Wegener's Continental Drift Evidence and Plate Tectonics

Continental Drift Theory

Wegener proposed that all land masses were once united into a single, large continent called Pangaea. The present continents are the result of Pangaea's division and the subsequent displacement of its fragments.

Wegener's Arguments

1) Geographic Arguments

The starting point was the shape of the continents, which permitted them to fit together like pieces of a puzzle. Wegener argued that processes such as coastal erosion and continuous sea-level changes impeded a perfect fit.

2) Paleontological Arguments

The distribution of many fossils, such as Mesosaurus or Lystrosaurus, and certain plants, presented a problem. Wegener noted that if evolutionary ideas were correct, the simultaneous presence of the same species in widely separated areas could not be explained unless the landmasses were once united.

3) Geological Arguments

He analyzed certain mountain ranges and other geological formations found on both sides of the Atlantic Ocean.

4) Paleoclimatic Arguments

Wegener noted the existence of glacial deposits in places now located far from polar regions. In their current positions, these areas would not have experienced the same climate in the past.

Cause of Displacement

Wegener considered the displacement of continents undeniable. However, he had many doubts about the forces causing these movements. He suggested two types of forces:

• Polar Flight: Due to the Earth's rotation, causing continents to shift toward the Equator.
• Tidal Braking: Caused by the attraction of the Sun and Moon, responsible for displacing the Earth's crust westward.

Current Assessment: From Continental Drift to Plate Tectonics

Wegener's theory had flaws:

• The causes suggested for continental movements were incorrect.
• Oceanic floors are not fixed beneath moving continents; sometimes the seabed moves along with the continents, and sometimes it moves independently.

Wegener knew very little about the Earth's interior and almost nothing about the deep ocean.

Plate Tectonics Theory

According to this theory, the lithosphere is divided into fragments, or plates, which move due to thermal agitation within the Earth. These movements originate volcanism, seismicity, mountain ranges, and changes in the distribution of lands and seas.

The Seabed

Oceanic Ridges

The Atlantic Ocean is traversed north to south by seamounts that rise 2–3 km above the surrounding plains. Iceland is an example where the ridge emerges. This feature is divided into the Mid-Atlantic Ridge, the Indian Ocean Ridge, and the Pacific Ridge. The dorsal (ridge) has a central groove, limited on both sides by normal faults, which are called Rift Zones. Periodically, the dorsal is interrupted by transverse fractures, called Transform Faults.

Distribution and Shortage of Sediment

Since materials are continuously deposited in the oceans, it was expected that sediments, along with their fossils, would be found there. Therefore, the absence of sediment on the ridges and their relative scarcity elsewhere on the ocean floor was a surprising finding. Sediments are not distributed uniformly; their thickness increases as one moves away from the ridge.