Earth’s Internal Properties

Continental Drift & Plate Tectonics

Science 2201

Chapter 17

October 17, 1989

May 18, 1980

Earthquakes and volcanoes: evidence that our planet is not at rest.

• global distribution of earthquakes and volcanic centers…above crustal plate boundaries

• The surface of the earth is constantly changing and no feature on earth is permanent.

Why is the Earth so restless?

• “Convection Conveyer-belt”

• The heat that drives the motion of the mantle comes from two sources:

---Radioactive decay of materials in the mantle

---Heat left over from the formation of the earth

James Hutton 1726-1797

• "catastrophism," earthly changes were sudden & caused by a series of catastrophes

• This belief prevailed until the 1700s

• "uniformitarianism • 1785 James Hutton• The present is the key

to the past

Uniformitarian Principle

• the geologic forces and processes -- gradual as well as catastrophic -- acting on the Earth today are the same as those that have acted in the geologic past.

Alfred Wegener 1880-1930

Theory of Continental Drift

• (Wegener, 1915)

• continents plowed through crust of ocean basins, which would explain why the outlines of many coastlines(like South America and Africa) look like they fit together like a puzzle

Continental Drift: a theory ahead of its time

• impossible for a large mass of solid rock to plow through the ocean floor without breaking up

• mid 20th Century, new evidence from ocean floor exploration rekindled interest in Wegener's theory, ultimately leading to theory of plate tectonics.

Plate Tectonics: A modern version of

Wegener’s old idea • the outer rigid lithosphere consists of about

20 rigid segments called plates

• Most plates contain both continental and oceanic crust—a major departure from the continental drift theory

• The word tectonics comes from the Greek root "to build."

Folding and Faulting

• Compressive forces from moving plates can cause rock to bend & wrinkle into wavelike folds

• The crack or break in the rock is called a fault and the movement of the rock is “faulting”

Earthquakes

• Great forces of stress occurs as plates of rock pushed in opposite directions

• sudden movement as the blocks slide and then come to rest in a new position that eases the pressure

The entire San Andreas fault system is more than 800 miles long and extends to depths of at least 10miles within the Earth. In detail, the fault is a complex zone of crushed and broken rock from a few hundred feet to a mile wide.

Seismograph

• Richter Scale based on the amount of seismographic movement

• An earthquake with a number of 2 is just strong enough to be felt, 5 can cause much damage, 7 or more is classified as a major earthquake

Plate Boundaries

• 3 Main Types–Divergent

–Convergent

–Transform

Divergent Plate Boundaries

• plates are being pulled apart

• sea floor spreading at Mid-Atlantic Ridge

• Eurasia and North America are separating at a rate of about 5 cm per year

• East African Rift Valley

Convergent Plate Boundaries

• two plates are coming together

• 3 associated surface feature– deep ocean trench and chain of volcanic

islands – folded mountains – ocean plate subduction beneath continent

Transform Plate Boundaries

• one plate scrapes past the other

• San Andreas fault in California

• No mountain building or volcanism is associated with transform boundaries.

Mountains

• great masses of rock pushed high into the air by forces inside the earth

• may be folded, tilted, shaped into domes, or built up from volcanic material

Forming Mountains

• Folded mountains are formed when layers of rock are pushed into a series of wavelike folds by tremendous sideward forces

• Appalachian Mountains & Rocky Mountains

Forming Mountains

• Fault-block mountains formed when folded layers of rock crack, producing a fault

• Sierra Nevadas

Forming Mountains

• Domed mountains formed when magma flows up between two layers of rock

• Black Hills of shown rising from the Great Plains of South Dakota

Forming Mountains

• Volcanic mountains formed by the build up of lava and other materials thrown up when a volcano erupts

• Hawaiian Islands

Volcanoes

• A mountain formed around a crack in the earth’s crust

• Cracks in crust allow magma to rise up

• Some erupt quietly, others violently

• Magma lava

• Obsidian

• Pumice

3 main types of volcanoes

• Shield volcanoes link –Kilauea in Hawaii

3 main types of volcanoes

• Cinder cone link—Paricutin in Mexico

3 main types of volcanoes

• Composite or Stratovolcano link

– Ranier in U.S.

Hot Springs

• Formed when underground water is heated by rock, and then flows to surface before cooling

Geysers

• A hot spring that throws its water into the air from time to time

• Almost all geysers are on 3 places: Yellowstone National Park, Iceland, and New Zealand

• How geysers work: – (understanding boiling)