates.

Formation of Planet Earth

Approximately 15 billion years ago, a great explosion called the Big Bang occurred. As a result, the Milky Way was formed, along with a central sphere and masses that gave rise to the Sun, planets, and satellites. One of these planets generated water and gases. The temperature was low enough to allow the formation of a crust. Initially, there was no atmosphere. Volcanic activity was constant, increasing the thickness of the crust and forming a large amount of gases, creating the first atmosphere. Over time, liquid water accumulated in deeper areas, and due to rains, seas and oceans were formed, creating the hydrosphere.

Physical and Chemical Characteristics of Earth

Using Newton's law, the Earth's mass is recalculated to be 5.98 x 10²⁴ kg. The crust makes up 1% of the mass, the mantle 68%, and the core 31%. The Earth's volume is 1.08 x 10¹² km³. Applying this formula, we obtain the Earth's density as 5.517 g/cm³. The Earth is a sphere flattened at the poles, with an equatorial radius of 6,378 km and a polar radius of 6,356 km. The Earth has a bipolar magnetic field. The surface temperature varies, exceeding the boiling point in some places. The atmosphere is composed of nitrogen, oxygen, and other elements, and it rotates on its axis.

Seismic Waves

P-Waves (Primary Waves)

P-waves are the first to reach the surface. Their propagation speed is higher than other waves, and their vibration coincides with their direction of propagation.

S-Waves (Secondary Waves)

S-waves reach the surface after P-waves. Their propagation velocity is lower, and their vibration is transverse to their direction of propagation. S-waves do not propagate through fluid media.