Plate Tectonics Overview

Plate Tectonics

• Plate tectonics is the theory that Earth's surface is made up of large, rigid plates that move slowly over time.
• Alfred Wegener developed the continental drift hypothesis, proposing that continents were once joined together in a supercontinent called Pangaea and have since drifted apart.
• Evidence for continental drift includes the fit of continents, similar rock types and mountain ranges on different continents, and the distribution of fossil organisms.
• Seafloor spreading is a process where new oceanic crust is formed at mid-ocean ridges and moves away from the ridges, creating the mechanism for continental drift.
• Mid-ocean ridges are underwater mountain ranges where magma rises to the surface, creating new oceanic crust.
• Magnetometers tracked normal and reversed magnetic polarity found in the seafloor basalt, creating a pattern of alternating stripes on either side of mid-ocean ridges.
• The seafloor spreading hypothesis suggests that the oldest seafloor is coldest and lies deepest in the ocean because it is less buoyant than the hot, newly formed crust.
• The Earth's tectonic plates are made of the lithosphere.
• Plate boundaries are where plates meet and most geologic activity happens (earthquakes, volcanoes, and mountain building).
• Divergent plate boundaries occur where plates move apart, creating new crust.
• Convergent plate boundaries occur where plates collide, leading to subduction zones (where one plate dives beneath another) or mountain ranges.
• Transform plate boundaries occur when plates slide past each other.
• Intraplate boundaries are geologic activity not occurring at plate boundaries, such as hotspots.
• Hotspots are areas in the Earth's mantle where magma rises to the surface, creating volcanoes.
• The oldest oceanic crust is near the edges of continents or deep sea trenches, far less than 180 million years old.
• Plate movement and geological activity are driven by mantle convection.

Continental Drift

• Alfred Wegener proposed the continental drift hypothesis.
• Pangaea is the name of the supercontinent.
• Wegener suggested continents were once joined and have drifted apart.
• Identical rocks and fossils of similar species.

Seafloor Spreading Hypothesis

• Seafloor spreading is proposed to be the mechanism driving continental drift.
• New oceanic crust forms at mid-ocean ridges.
• As new crust forms, it is pushed away from the ridge.
• Oceanic crust is eventually destroyed in deep-sea trenches.
• Evidence of seafloor spreading comes from the pattern of magnetic stripes on the seafloor.
• The mechanism for this is mantle convection currents.

Theory of Plate Tectonics

• Theory of plate tectonics combines continental drift and seafloor spreading.
• Convection currents in the Earth's mantle drive the movement of tectonic plates.
• The movement of plates is measured in centimeters per year.

Plate Boundaries

• Plate boundaries are where plates meet and most geologic activity happens.
• Divergent boundaries: plates move apart.
• Convergent boundaries: plates collide.
• Transform boundaries: plates slide past each other.
• Intraplate boundaries: geologic activity not at a plate boundary.