EARTH…What lies beneath

WHAT LIES BENEATH…

CRUST: Top layer of Earth’s internal structure that has two parts…

1.Basalt-rich oceanic crust

2.Granite-rich continental crust (much thicker than oceanic)

• Relatively cold in temperature• Rocky and brittle so it can

fracture during earthquakes

WHAT LIES BENEATH…

MANTLE: Most of Earth’s mass comes from the mantle

•Comprised of iron, magnesium, aluminum, and silicon-oxygen compounds

• Upper 1/3 is known as the ASTHENOSPHERE• Asthenosphere is more plastic

in nature than rest of the mantle

WHAT LIES BENEATH…

CORE: Center of Earth that has two parts…

1. Inner Core2. Outer Core

-Comprised of mostly iron

Inner Core – Under EXTREMELY high pressure so it remains solid

Outer Core – Temperature is so hot that it remains molten

What is Plate Tectonics?

If you look at a map of the world, you may notice that some of the continents could fit together like pieces of a puzzle.

Plate Tectonic Theory

•Scientific theory which describes the large scale motions of Earth’s lithosphere

•Arose out of two separate observations:

•Continental Drift

•Seafloor Spreading

Seafloor Spreading

Continental Drift

Plate Tectonics

The Continental Drift Hypothesis

Proposed by Alfred Wegener in 1915.

Supercontinent Pangaea started to break up about 200 million years ago.

Continents "drifted" to their present positions.

Continents "plowed" through the ocean crust.

Continental Drift: Evidence

Geographic fit of South America and Africa

Fossils match across oceans

Rock types and structures match across oceans

Ancient glacial features

Tight fit ofthe

continents, especially

usingcontinental

shelves.

Continental

Drift:Evidence

Continental Drift: Evidence

Fossil critters and plants

Continental Drift:

Evidence

Correlation of

mountains with nearly

identical rocks and structures

Continental

Drift:Evidence

Glacial features

of the same age

restore to atight polar

distribution.

Seafloor SpreadingU.S. Navy mapped seafloor with echo sounding (sonar) to find and hide submarines. Generalized maps showed:

oceanic ridges—submerged mountain ranges

fracture zones—cracks perpendicular to ridges

trenches—narrow, deep gashes

abyssal plains—vast flat areas

seamounts—drowned undersea islands

How magnetic reversals form at a spreading center

Bands of seismicity—chiefly at trenches and oceanic ridges

Plate Tectonics•The Earth’s crust is divided into 12

major plates which are moved in various directions.

•This plate motion causes them to collide, pull apart, or scrape against each other.

•Each type of interaction causes a characteristic set of Earth structures or “tectonic” features.

•The word, tectonic, refers to the deformation of the crust as a consequence of plate interaction.

Tectonic Plates on Modern Earth

What are tectonic plates made of?

•Plates are made of rigid lithosphere.

The lithosphere is made up of

the crust and the upper part of the

mantle.

What lies beneath the tectonic plates?

•Below the lithosphere (which makes up the tectonic plates) is the asthenosphere.

Plate Movement•“Plates” of lithosphere are moved

around by the underlying hot mantle convection cells

What happens at tectonic plate boundaries?

The Theory of Plate Tectonics

Earth’s outer shell is broken into thin, curved plates that move laterally atop the asthenosphere

Most earthquakes and volcanic eruptions happen at plate boundaries.

Three types of relative motions between plates:

“group authorship” in 1965-1970

divergent convergent transform

•Divergent

•Convergent

•Transform

Three types of plate boundary

•Spreading ridges

•As plates move apart new material is erupted to fill the gap

Divergent Boundaries

Divergent boundaries: Chiefly at oceanic ridges

(aka spreading centers)

Age of Oceanic Crust

Courtesy of www.ngdc.noaa.gov

Divergent boundaries

also can rip apart (“rift”)

continents

How rifting of acontinent could

lead to formation of

oceanic lithosphere.

e.g., Red Sea

e.g., Atlantic Ocean

e.g., East Africa Rift

• Iceland has a divergent plate boundary running through its middle

Iceland: An example of continental rifting

Presumably,Pangea was ripped apart

by such continental

rifting & drifting.

•There are three styles of convergent plate boundaries

•Continent-continent collision

•Continent-oceanic crust collision

•Ocean-ocean collision

Convergent Boundaries

•Forms mountains, e.g. European Alps, Himalayas

Continent-Continent Collision

Himalayas

•Called SUBDUCTION

Continent-Oceanic Crust Collision

• Oceanic lithosphere subducts underneath the continental lithosphere

• Oceanic lithosphere heats and dehydrates as it subsides

• The melt rises forming volcanism

• E.g. The Andes

Subduction

• When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone.

• The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench.

• The worlds deepest parts of the ocean are found along trenches.

• E.g. The Mariana Trench is 11 km deep!

Ocean-Ocean Plate Collision

•Where plates slide past each other

Transform Boundaries

Above: View of the San Andreas transform fault

Most transform boundariesare in the oceans.

Some, like the one in California, cut continents.

The PAC-NA plate boundary is MUCH more complex than this diagram

shows.