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Crustal Plate Basics
The theory of plate tectonics began to take shape in the early 1960s. According to this theory, scientists recognized that the Earth's surface, or outer crust, was fractured into approximately 14 major plates and a handful of minor ones. These plates represent large, solid slabs of rock. There are two primary types of plates. Oceanic plates comprise the seafloors, while continental plates represent landmasses that have risen above sea level.
Oceanic Plates
Oceanic plates have an average thickness of around 4.5 miles and an average density of 1.73 oz. per cubic inch. These plates are very young, with the oldest oceanic rock dating to around 180 million years ago. This is due to the fact that oceanic crust is constantly being produced at mid-ocean ridges. Oceanic plates are primarily composed of mafic rocks, including basalt and gabbro. These rocks are rich in magnesium and iron, raising the plates' density. The primary minerals found in oceanic crust are pyroxene and olivine.
Continental Plates
Continental plates have and average thickness of between six and 44 miles. They are also less dense than oceanic plates, with an average density of only 1.56 oz. per cubic inch. Continental crust is comprised of core rocks that are very old, dating as far back as 4 billion years ago during the Precambrian period. These core, or shield, rocks are comprised of granite, gneiss, schist and volcanic rocks. On top of the shield layer, continental crust has a platform layer of sedimentary rock. These rocks are as old as 600 million years. Together, the two continental rock layers are known as the craton. These felsic rocks are rich in potassium, sodium, aluminum and silica. The primary minerals found in continental crust are feldspar and quartz.
Crustal Plate Interaction
Crustal plates ride atop the Earth's fluid mantle, comprised of molten magma. Convective forces cause these plates to move and interact with each other. Originally, all the Earth's continental crust was combined, forming the single supercontinent of Pangaea. Approximately 200 to 225 million years ago, the convective forces of the mantle caused Pangaea to break apart, forming today's collection of separate continents. As these continental and oceanic plates continue to move, they interact in three primary ways. When plates collide, they form convergent plate boundaries. Oceanic plates spread apart at divergent zones, where new oceanic crust is formed. Finally, when plates slide past one another, they form transform boundaries.