Tectonics, Continental Drift & Mountain Building

1. Continental Drift

Proposed by Alfred Wegener in 1912, the theory suggested that a supercontinent called Pangaea existed ~250 million years ago, surrounded by a global ocean called Panthalassa.

Evidences for Drift:

• Jigsaw fit: Matching coastlines of South America and Africa.
• Fossil evidence: Identical fossils of the reptile Mesosaurus and the fern Glossopteris found across oceans.
• Geological matching: Appalachian mountains align with the Scottish Highlands.
• Paleoclimatology: Glacial striations (tillites) found in tropical India, Africa, and Australia.

Wegener's mechanism (centrifugal forces and tidal pull) was rejected by geophysicists, leading to the temporary dismissal of the theory until it evolved into Plate Tectonics.

2. Plate Tectonics

Formulated in the late 1960s, it successfully explained continental drift through Sea-Floor Spreading (proposed by Harry Hess) and convection currents in the mantle (Holmes).

• The lithosphere is broken into rigid plates floating on the plastic asthenosphere.
• Divergent Boundaries: Plates pull apart. Forms mid-ocean ridges (e.g., Mid-Atlantic Ridge) and rift valleys (e.g., East African Rift).
• Convergent Boundaries: Plates collide. Causes subduction (forming trenches and volcanic arcs, e.g., Andes, Japan) or mountain folding (Himalayas).
• Transform Boundaries: Plates slide past each other (e.g., San Andreas Fault), causing earthquakes without volcanism.

3. Geosynclines

Before plate tectonics, the concept of geosynclines was used to explain mountain building. Evaluated by Hall and Dana, a geosyncline is a massive, long, narrow subsiding trough in the Earth's crust, into which sediments from adjacent landmasses are deposited.

• Over millions of years, the accumulation of sediments causes immense pressure and subsidence.
• The compressive forces eventually squeeze these sediments, folding them upward into mountain ranges (orogenesis).

4. Isostasy

Isostasy (from Greek 'iso-stasios', equal standstill) refers to the state of gravitational equilibrium between the Earth's lithosphere and asthenosphere.

• Airy’s Hypothesis (Uniform density, varying thickness): Suggested that mountains have deep, low-density roots extending into the denser mantle, much like an iceberg floating in water.
• Pratt’s Hypothesis (Varying density, uniform depth): Proposed that higher elevations are made of less dense rock, while ocean basins are made of denser rock. Thus, they all achieve equilibrium at a uniform "depth of compensation".

5. Recent Views on Mountain Building

• Orogeny is now entirely understood through the lens of Plate Tectonics.
• Continent-Continent Collision: When two continental plates meet, neither subducts fully due to low density. They crumple and fold, creating massive fold mountains (e.g., the Indian plate colliding with the Eurasian plate to form the Himalayas).
• Ocean-Continent Convergent (Andean type): Subduction of dense oceanic plate causes partial melting and inland volcanic mountain chains.
• Island Arcs: Convergence of two oceanic plates leading to subduction and chain of volcanic islands (e.g., Aleutian Islands).