Tectonic Processes: Theories and Plate Boundaries

Continental Drift Theory

• In 1921 Alfred Lothar Wegener proposed the existence of supercontinent called Pangaea. Suggested that the continents were all originally part of huge landmass and had been spilt apart.

• He noticed the close correlation of the coastline shapes of eastern South America and Western Africa.

Evidences Supporting the Theory

•Distribution of Ancient Fossils•Geological Formations• Ice•Drifting of Continents (Plate Tectonics)

Distribution of Ancient Fossils

• They found out that the deposits fossil remains of a certain plant species in Argentina, Brazil, South Africa, Antarctica, India and Australia follow in the same succession.

• He noted that the mesosaurus a small reptile was both found in South Africa and Brazil.

• The Identification of Lystrosaurus reptile skull in a sandstone layer in the Alexandria mountain range of Antarctica.

Geological Formations

Wegener said that all continents are moving apart. He noticed that Africa and South America fit together almost like pieces in a giant jigsaw puzzle. He thought that all the continents might fit together. Rock formations from Africa and South America are of the same age and type.

Ice and Plate Tectonics • Three hundred million years ago, ice sheets covered many parts of South

America Evidence has shown, that glaciers moved from the South America.

• Plate tectonics can explain the phenomena of continental drift. Plates may either be carry a continental or an oceanic crust or both. Convection currents generated by heat from the center of the earth move these plates. This movement deep underground is transferred to the earths dramatic effect.

• One of the objections to the continental drift theory is its inability to provide a mechanism that explains the movement of the continents across the globe.

Plate Tectonic Theory supported by the following evidences:

• Paleomagnetism or fossil magnetism – refers to fossil (especially rocks) formed million years ago contain record of the magnetic poles at the time of their formation. A study of rock magnetism.• Distribution of Earthquakes – Earthquakes often

occur along faults. Faults are breaks in a rock mass where plate movement has occurred , associated with plate boundaries.

Plate Boundaries

Is a fracture separating one plate from another , the region where plates meet. All major interactions among the individual plates occur along their boundaries. There are three distinct types of boundaries based on the type of movement.. These are:

•Convergent Boundary•Divergent Boundary•Transform Boundary

Convergent Boundary

• Occurs when two plates move toward each other. Convergent boundaries are also subduction zone.

I. Oceanic-continental convergence- forms trenches, destructive earthquakes, uplift of mountain ranges, and building of volcanic arc.

II. Oceanic-oceanic convergence – forms trenches(Marianas Trench) and volcanic arc

III.Continental-continental convergence – forms mountain ranges like Himalayan trench.

Divergent Boundary

• Occurs when two plates move away. Most divergent boundaries occur along the crest of oceanic ridges. Divergent can also occur within a continent forming a rift.

Transform Fault Boundary

• Occurs when plates slide horizontally past one another. Most within the ocean basin , but there are a few that can be found in continental plates.

Ocean Basin:Evolution and Characteristics

• Continental Shelf – is a relatively shallow gently sloping part of the continental crust that borders the continent. (average depth of 130 m with a maximum width about 1300 km

• Continental slope – after continental shelf, leads to deep water. (presence of submarine canyons)

• Continental break – boundary between continental shelf and slope • Continental rise – more gradual incline , links the deep ocean

basin floor to the continental slope

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