Density: 6.6B

Variables involved  Density (g/mL), Mass (grams), and Volume (mL or cm3)

D = M / V

Plate Tectonic Theory: 8.9A

Alfred Wegener (early 1900s): Proposed the idea that the continents were once joined together that he called Pangaea.

  • Wegener called his theory Continental Drift.
  • Evidence: Fit together like a puzzle (South America and Africa), similar plant and animal fossil record, and similar rock layers.
  • People did not support his theory because he could not explain how the continents moved.

After seafloor spreading was discovered by Harry Hess a few decades later, convection currents within the mantle explained how the continents moved, and Wegener’s theory was brought back to life. They changed the name to the Theory of Plate Tectonics.

Earth’s Structure 6.10A

Lithosphere: Earth’s crust

Asthenosphere: upper most part of mantle that the crust floats on.

Convection 6.9A

Convection in the Mantle

  1. The core heats the magma in the mantle.
  2. The high heat makes the magma less dense causing it to rise.
  3. When the magma reaches the lithosphere, it is forced to travel horizontally causing friction underneath the crust. The friction created forces the plates to move together, apart, or slide past each other.
  4. While the magma is further away from the core, its temperature decreases which cause the density to get higher.
  5. The more dense magma will sink back down towards the core and the cycle will repeat.

Plate Boundaries: 8.9B

Types of Plates

  • Oceanic Plates: relatively thin, but VERY dense.
  • Continental Plates: relatively thick, but LESS dense.

Reminder: Less dense materials tend to rise, and more dense materials tend to sink.

Plate Boundaries and Crustal Features

Convergent Boundaries: When plate collide. Earthquakes can result.

Oceanic/Oceanic Convergent Boundaries

  • Oceanic/Oceanic Boundaries: the more dense plate will sink beneath the other in a process called subduction. Tsunamis may result.
  • Crustal features that form as a result are deep ocean trenches, underwater volcanoes, volcanic island arches.

Oceanic/Continental Convergent Boundaries

  • Oceanic/Continental Boundaries: the oceanic plate will subduct under and the continental plate will push up in a process called uplift. Tsunamis may result.
  • Crustal features: mountain ranges and volcanoes on the continental plate, and deep ocean trenches.

Continental/Continental Convergent Boundaries

  • Continental/Continental Boundaries: the less dense continental plates will be forced upward in a process called uplift.
  • Crustal features: mountains. NO VOCANIC ACTIVITY WILL OCCUR.

Divergent Boundaries: When plates separate. Earthquakes can result.

Oceanic/Oceanic Divergent Boundaries

  • Oceanic/Oceanic Boundaries: as plates move apart, magma rises to fill in the gap. The process is called seafloor spreading.
  • Crustal Features: As magma cools, it forms underwater mountain ranges (Mid-Ocean Ridges). Volcanic activity will continue to occur at the boundary.

Continental/Continental Divergent Boundaries

  • Continental/Continental Boundaries: as stress on plate created from the convection currents in the mantle occurs, plate can crack in two places causing the middle section to sink (subsidence).
  • Crustal Features: rift valleys

Transform Boundaries: When two plates slide past each other. Earthquakes are common.

Transform Boundaries

  • Crustal Features: faults are formed (large cracks)
  • NO VOLCANIC ACTIVITY WILL OCCUR.