CHAPTER 6 PLATE TECTONICS

6.1 EARTH HAS SEVERAL LAYERS Objectives Learn about the different properties of Earth’s layers Learn about the plates that make up Earth’s outermost layers.

What are the layers of the Earth  Crust  Thin layer of rock  Surrounds Earth like a shell surrounds an egg  Two types of crust Continental crust Makes up the continents and some major islands Oceanic crust Makes up the ocean floor  Between 6-70 km thick (about 3.7 – 43.5 miles)  Furthest we have ever been in 12.3 Km (7.6 miles)

Two Kinds of Crust  Continental Crust  Thicker of the two  Less Dense  Composed mostly of Granite  Avg. of 30 – 40 Km thick (19 – 24.9 Miles)  Oceanic Crust  Thinner of the two  More Dense  Composed mostly of Basalt  Avg. of 6 – 11 Km thick (3.7 – 6.8 Miles

Mantle  Thickest Layer of Earth  Made mostly of Hot Molten Rock  Very, Very top is cool ridged rock  About 2900 Km (1802 miles)  870 – 4400 C (1598 – 7952 F)

Outer Core  Layer of liquid metal  Iron & Nickel  2300 Km (1429 Miles)  4400 – 6100 C (7952 – 11,012 F)  There is more pressure than the mantle but less pressure than the inner core

Inner Core  Solid ball of metal  Iron & Nickel  2400 Km in diameter (1491 Miles)  7000 – 8000 C (12,632 – 14,432 F)  Extreme pressure keeps it solid  Densest layer of the Earth

Lithosphere  Made up of the crust and very top rigid part of the mantle  This is what the tectonic plates are made of  Most plates have both continental and oceanic crust  On continents it is believed to be an average of 150 Km (93 miles)  In ocean it is believe Km (31 – 62 miles)

Asthenosphere  The hot soft layer of molten rock in the middle mantle on which the lithosphere rests  Flows slowly like hot tar  Estimated to be 85- several hundred Km thick (50 – several hundred miles)

6.2 Continents change position over time  Objectives 1. Learn how the continental drift hypothesis was developed 2. Learn about evidence for plate movement from the sea floor 3. Learn how scientists developed the theory of plate tectonics

Continental Drift  Hypothesis proposed by Alfred Wegener in 1915  The idea that Earth’s continents were once joined in a single landmass and gradually drifted apart.  This was not believed until the mid-1900’s  Wegener found many things that he claimed proved his hypothesis

Evidence for Continental Drift  Fossils  Ancient reptile found in both South America and Western Africa - Mesosaurus

Evidence for Continental Drift  Geology  Rocks in Brazil matched perfectly to rocks in Africa  Limestone in Appalachian Mts. (N.A.) same as Scottish Highlands

Evidence for Continental Drift  Climate  The same glacial till of the same age is found in southern Africa, South America, India and Australia  Large coal deposits were formed from tropical swamps in both N. America and Europe at the same time

Super Continent  Wegener named his super continent  Pangaea From the Greek word meaning “all lands” Was a continent that reached from pole to pole  Began to split apart 200 million years ago

New Evidence  In the 1960’s scientists really started studying the sea floor  Found underwater mountain ranges  Called mid-ocean ridges  Found in every ocean  Seemed to circle the Earth like the seams of a baseball  Sea-floor Spreading  Where the ridges form  Cracks in the crust where molten rock rises, cools, and forms new crust

New Evidence  Age of the Sea Floor  Rock near the cracks are younger then rocks further away  Ocean Trenches  Where the oceanic crust sinks back into the Asthenosphere  Scientists put this new evidence together with Wegener's hypotheses to create the Theory of Plate Tectonics

What causes Plate Movement  Convection Current  Motion that transfers heat energy in a material  As molten rock heats up it becomes less dense, as it rises it starts to cool becoming more dense and sinking again

Convection Currents Create  Ridge Push  Happens at mid-ocean ridges  Pushes plates apart  Creates new sea floor  Slab Pull  Happens at Ocean Trenches  The denser plate sinks  Melts back into the Asthenosphere

Theory of Plate Tectonics  States that Earth’s lithosphere is broken into huge plates that move over the surface of the Earth  Driving force of this movement is the convection current in the Asthenosphere  Most major earthquakes, volcanoes, and mountain ranges are where two plates meet.

6.3 Plates Move Apart  About different plate boundaries  What happens when plates move apart  How the direction and speed of plates can be measured

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

Magnetic Reversal  Recorded in the sea floor  Magnetic minerals in igneous rock lines up according to the direction of the poles  Last one was about 760,000 yrs ago  Flipped 170 times over the last 100 million years  The prediction for it happening again is 1,500 – 2,000 yrs from now

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

Hot Spots  Area where hot molten rock rises to the surface in plumes (columns) from the mantle  Usually found in the middle of tectonic plates but can be near boundaries  Hawaiian Islands  Can tell the direction and speed of plate movement  Yellowstone National Park

6.4 Plates Converge or Scrape past each other What you will learn  What Happens when two continental plates converge  What happens when an oceanic plate converges with another plate  What happens when one plate scrapes past another plate

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  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

As long as their plates keep moving these mountains keep getting bigger!

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 O-O 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 O-O 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  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