Continental Drift and Plate Tectonics: Earth’s Shifting Surface
Item 4: Earth’s Dynamic Crust
Introduction
The movement of the Earth’s crust (lithosphere) is continuous.
The Theory of Continental Drift
In 1912, Alfred Wegener, a German meteorologist, gathered a large amount of geological, paleontological, and climatic data. These data indicated that the continents had moved through time. Wegener showed that all the continents were once united into a single supercontinent called Pangaea. Pangaea later fragmented into several continents that drifted to their present positions.
The hypothesis of continental drift explains:
- The fit of coastlines of different continents.
- The continuity of long mountain ranges, today fragmented.
- Identical fossil fauna and flora on distant continents.
- Glacial rocks in tropical areas.
However, there was a problem: it did not properly account for the movement of continents over the oceanic crust.
The Theory of Ocean Floor Spreading
This hypothesis suggested that the continents and oceanic crust segments moved together in blocks as a unit and that some kind of thermal convection cells operating inside the Earth were the mechanism responsible for the movement of these blocks.
The Theory of Plate Tectonics
This theory holds that the lithosphere is divided into fragments, the lithospheric plates, that move over the asthenosphere, which consists of plastic materials. Under that scenario, the plates are moved from the ridges to the deep-sea trenches. Also, new crust is formed along the ridges and is destroyed along ocean trenches. Mountain ranges are formed adjacent to said plate boundaries.
Plate Boundaries
The lithosphere is divided into seven large plates, all of which move more or less independently on the asthenosphere.
- At divergent plate boundaries, plates move apart when magma rises to the surface from the asthenosphere. The limit of divergent plate boundaries is marked by the Mid-Ocean Ridge in the oceanic crust.
- Plates move against each other and collide at convergent plate boundaries. As the oceanic plate sinks into the Earth, it gradually warms until partially melted, generating magma. This magma may erupt on the surface of the Earth, forming a volcanic mountain belt, as in the Andes.
- Transform faults are places where plates slide against each other, as in the San Andreas Fault in California, where frequent earthquakes originate as a result of the relative displacement of both plates.
The theory of plate tectonics has produced significant and powerful effects in all fields of geology because it provides the basis for many seemingly unrelated phenomena.
Basic Processes
- Continental fragmentation: It seems that any large continent fragments in a short period of time, while the mantle prevents it from radiating toward the exterior.
- Ocean floor spreading: This is given as a result of accretion at divergent plate boundaries, the oceanic ridges.
- Subduction at various stages: Subduction occurs at convergent plate boundaries and involves an understanding that a mixed plate ruptures at its weakest area: the continent/ocean boundary, which leads to the formation of two new plates, one of which will be sinking progressively.
- Obduction or continental collision: A collision is reached as a result of prolonged subduction of oceanic lithosphere, which washes away and changes the meaning of the expansion of one of the plates.