Plate Tectonics - Theory

Plate tectonics

Plate tectonics is a theory of geology developed to explain the phenomenon of continental drift and is currently the theory accepted by the vast majority of scientists working in this area.

In the theory of plate tectonics the outermost part of the Earth's interior is made up of two layers: the outer lithosphere and the inner asthenosphere.

The lithosphere essentially "floats" on the asthenosphere and is broken-up into ten major plates: African, Antarctic, Australian, Eurasian, North American, South American, Pacific, Cocos, Nazca, and the Indian plates..
 
Theory of Plate Tectonics

In the early 1960’s, the theory of plate tectonics rocked the geologic world. Before the concept of plate tectonics had been introduced, geologists puzzled over earthquakes, mountain ranges such as the Andes Mountains, volcanoes, deep sea trenches, and fault lines such as San Andreas Fault, tossing about a number of incomplete ideas as to how such rock formations occurred in nature. Alfred Wegener, expanding on the earlier theory of continental drift, presented the theory of plate tectonics as an all-encompassing explanation for earthquakes, volcanoes, mountain ranges, and most other geologic rock formations.

Plate Boundaries

The place where the two plates meet is called a plate boundary. Boundaries have different names depending on how the two plates are moving in relationship to each other

• Divergent Boundary
• Convergent Boundary
• Transform Boundary

Divergent Boundary
A divergent boundary is a form of plate tectonics where plates separate and move in opposite directions, forming a rift or massive crack in the surface. This rift allows for magma to well up from the asthenosphere, fill the crevice, solidify, and become new lithosphere. Earthquakes and volcanoes are present at the divergent boundary as plates move apart. The further the plates “drift,” the more magma solidifies as lithosphere in the valley formed.

The Mid-Atlantic Ridge is a famous example of sea-floor spreading where new sea floor is created as plates move apart at a divergent boundary. Earth is constantly recycling old rock matter and creating new rock surface. The divergent boundary is the form of plate tectonics responsible for all new rock lithosphere on the face of the planet.


Convergent Boundary
If plates are moving apart anywhere on the globe, it would stand to reason that elsewhere they are being pushed together. At a convergent boundary, two plates collide and massive earthquakes will ensue as one plate is forced beneath the other—this process of plate tectonics is called subduction. Subduction is responsible for most of Earth’s mountain ranges, volcanoes, earthquakes, and deep sea trenches.

In the case of the Andes Mountains, the oceanic Nazca Plate plunged below the South American Continental Plate creating a deep sea trench on the oceanic side and the crumpled, upturned Andes Mountains on the continental side. Matter from the oceanic plate was scrapped away by the upward movement of the mountain range, accounting for the discovery of deep-sea fossils on the highest peaks of the Andes Mountains.

When the lithosphere moves down into the asthenosphere at a convergent boundary, it gets melted and recycled back into the mantle. The pools of magma that form while the lithosphere melts into the mantle may rise upward, exploding to the surface as volcanoes. The convergent boundary of the Andes Mountains in South America is home to some of the deadliest volcanoes in the world.

Transform Boundary
 A transform boundary is a form of plate tectonics where two plates slide past one another in opposite directions, creating a transform fault. The San Andreas Fault of California is one such example of a transform boundary. The rocks facing each other on either side of the San Andreas Fault are of differing age and composition.

The lithosphere plates at a transform boundary do not smoothly slide alongside one another; snags and folds create friction that accounts for some of the world’s deadliest earthquakes—The San Andreas Fault was responsible for the earthquakes that tore down San Francisco in 1906.