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 Crust  Made 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

Ocean – Ocean Convergent Boundaries  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.