The Theory of Continental Drift
Continental Drift Theory Alfred Wegener proposed in 1912 that the continents had once been joined together in a single landmass and have drifted apart since. Wegener named this supercontinent Pangaea. Wegener s theory was rejected by scientists because he could not explain what force pushes or pulls continents.
Evidence To Support Continents look like they could be part of a giant jigsaw puzzle
Plant and animal fossils found on the coastlines of different continents Evidence To Support Distribution of Fossils
Evidence To Support Same rock patterns found in South America, India, Africa, Antarctica and Australia Sequence of Rocks
Evidence To Support Tropical plant remains (coal deposits) found in Antarctica Glaciation in Africa, South America, India, and Australia during the same time Ancient Climates
Plate Tectonics
What is Plate Tectonics The Earth s crust and upper mantle are broken into sections called plates Plates move around on top of the mantle like rafts
What is the Lithosphere? The crust and part of the upper mantle = lithosphere 100 km thick Less dense than the material below it so it floats
What is the Asthenosphere? The plastic layer below the lithosphere = asthenosphere The plates of the lithosphere float on the asthenosphere
Lithosphere : - broken in to 7 large rigid pieces 1. African 2. North American 3. South American 4, Australian 5. Antarctic, 6. Pacific plates. 7. Eurasian -Several minor plates also exist ( the Arabian, Nazca, and Philippines plates )
2 Types of Plates Ocean plates - plates below the oceans Continental plates - plates below the continents
Questions... What is the theory of plate tectonics? What is the lithosphere? What is the asthenosphere? What is the connection between the two? What are the two types of plates?
Plate Boundaries
Plate tectonics The plates are all moving in different directions and at different speeds Plates only move a few centimeters each year, so collisions are very slow and last millions of years ~2 to10 cm per year (about the speed at which your fingernails grow) Mountains, earthquakes, and volcanoes form where plates collide.
Divergent Boundaries Boundary between two plates that are moving apart or rifting RIFTING causes SEAFLOOR SPREADING
Divergent Boundaries Places where plates are coming apart The Lithosphere is pulled apart Plates separate along that boundary A block between the faults cracks and drops down into the soft, plastic interior
Divergent Boundaries Places where plates are coming apart A rift is formed Magma seeps upward to fill the cracks New crust is now formed along the boundary Earthquakes occur along the faults volcanoes form where the magma reaches the surface.
Features of Divergent Boundaries Mid-ocean ridges Rift valleys Fissure volcanoes
Seafloor spreading Movement of two oceanic plates away from each other Ocean floor spreading was first suggested by Harry Hess and Robert Dietz in the 1960's. Zone of divergence: where the oceanic plates are moving away from each other
Seafloor spreading
Seafloor spreading Mid-ocean Ridge Results in the formation of new oceanic crust from magma that comes from within the Earth's mantle along a a mid-ocean ridge.
Mid Atlantic ridge Rift valley: Deep crack that runs along the top of this chain of mountains This is where new ocean floor is continuously created. Divergence along the Mid Atlantic ridge causes the Atlantic Ocean to widen at only about 2 centimeters per year
Divergent Boundaries: Sea floor spreading Mid Atlantic ridge The Earth's longest mountain chain is an underwater chain of mountains 47,000 miles long Runs down the middle of the Atlantic Ocean (surfacing at Iceland), around Africa, through the Indian Ocean, between Australia and Antarctica, and north through the Pacific Ocean.
SEAFLOOR SPREADING The image shows the surface of the Earth minus its water Notice the distinct mountain chains which circles the world It wasn't until the 50's and 60's that this detail became known
subduction New crust is continually being pushed away from divergent boundaries (where sea-floor spreading occurs), increasing Earth's surface. But the Earth isn't getting any bigger. What happens, then, to keep the Earth the same size? subduction PLATES ARE DESTROYED Subduction causes partial melting of both the ocean crust and mantle as they slide past one another Most of the subducting plate continues into the mantle, perhaps to reappear much later at a distant divergent boundary.
Convergent Boundaries * Places where plates crash or crunch together Think about it like this : 2 plates colliding is like 2 cars crashing into each other The front ends of cars fold and bend in a collision, so do the "front ends" of colliding plates.
Convergent Boundaries There are 3 types 1) Ocean plate colliding with a less dense continental plate 2) Ocean plate colliding with another ocean plate 3) Continental plate colliding with another continental plate
1) oceanic plate continental plate Ocean plate is less dense continental plate Subduction Zone: where the less dense plate slides under the more dense plate VOLCANOES occur at subduction zones
subduction Convergent: Places where plates crash or crunch together The edge of this continental plate has folded into a huge mountain range The edge of the oceanic plate has bent downward and dug deep into the Earth.= Trench oceanic plate continental plate
subduction All that folding and bending makes rock in both plates break and slip, causing earthquakes. As the edge of the oceanic plate digs into Earth's hot interior, some of the rock in it melts. The melted rock rises up through the continental plate, causing more earthquakes on its way up, and forming volcanic eruptions where it finally reaches the surface.
Convergent Boundaries Subduction If the activity continues, the volcano may grow tall enough to breech the surface of the ocean creating an island. West coast of South America : Oceanic Nazca Plate is crashing into the continent of South America. The crash formed the Andes Mountains, the long string of volcanoes along the mountain crest, and the deep trench off the coast in the Pacific Ocean. Andes Mountains
2) oceanic plate oceanic plate The less dense (older) plate slides under the more dense plate creating a subduction zone called a TRENCH
The plate with the older (cooler, more dense) crust descends into the subduction zone. A chain of volcanic islands (island arc) forms on the overriding plate. Trenches often lie adjacent to chains of islands (island arcs) formed by magma from the subducted plate.
Aleutian Islands
3) 3) Continental plate Continental plate Have Collision Zones: a place where folded and thrust faulted mountains form.
Collisional bounderies When two land masses meet, neither will slide under the other They crumple and fold. Some pieces of land are thrust over or under other pieces An ocean floor pushed toward a land mass will always slide under the land mass. This is because the land mass is more buoyant, or lighter, than the ocean floor. The result is a mountain range.
Collisional bounderies
The highest mountains in the world The Himalayas All these mountains were formed when two land masses came together. European Alps Appalachian Mountains
Transform Fault Boundaries Boundary between two plates that are sliding past each other EARTHQUAKES along faults
Transform Boundaries Cause: plates slide past each other Linear valleys along the boundary where rock has been ground up by the sliding. Stream beds that have been split in half and the two halves have moved in opposite directions.
Transform Boundaries San Andreas fault The slice of California to the west of the fault is slowly moving north relative to the rest of California Motion along the fault is sideways Los Angeles is creeping towards San Francisco at about 6 cm per year. In about ten million years, the two cities will be side by side!
San Andreas fault
The strongest and most famous earthquake along the San Andreas fault hit San Francisco in 1906. Many buildings were shaken to pieces by the quake, and much of the rest of the city was destroyed by the fires that followed. More than 600 people died as a result of the quake and fires.
Questions 1. What are the three types of boundaries? 2. What direction do plates go for each? 3. Which boundary has a subduction zone what occurs at a subduction zone? 4. What type of boundary is involved with seafloor spreading? 5. What type of boundary is at the San Andreas fault? 6. The Mid Atlantic Ridge is an example of what boundary?
Causes of Plate Tectonics
Convection Currents Hot magma in the Earth moves toward the surface, cools, then sinks again. Creates convection currents beneath the plates that cause the plates to move.
Questions... 1. What causes plates to move? 2. How is a convection current formed?