Theory of Plate Tectonics The theory states that

Theory of Plate Tectonics

The theory states that the Earth’s crust is broken up into plates that are in motion, driven by convection currents in the mantle.

Three types of plate boundary • Divergent • Convergent • Transform

Divergent Boundaries • Spreading ridges – As plates move apart new material is erupted to fill the gap

Ocean – Ocean Divergence P. 121 • Rising convection currents in the asthenosphere push the plates apart forming a mid-ocean ridge in the oceanic crust. • Magma erupts out of the mid-ocean ridge

MID-ATLANTIC RIDGE • https: //www. geolsoc. org. uk/Plate. Tectonics/Chap 3 -Plate. Margins/Divergent/Mid-Atlantic-Ridge

Mid-Ocean Ridges • Areas of high heat flow and elevation. • Form new crust where the plates separate. • Black smokers or vents form near the ridge. • Sea floor spreading.

• Pillow lavas form as the lava cools quickly in the water.

Iceland is an O-O divergent boundary • The Mid-Atlantic Ridge is going through the country • The country is spreading 1 inch a year • The country is above sea level • More active volcanoes than anywhere else in the world. .

Iceland: An example of continental rifting • Iceland has a divergent plate boundary running through its middle

Convergent Boundaries • There are three styles of convergent plate boundaries – Continent-continent collision – Continent-oceanic crust collision – Ocean-ocean collision

C-C Divergent p. 122 • Rift Valleys form where the two continental plates are separating. • The Great Rift Valley of East Africa

East African Rift System

TRIPLE JUNCTION: THE RED SEA/EAST AFRICA • https: //www. geolsoc. org. uk/Plate. Tectonics/Chap 3 -Plate. Margins/Divergent/Triple-Junction

C-C Convergent p. 123 • Plates of similar density are converging creating folded mountains. • No volcanoes are created.

Continent-Continent Collision • Forms mountains, e. g. European Alps, Himalayas

Himalayas

p. 114 The Appalachian Mtns. in the eastern U. S. is a folded mountain range.

O-C Convergent p. 124 • An oceanic and continental plate are colliding and the denser oceanic plate subducts (dives) beneath the less dense continental plate.

Continent-Oceanic Crust Collision • SUBDUCTION

p. 124 Trenches • Trenches form deep valleys at the point where the plate dives down. These are the deepest points on Earth.

Subduction • 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 • As the plate subducts, friction generates heat which melts the subducting plate, magma rises to create volcanic mtns that parallel the boundary. –Cascade Mtns (Wash, Oregon) –Andes Mtns. (S. America)

Subduction • Crust is destroyed in the process of subduction. • Creates the world’s most explosive volcanic eruptions.

O-O Convergence p. 125 • The denser older of the two oceanic plates subducts. • Marianas Trench – 33, 035 feet below sea level.

Subduction • Trenches form at the boundary. • Volcanic islands (called island arcs) form at the surface where the magma rises. – Philippines – Japan – Aleutian Islands (off Alaska) – Indonesian Islands

Ocean-Ocean Plate Collision • 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!

p. 126 Transform Boundaries • Plates slide past each other • Shearing : Stress that pushes a mass of rock in opposite directions. Above: View of the San Andreas transform fault

What will happen in the future? • The Atlantic Ocean will continue to expand. • The Pacific Ocean will close. • The Mediterranean will disappear connecting Africa with Europe. • India will continue to push into Eurasia pushing the Himalaya Mtns. higher. • Los Angeles will continue to the northwest joining with San Francisco.

Age The age of the ocean crust. Courtesy of www. ngdc. noaa. gov

Volcanoes and Plate Tectonics… …what’s the connection?

Pacific Ring of Fire Volcanism is mostly focused at plate margins

Volcanoes are formed by: - Subduction - Rifting - Hotspots

Pacific Ring of Fire Hotspot volcanoes

What are Hotspot Volcanoes? • Hot mantle plumes breaching the surface in the middle of a tectonic plate The Hawaiian island chain are examples of hotspot volcanoes. Photo: Tom Pfeiffer / www. volcanodiscovery. com

The tectonic plate moves over a fixed hotspot forming a chain of volcanoes. The volcanoes get younger from one end to the other.

Earthquakes and Plate Tectonics… …what’s the connection?

• As with volcanoes, earthquakes are not randomly distributed over the globe Figure showing the distribution of earthquakes around the globe • At the boundaries between plates, friction causes them to stick together. When built up energy causes them to break, earthquakes occur.

Where do earthquakes form? Figure showing the tectonic setting of earthquakes

Plate Tectonics Summary • The Earth is made up of 3 main layers (core, mantle, crust) • On the surface of the Earth are tectonic plates that slowly move around the globe • Plates are made of crust and upper mantle (lithosphere) • There are 2 types of plate • There are 3 types of plate boundaries • Volcanoes and Earthquakes are closely linked to the margins of the tectonic plates