Transcript - Frost Middle School

Types of plate boundaries
• Divergent boundary
• Where two plates move apart from each other
• Convergent boundary
• Where plates push into each other
• Transform boundary
• Where plates scrape past each other
Sea Floor Spreading
• Spreading Center
• Another name for a divergent boundary in the sea floor
• What can form here
• Rift Valley
• Gap between mid-ocean ridges
• Mid-ocean ridges
• Longest chains of mountains in the world
• Mid-Atlantic Ridge – Runs from Iceland to Antarctica
• 11,000 Km ( 6,214 mi) long
• Rift Valley – 24 Km (15 mi) wide, 9 Km (6 mi) deep
Divergent boundaries on
Continents
• Crust is to thick to pull apart like in the ocean
• Two plates still pull apart
• Faults are created at the edges of these plates
• Causes lots of earthquakes to occur
• Middle sinks down
• Eventually it will sink below sea level and fill with water
• Red sea
Divergent boundaries on
Continents
• The Great Rift Valley in Africa
• Thousands of Km long and 1800 m deep
• Iceland
• Being split into two islands
• Process takes millions of years
Tectonic Plates push together
at convergent boundaries
• Crust is either folded or destroyed
• Subduction
• When one plate sinks beneath another
• Major geologic events occur at all convergent boundaries
Continental-Continental
Collision
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When two plates carrying continental crust push together
Because both crusts are the same density neither sinks
Edges crumple and fold creating mountains
Some of the worlds larges mountain chains are formed along
these boundaries
• Alps and Himalayas are examples
Oceanic-Oceanic Subduction
• This is where one plate with oceanic crust sinks beneath
another plate with oceanic crust
• The older plate sinks
• It is more dense and colder than the younger plate
• When the plate reaches the asthenosphere it melts into
magma
What main features form at OO Subduction
• Deep-Ocean Trenches
• Like deep canyons that form in the ocean floor as a plate sinks
• Most are found in the Pacific Ocean
• The Mariana Trench is where the Pacific Plate is sinking under the
Philippine Plate
• Deepest place in the ocean
• Nearly 11,000 meters (36,000 ft) down
• Oldest crust is found at these trenches
• Earthquakes can occur here
What main features form at OO Subduction
• Island Arcs
• Chain of volcanic islands
• These are violent volcanoes
• Examples
• Philippine Islands
• Aleutian Islands of Alaska
• Islands of Japan
• Earthquakes often happen here
Oceanic-Continental
Subduction
• This is when oceanic crust sinks under continental crust
because it is colder and denser
• Deep-Ocean Trenches also form here
• These are younger trenches than at O-O subduction
• Earthquakes also happen here
Oceanic-Continental
Subduction Continued
• Coastal Mountains
• When the oceanic crust sinks it can cause the continental crust to
buckle to form mountains
• Run parallel to the deep-ocean trench
• Many of these mountains will also be volcanic
• Cascade Mountains in Oregon and Washington
• Mount St. Helens – Active Volcano
Transform Boundaries
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The only boundary where crust is neither created or destroyed
Plates scrape past each other at these boundaries
Site of most major earthquakes
Most occur on the sea floor but some are on land
Transform Boundaries
Continued
• San Andreas Fault in California
• Runs from the Gulf of California through San Francisco
• Pacific Plate and part of the North American Plate are moving in
opposite directions
• If it keeps moving at its current rate
• Los Angeles will be a suburb of San Francisco in about 10 million
years