Understanding Earth's Plate Tectonics

Key Concepts in Plate Tectonics

Isostasy

A theory explaining the vertical movements of the lithosphere in accordance with the principle of Archimedes. According to this principle, if a terrestrial area is overloaded, it will sink, while if it is unloaded, it will rise.

Continental Drift Theory

Alfred Wegener believed that all landmasses had once existed together in a large supercontinent called Pangaea. The present continents are the result of the breakup of Pangaea.

Arguments for Continental Drift

• Geographic Arguments: The shape of the continents allowed them to fit together like puzzle pieces. Coastal erosion processes and continuous changes in sea level prevented a perfect fit.
• Paleontological Arguments: The presence of identical fossil species on widely separated continents can only be explained by the existence of Pangaea.
• Geological Arguments: Analysis of certain mountain ranges and other geological formations showed continuity across the Atlantic.
• Paleoclimatic Arguments: Evidence of ancient glacial deposits (tillites) of the same age is found on continents now separated by oceans.

Mid-Ocean Ridge

Underwater mountain ranges where new oceanic crust is generated. They are typically over 60,000 km long and rise about 3,000 meters above the surrounding plains. There is intense magmatic activity along these ridges.

Seafloor Spreading Theory

According to this theory, mid-ocean ridges are places where new oceanic lithosphere is generated from upwelling magma from the mantle. The newly created lithosphere moves away from the ridge axis, and the space is filled by new magma.

Thermal Subsidence

Subsidence occurs on the ocean floor as it moves away from the ridge due to the gradual cooling of the newly formed lithosphere.

Subduction

The process of the descent of oceanic lithosphere into the Earth's mantle.

Plate Convergence

• Continental-Oceanic Convergence: An oceanic plate is subducted beneath a continental plate. Friction and heating of the subducting oceanic lithosphere release water, which lowers the melting point of the overlying mantle wedge, causing partial melting.
• Oceanic-Oceanic Convergence: As oceanic lithosphere ages, it acquires a higher density, becomes denser than the underlying asthenosphere, and subducts.
• Continental-Continental Convergence: Following the collision of two continental plates, neither plate subducts significantly; instead, the crust is thickened and uplifted, leading to the formation of large mountain ranges.

Transform Fault

A conservative plate boundary where lithosphere is neither created nor destroyed.

The thermal energy generated inside Earth's mantle convection currents constitute the primary cause of plate movement, aided by gravitational forces (like ridge push and slab pull).

The lithosphere is divided into a set of rigid fragments called plates.

Plate Boundaries

The boundaries or edges of the plates can be:

• Mid-ocean ridges (Divergent boundaries)
• Subduction zones (Convergent boundaries)
• Transform faults (Conservative boundaries)