Chapter Four – Earth Science

Plate Tectonics

Section 4-1-Earth’s Interior

• Three main layers:– The Crust– The Mantle– The Core: inner core/outer core.

Earth’s Layers

The Crust

The Crust: the layer of rock that forms Earth’s outer skin. In includes both dry land and the ocean floor. The crust beneath the ocean is the oceanic crust – made mostly of basalt. The crust that forms the continents, called continental crust, is made mostly of granite.

The Mantle

• Below the crust is the Mantle. This is a hot layer of rock. The crust and the uppermost part of the mantle together form the lithosphere. Beneath the lithosphere is a softer layer called the asthenosphere.

The Core

• Earth’s core consists of two parts:– The outer core is made of metals iron and

nickel. The outer core is a layer of molten metal that surround the inner core.

– The inner core is also made of iron and nickel. The inner core is a dense ball of of solid metal.

– The movement of both the inner/outer core creates Earth’s magnetic field, which causes the planet to act like a giant bar magnet.

Convection Currents

• The movement of energy from a warmer object to a cooler object is call heat transfer. There are three types of heat transfer:– Radiation– Conduction– Convection

Radiation

• The transfer of energy through empty space. Heat transfer by radiation takes place with no direct contact between a heat source and an object. – Example: Sunlight warms the Earth’s Surface

Conduction

• Heat transferred by direct contact of particles of matter. In conduction, the heated particles of a substance transfer heat to other particles through direct contact.– Example: Use a spoon to eat soup. The hot

liquid transfers heat to the spoon and the spoon becomes hot.

– Example: Stirring coffee with the spoon.

Convection

• The transfer of heat by the movement of a heated liquid. Fluids include liquids and gases. During convection, heated particles of a fluid begin to flow, transferring heat energy from on part of the fluid to another.– Example: Hot Tub– Example: Heating a pot of soup. As the soup on the

bottom gets hot, it expands and becomes less dense. The warm, less dense soup moves upward, floating over cooler, denser soap. Then gravity pulls this cooler, denser soup down to the bottom and the process continues.

Convection Currents

• The process of the heating and cooling of fluid, changes in the fluids density, and the force of gravity that combine and cause this flow is a convection current.

• Convection Currents flow in the asthenosphere. The heat source for these currents is heat from Earth’s core and from the mantle itself. Hot columns of mantle material rise slowly. At the top of the asthenosphere, the hot material spreads out and pushes cooler material out of the way. The cooler material sinks back into the asthenosphere.

Convection Currents

• Convection Currents: large convection systems in the mantle may carry along the plates of the lithosphere like a conveyor belt.

Section 4-3 – Drifting Continents

• Alfred Wegener wondered why the coasts of several continents matched so well.– Wegener’s hypothesis was that all the continents had

once been joined together in a single landmass and have since drifted apart. The Super-continent was named Pangaea – “all lands”.

– Pangaea existed 300 million years ago, and over tens of millions of years, Pangaea broke apart and drifted to their present-day locations, becoming the seven continents today: North America, South America, Africa, Australia, Antarctica, Europe, and Asia.

Alfred Wegener

Pangaea

Continental Drift

Continental Drift

• Simulation of moving plates:

http://www.enchantedlearning.com/subjects/astronomy/planets/earth/Continents.shtml

The Continental Drift

• The idea that the continents slowly moved over Earth’s Surface became known as the continental drift. Wegener presented evidence to support his theory:

1. Mountain Ranges and landforms provide evidence for continental drift. Mountain ranges in South America and Africa line up. Also, European coal fields match with similar coal fields in North America. We know coal is a product of once living, animals and plants.

Evidence

2. Evidence from Fossils: Fossils of the plant Glossopteris has been found in rocks on widely separated landmasses. Additionally, fossils of the reptiles Mesosaurus and Lystrosaurus also have been found on widely separated landmasses.

