Internal Geological Agents and Plate Tectonics

1. Internal Geologic Agents

Internal geologic agents, such as the movements of lithospheric plates, earthquakes, and volcanic eruptions, shape the Earth's relief. The energy source that drives these processes is the heat stored inside the Earth. Evidence of this heat is the geothermal gradient, defined as the increase in temperature with depth. This heat originates from:

• The collision of planetesimals during the Earth's formation
• The disintegration of radioactive elements

2. Movement of Lithospheric Plates

From a dynamic perspective, the Earth can be divided into three layers:

• Lithosphere: The rigid outermost layer composed of the crust and a portion of the upper mantle (from 0 to 100 km).
• Mesosphere: The rest of the mantle, a semi-liquid layer.
• Endosphere: Consists of the core (outer and inner core).

The lithosphere is not a continuous layer but is divided into pieces, much like a puzzle, forming the lithospheric plates. There are three types of lithospheric plates, categorized by the kind of crust they are formed by:

• Continental Plates: Formed by continental crust and a portion of the upper mantle (e.g., Arabian Plate).
• Oceanic Plates: Formed by oceanic crust and a portion of the upper mantle (e.g., Pacific Plate, Nazca Plate).
• Continental/Oceanic Plates: Formed by both continental and oceanic crust and a portion of the upper mantle (e.g., Indo-Australian Plate, North American Plate).

Plate Boundaries

There are three main types of plate boundaries:

• A) Divergent Boundary: Two plates move away from each other. Volcanic activity is common in these areas as mantle material easily rises to the surface through the thin, fractured rock. This occurs along the mid-ocean ridge, where magma constantly flows to the surface, creating new ocean floor as the plates separate (e.g., the Atlantic Ridge between the American Plate and the African and Eurasian Plates).
• B) Convergent Boundary: Two plates push into each other. Depending on the type of crust colliding, two outcomes are possible:
  • Oceanic Plate-Continental Plate: The denser oceanic crust slides under the continental crust (subduction), causing earthquakes due to friction. The subducted oceanic crust melts, and the molten material rises, forming a line of volcanoes on the continental crust (e.g., Mariana Trench).
  • Continental Plate-Continental Plate: High mountains are formed in these areas (e.g., Himalayas).
• C) Transform Boundaries: Plates slide past each other, causing earthquakes.

Hotspots

: there are places far from the plate boundaries where magma rises from the mantle to the surface forming volcanoes.