PLATE TECTONICS Alfred Wegner hypothesized that the continents

PLATE TECTONICS

• Alfred Wegner hypothesized that the continents were once joined into a single continent called Pangaea (meaning “all land”). He further suggested that this supercontinent later broke into smaller pieces and drifted to their present positions.

He referred to his hypothesis as “continental drift”. To support his hypotheses, Wegner and others collected evidence from around the world. Some of the evidence to support continental drift include:

• The coastlines of some of present day continents seem to fit together like a jigsaw puzzle

• Identical fossil organisms are found on continents separated by thousands of miles of ocean

• Rock types and mountain ranges match up when the continent puzzle is put together • Glacial deposits are found in areas which are now located in the tropics

• The one thing that Wegner could not do was explain a mechanism that could move continents!

• Scientists now have seismic data to support Wegner’s hypotheses. Today’s theory is referred to as plate tectonics.

• Plate tectonics describes plate motion and the effects of the motion. Plate tectonics says that Earth’s outer shell is made of about 20 plates. The plates are rigid and in constant motion relative to one another.

• The largest plates consist of two different kinds of crust; oceanic and continental. Many of the smaller plates contain only oceanic crust.

• Continental crust is thicker and less dense than oceanic crust. Continental crust is composed mostly of the igneous rock granite, while ocean crust is mostly the igneous rock basalt.

The plates sit on the hot asthenosphere. While the rock in the asthenosphere is solid, it is less rigid than the plates, and therefore moves. The movement within the asthenosphere is caused by heat from the core of the earth.

The heat rises slowly through the earth creating convection cells within the asthenopsphere. Scientists do not fully understand the forces that move the plates.

• The most accepted theory is the idea that the material carried by convection cells in the asthenosphere drag the plates along.

• The interaction between plates results in different effects on the earth’s surface. One of the effects of plate motion is the Ring of Fire. The Ring of Fire is an area of volcanoes that form a ring around the Pacific Plate.

• Along these areas, plates are colliding and ocean crust is subducted, or pulled into the mantle, and melted. This process destroys plate material.

• Along the Mid. Atlantic ridge, molten material wells up between plates that are moving apart. This magma cools and hardens to form new crust.

• By destroying and creating plate material, plate tectonics constantly recycles Earth’s materials.

• Each plate moves as a distinct unit. The borders between plates are called plate boundaries.

Divergent - plates move away from one another • New crust material is created • Shallow earthquakes along the plate boundary • Ocean/ocean crust

Landforms • Creates Mid-ocean Ridge • Rift Valleys Places • Mid-Atlantic Ridge, Atlantic Ocean

Convergent– plates move toward one another Ocean / Continental

• Ocean crust is pulled beneath the continental crust into the mantle and melts • Old crust is destroyed • Tight lines of volcanoes form on the continental crust • Deep Earthquakes on one side of the plate boundary and shallow on the

Landforms: • Deep Ocean Trenches • High Volcanic Mountains Places • Cascade Mountain Range, Oregon and Washington • Mt. Saint Helens

• One plate descends beneath the other (subduction) • Tight lines of Volcanoes on the ocean floor • Earthquakes Ocean / Ocean Crust

Landforms • Island Volcanic Arcs • Ocean Trench Places • Mariana Trench (Pacific Ocean) • Japan

Continental / Continental Crust • Neither plate is subducted • Plates crash into one another • No volcanic activity

Landforms • Mountain Ranges Places • Himalaya Mountains, India

Transform—plates grind past one another • Shallow Earthquakes • Some volcanoes, but spread out Mostly Ocean Crust Some Continental

Landforms • Faults (breaks in rock) Places • San Andreas Fault, California