3. Evidence from the Climate: An island in e Artic Ocean contains fossils of tropical plants. The island at one time must have been close to the equator. Also, scratches in rocks made by glaciers have been found in South Africa. Wegener stated Earth’s climate has not changes, but the positions of the continents have changed.

Continental Drift

• Wegener could not provide a satisfactory explanation for the force that pushes or pulls the continents. Most geologists rejected his theory. Due to new evidence, Wegener’s theory is being reconsidered.

4-4 Sea Floor Spreading

• Scientists mapped the mid-ocean ridge using sonar.– Mid-Ocean ridge is the longest chain of

mountains in the world, curving along the sea floor of all Earth’s oceans.

– Sonar is a device that bounces sound waves off underwater objects and then records echoes of these sound waves.

4-4 Sea Floor Spreading

Earth’s ocean floors move like a conveyor belt, carrying the continents with them. This movement begins at the mid-ocean ridge.

The ridge forms along a crack in the oceanic crust. At the mid-ocean ridge, molten material rises from the mantle and erupts. The molten material then spreads out, pushing older rock to both sides of the ridge.

4-4 Sea Floor Spreading

• Then more molten material splits apart the strip of rock, pushing it aside. This process, called sea-floor spreading, continually adds new material to the ocean floor.

Sea-floor spreading

4-4 Evidence

1. Scientists have found strange rocks shaped like pillows in the central valley of the mid-ocean ridge. Such rocks can form only if molten material hardens quickly after erupting under water.

2. Scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized strips.

3. Lastly, the last piece of evidence came from rock samples, the farther from the ridge the rocks were taken, the older they were.

Evidence – Further Reading

http://kids.earth.nasa.gov/archive/pangaea/evidence.html

4-4 – Sea Floor Spreading

• The ocean floor does not just keep spreading, it also sinks beneath deep underwater canyons called deep-ocean trenches.

• Where there are trenches, subduction takes place. Subduction is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle. This happens over tens of millions of years.

4-4

• The process of subduction and sea-floor spreading can change the size and shape of the oceans. Because of these processes, the ocean floor is renewed about every 200 million years.– The Pacific Ocean is shrinking.– The Atlantic Ocean is expanding.

4-5 Theory of Plate Tectonics

• Earth’s lithosphere is broken into separate sections called plates. The plates fit closely together along cracks in the crust.

The Theory of Plate Tectonics

The Theory of Plate Tectonics

• Plate Tectonics is the geological theory that states pieces of the Earth’s lithosphere are in constant, slow motion, driven by convection currents in the mantle. The theory of plate tectonics explains the formation, movement, and subduction of Earth’s plates.

Divergent Boundary

• Two plates move apart – these occur mostly at the mid-ocean ridge. (creation of valleys)

Divergent Boundary

Convergent Boundary

• Two plates come together causing a collision.

Convergent

Transform Boundary

• A place where two plates slip past each other, moving in opposite directions. Earthquakes occur frequently along these boundaries.

Today – NASA’s Role

• The LAGEOS II satellite, built by the Italian Space Agency, Agenzia Spaziale Italiana (ASI), and managed by NASA's Goddard Space Flight Center, measures crustal movement by using Satellite Laser Ranging technology

Today

• Residents of warm southern California are moving to Alaska whether they like it or not. Scientists, using the theory of plate tectonics, say that southern California is moving north and will collide with Alaska in approximately 150 million years.

Predictions for the Future:

• What can scientists predict about some of the features of the Earth existing today? Quite a lot. The Atlantic Ocean will continue to expand, while the Pacific Ocean (originally Panthalassa Ocean) will shrink. Eventually, the Mediterranean Sea (remnant from the Tethys Sea) will disappear, connecting Africa with Europe. India will continue to push into the southern Asian continent, pushing the Himalayas higher. Meanwhile the city of Los Angles will continue its journey north to join with the city of San Francisco. This will take several million years to occur. Maybe in another 200 million years there will be no need for a transoceanic trip from America to Asia.