Earth’s Crust, Plate Tectonics, and Pangaea
The top layer of the Earth’s surface is called the crust (it lies on top of the plates). Oceanic crust (the thin crust under the oceans) is thinner and denser than continental crust. Crust is constantly being created and destroyed; oceanic crust is more active than continental crust.
Type of Crust | Average Thickness | Average Age | Major Component |
Continental Crust | 20-80 kilometers | 3 billion years | Granite |
Oceanic Crust | 10 kilometers | Generally 70 to 100 million years old | Basalt |
Divergent Plate Movement: Seafloor Spreading Seafloor spreading is the movement of two oceanic plates away from each other (at a divergent plate boundary), which results in the formation of new oceanic crust (from magma that comes from within the Earth’s mantle) along a mid-ocean ridge. Where the oceanic plates are moving away from each other is called a zone of divergence. Ocean floor spreading was first suggested by Harry Hess and Robert Dietz in the 1960’s.
Convergent Plate Movement: When two plates collide (at a convergent plate boundary), some crust is destroyed in the impact and the plates become smaller. The results differ, depending upon what types of plates are involved.
Oceanic Plate and Continental Plate– When a thin, dense oceanic plate collides with a relatively light, thick continental plate, the oceanic plate is forced under the continental plate; this phenomenon is called subduction.
Two Oceanic Plates– When two oceanic plates collide, one may be pushed under the other and magma from the mantle rises, forming volcanoes in the vicinity.
Two Continental Plates– When two continental plates collide, mountain ranges are created as the colliding crust is compressed and pushed upwards.
Lateral Slipping Plate Movement: When two plates move sideways against each other (at a transform plate boundary), there is a tremendous amount of friction which makes the movement jerky. The plates slip, then stick as the friction and pressure build up to incredible levels. When the pressure is released suddenly, and the plates suddenly jerk apart, this is an earthquake.
ALFRED WEGENER AND PANGAEA
In 1915, the German geologist and meteorologist Alfred Wegener (1880-1930) first proposed the theory of continental drift, which states that parts of the Earth’s crust slowly drift atop a liquid core. The fossil record supports and gives credence to the theories of continental drift and plate tectonics.
Wegener hypothesized that there was an original, gigantic supercontinent 200 million years ago, which he named Pangaea, meaning “All-earth”. Pangaea was a supercontinent consisting of all of Earth’s land masses. It existed from the Permian through Jurassic periods. It began breaking up during the Jurassic period, forming continents Gondwanaland and Laurasia, separated by the Tethys Sea.
Pangaea started to break up into two smaller supercontinents, called Laurasia and Gondwanaland, during the Jurassic period. By the end of the Cretaceous period, the continents were separating into land masses that look like our modern-day continents.
Wegener published this theory in his 1915 book, On the Origin of Continents and Oceans. In it he also proposed the existence of the supercontinent Pangaea, and named it (Pangaea means “all the land” in Greek).