Chapter 2 - Plate Tectonics

Continents were formerly in different positions on the Earth and have shifted to their present locations over time. **Wegener believed that the continents had once been part of a supercontinent that he named Pangaea.

Fossils of identical organisms were found on several continents, indicating the continents were once joined. **When organisms are separated into two smaller groups by a physical barrier (i.e., the Atlantic Ocean), then they follow different evolutionary paths. The organisms Wegener cited were identical on both continents.

The climate would have been a warm, humid climate near the Equator. **Swamp vegetation needs a lot of moisture and plenty of warm temperatures to thrive. The conditions would have been (and are today) present near the Equator.

The continental shelves are the edges of the continent. The shoreline simply shows where the sea level is in relation to topography. **Depending on sea level, continental shelves are often underwater and below the erosive effects of wave action.

Striations **Striations are scratches or grooves created by glacial debris as it moves. The striations found on the southern continents all radiate from the center of the south part of Pangaea.

Alfred Wegener

Pangaea

The rigid lithosphere plates overlay the hotter and weaker asthenosphere.

Divergent boundaries **Divergent boundaries form because of tensional stress.

Transform faults **Transform faults account for the lateral offset that is seen along divergent boundaries.

Normal Faults, long-wide vallies, and lava flows **All of these physical features are evidence that the landscape is experiencing tensional stress from rifting.

Convection is a cycle of moving material formed by the rise of less-dense material and the sinking of denser material.

Forces at subduction zones account for most tectonic plate motion.

In general, warmer materials are less dense, and colder materials are more dense.

Diffuse

A plume is a zone of up-welling material originating at the outer core-mantle boundary.

Hot spots provide evidence of plumes.

Plates move apart at divergent plate boundaries.

Plates move toward one another at convergent plate boundaries.

Plates slide past one another at transform plate boundaries.

In general,earthquakes and volcanic eruptions occur at both divergent plate boundaries AND convergent plate boundaries.

New oceanic lithosphere form at divergent plate boundaries.

In general, volcanoes form on the overriding plate, away form the convergent boundary.

Volcanoes are not found at transform boundaries because they do not cause changes to the pressure, temperature, or composition of the mantle.

an ocean

The Cascade Mountain Range

Convergent boundary **Convergent boundaries form as a result of compressional forces pushing two plates together.

The oceanic lithosphere would subduct because it has a higher density at 3.0 grams per cubic centimeter. **Oceanic lithosphere, which is basaltic, has a higher density than continental lithosphere.

Partial melting occurs due to subduction. **Partial melting is triggered by water loss in the plate. This melts mantle material, which rises to the surface.

Plate B will subduct because it is older—and therefore colder—and more dense. **The older an oceanic plate is, the colder it is, making it more dense.

Continental plates are too buoyant to subduct. **Continental lithosphere has a density of 2.7 grams per cubic centimeter, making it unlikely to subduct.

The San Andreas Fault

Since oceanic plates are denser than the continental plates, they will subduct to great depths when they converge with continental plates. When two oceanic plates converge, the denser of the two will subduct; whereas when two continental plates converge, neither will subduct. Therefore, the resulting geologic feature depends on the types of plates that converge.

Once the volcanoes become completely dormant, they can be called mountains. Similarly, when the volcanic activity in the volcanic arc islands stops, they would appear and have the characteristics of islands.

Core **The field is generated in the outer part of the core, which is believed to be a liquid iron alloy.

700,000 years ago

Iron particles in magnetically susceptible minerals within basaltic lava align with the magnetic field of the Earth while the lava is still liquid and then freeze in position when the lava solidifies. **Particles of iron will align while the material is still molten, but are locked into place once the lava hardens.

Small marine fossils in deep-sea sediments give the age of the portions of the seafloor **Fossils are very useful in determining geologic ages.

divergent

Earth's magnetic field switches polarity at irregular time intervals.

every 100,000 to several million years

A compass points toward the North Pole during normal polarity and to the South Pole during reverse polarity.

A record of Earth's magnetic field is recorded in oceanic rocks, which show a clear pattern of changes in Earth's magnetic polarity.

on both sides of the mid-ocean ridge