Tectonic Plates Flashcards

Tectonic Plates Overview

• Tectonic plates are rigid blocks of the Earth's lithosphere, akin to a cracked eggshell, comprised of the crust and upper mantle.
• Earth's lithosphere consists of approximately seven to eight major plates and many minor ones.

Theory of Plate Tectonics

• Plate tectonics theory explains that the outer shell of Earth is divided into plates that glide over the mantle, which is the layer above the core.
• The lithosphere is the strong outer layer, contrasting with the more fluid mantle underneath.

Number of Plates

• There are currently 12 large semirigid tectonic plates with various shapes and sizes, separated by boundaries.
• The seven major tectonic plates account for 94% of Earth's surface area.

Movement of Tectonic Plates

• Plates move relative to each other at three types of boundaries:
  • Convergent boundaries: Plates collide.
  • Divergent boundaries: Plates separate.
  • Transform boundaries: Plates slide past one another.

Composition of Tectonic Plates

• Tectonic plates are approximately 100 km (62 miles) thick, primarily consisting of two types of crust:
  • Oceanic crust (sima): Dense, made mainly of basalt and gabbro.
  • Continental crust (sial): Less dense, mainly composed of granitic rocks.

Earth's Crust

• The outermost rock layer of Earth, known as the crust, is characterized by low-density, easily melted rocks.
• The crust extends 50 km (30 miles) beneath continents and only 5-10 km (3-6 miles) beneath ocean floors.

Mantle and Lithosphere

• The mantle is located beneath the crust, composed of denser rocks and exhibiting viscous fluid behavior under stress.
• Together, the uppermost mantle and crust form the lithosphere, which acts as a single rigid layer.

Examples of Tectonic Plates

• Oceanic Plate: The Pacific Plate, which extends from the East Pacific Rise to the deep-sea trenches around the Pacific basin.
• Continental Plate: The North American Plate, which includes North America and parts of the oceanic crust.

Asthenosphere

• The asthenosphere lies beneath the lithosphere and is believed to be hotter and more fluid, extending from about 100 km (60 miles) to 700 km (450 miles) deep.

Rift Valleys

• A rift valley is an elongated trough formed by the subsidence of the Earth's crust between normal faults.
• Rift valleys occur in divergence zones, where two tectonic plates are moving apart and can be found on continents and ocean floors.