Plate Tectonics

Mid-Ocean Ridges

Improved sea floor mapping led to the discovery of large undersea mountain belts, called mid-ocean ridges, that twist in between the continents like the seams on a basketball. Named for specific geographies

The USS Glomar Challenger For more than 15 years,

scientists aboard the Glomar Challenger traveled more than 575,000 km and collected rocks from 635 drill sites.

Robert Deitz analyzed the data from the mid 1950’s into the 1960’s, but did not know what to make of it.

Enter Admiral Harry Hammond Hess A professor of geology at

Princeton University.

Analyzed the rocks and data that Bob collected from his boat.

Hypothesized that the mid-ocean ridges were not static mountain belts!

A New Idea - Sea Floor Spreading Mid-Ocean Ridges are active

volcanoes.

Molten rock (magma) oozes up from the Earth's interior along the mid-oceanic ridges.

Younger (newer) rocks found at the ridge

Older rocks found where the ocean floor meets the continents.

This is the hypothesis of sea

floor spreading

Theory of Plate Tectonics After the hypothesis of seafloor spreading it was tested

and retested through data collection and experimentation. In 1964 the hypothesis of seafloor spreading gave way to the theory of plate tectonics.

The Theory of plate tectonics states that”

The lithosphere is broken into 7 major plates that are in motion and constantly changing size and shape as they “float” on top of the asthenosphere.

How do we know these plates exist?

Major interactions occur along the plate boundaries such as volcanoes and earthquakes.

By tracking volcanoes and earthquakes, scientists can get a good idea of the plate boundaries.

The 7 major plates

1. N. American Plate

2. S. American Plate

3. Eurasian Plate

4. Antarctic Plate

5. African Plate

6. Indo-Australian Plate

7. Pacific Plate

All Done!

Two Types of Crust Continental Crust

Made primarily from granite. Less dense than oceanic crust.

Oceanic CrustMade primarily from basalt.More dense than continental crust.

What do these densities mean?

TYPES OF PLATE BOUNDARIES

1. DIVERGENT

2. CONVERGENT

3. TRANSFORM

Divergent Plate Boundaries What does it mean to diverge?

Plates move apart

2 Types:

a) Ocean – Ocean

b) Continent – Continent

Predict what would happen as a result of

a) Ocean-ocean divergent motion

b) Continent-continent divergent motion

Ocean – Ocean Divergent Boundaries

Sea floor spreading creates a Mid-Oceanic Ridge. Basalt Undersea volcanoes

Continent – Continent Divergent Boundaries Hot magma rises and

causes the crust to split.

Causes the land to drop and form a deep valley. Rift valley

As spreading continues, the rift valley will lengthen, deepen, and make a sea. Red Sea

Africa’s Rift Valley & The

Red Sea

Convergent Plate Boundaries What does it mean to

converge? Plates are moving

together. 3 Types:

1. Ocean – Ocean

2. Continent – Continent

3. Ocean - Continent

Based on the model below Make some observations

about what happens at convergent plate boundaries

Predict what would happen at these boundaries

Oce

an –

Oce

an C

onve

rgen

t B

ound

arie

s

One oceanic crust dips below the other – subduction.

Forms an ocean trench. Examples include volcanic island arcs such as

Japan and the Caribbean.

Ocean – Continent Convergent Boundaries

Denser oceanic crust subducts below continental crust.

Causes mountains with volcanic activity

Examples—Cascade Mts. in Washington, Oregon, and California & Andes Mountains.

Continent – Continent Convergent Boundaries

Continents will smash together, NO SUBDUCTION, making mountain ranges.

Examples include the Himalayas.

TRANSFORM BOUNDARIES

Plates are sliding past each other. Examples include the San Andres Fault in California.

Transform Boundaries Occurs when two

plates slide past each other.

Examples include the San Andres Fault in California.

SAN ANDREAS

ALPINE FAULT BOUNDARY IN NEW ZEALAND

How do these plates move? Plate-driving force is the

slow movement of hot, softened mantle that lies below the rigid plates in the Asthenosphere.

These slow moving “convection cells” act as a moving walkway for the plates.