4.5 Supercontinents

Early Continents

• The first crust consisted of basaltic rock, similar to the current oceanic crust.
• Partial melting of the lower basaltic crust occurred over 4 billion years ago.
• This melting formed silica-rich crust, which became the felsic continents.

Craton

• Cratons are ancient cores of continents where the earliest felsic continental crust is found.
• Rapid plate motions caused cratons to experience frequent continental collisions.
• Cratons contain felsic igneous rocks, remnants of the first continents.
• Cratonic rocks hold rounded sedimentary grains, indicating erosion from earlier rocks and the presence of rivers or seas.
• Greenstone, a metamorphosed volcanic rock, is found in cratons.
• Greenstones in ancient rocks indicate the presence of subduction zones.

Shield

• Shields are areas where the craton is exposed at the surface.
• Cratons from the Precambrian period are called Precambrian shields, many around 570 million years old.
• Cratons, in most places, are covered by younger rocks, which together create what is called a platform.
• Erosion sometimes exposes the Precambrian craton.

Early Convection

• Earth's interior was warmer during the Pre-Archean and Archean periods.
• Mantle convection and plate tectonic processes were more vigorous, leading to more common subduction zones and smaller crustal plates.
• Earth has been cooling since its molten state.
• About half of Earth's initial internal heat remains and is the source for the core and mantle's heat today.
• Zircon crystals reveal the presence of water on early Earth in the ancient past.

Supercontinent Cycle

• Wegener's supercontinent Pangaea's existence is widely accepted.
• Continental movements explain observed geological activity.
• There were supercontinents before Pangaea, like Rodinia, which existed 750 million to 1.1 billion years ago, and Columbia, which existed 1.5 to 1.8 billion years ago.
• If continental drift continues, a new supercontinent may form on the opposite side of the planet in roughly 200 million years.
• The supercontinent cycle involves continents colliding on opposite sides of the planet roughly every 500 million years.
• Supercontinent formation accounts for many long-gone and current geological features.
• The Appalachian Mountains formed from the collision of eastern North America at a convergent plate boundary approximately 200 million years ago when they were potentially as tall as the Himalayas.
• Since about 250 million years ago, Pangaea broke up because of plate boundaries, and the continents are still moving apart.
• The Pacific Ocean is shrinking, and the Atlantic Ocean is growing.