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Plate Tectonics What is it and what makes it work?
The Earth’s Layers The Earth is made of many different and distinct layers. The deeper layers are composed of heavier materials; they are hotter, denser and under much greater pressure than the outer layers. Natural forces interact with and affect the earth’s crust, creating the landforms, or natural features, found on the surface of the earth.
Before we start to look at the forces that contribute to landforms, lets look at the different layers of the earth that play a vital role in the formation of our continents, mountains, volcanoes, etc.
Crust Mantle Outer Core Inner Core crust - the rigid, rocky outer surface of the Earth, composed mostly of basalt and granite. The crust is thinner under the oceans.
mantle - a rocky layer located under the crust - it is composed of silicon, oxygen, magnesium, iron, aluminum, and calcium. Convection (heat) currents carry heat from the hot inner mantle to the cooler outer mantle. outer core - the molten iron-nickel layer that surrounds the inner core - the solid iron-nickel center of the Earth that is very hot and under great pressure.
Lithosphere: the layer of earth made up of the crust and upper mantle Asthenosphere: a partly molten layer just below the lithosphere in the upper mantle
The Earth’s interior 1000 C 4000 C 7000 C
The Earth’s Crust The crust is divided into sections called plates The two types of crust: oceanic and continental Continental crust is made of “lite” granite Oceanic crust is made of dense basalt Both the continental and oceanic crust floats on a layer of mantle
Plates of the Earth’s crust
mantle convection Heat transfer from the core to the mantle produces slow convection of the mantle material (in the order of centimetres per year)
Mantle convection produces lateral forces at the boundary between the mantle and the crust
Continental Drift Theory When the tectonic plates under the continents and oceans move, they carry the continents and oceans with them. • Alfred Wagner proposed the continental drift theory. He proposed that there was once a ingle “supercontinent” called Pangaea. • Over time pangea broke into Laurasia and Gondwanaland
Continental drift: An idea before its time Continental drift hypothesis Continents "drifted" to present positions Evidence used in support of continental drift hypothesis Fit of the continents Fossil evidence Rock type and structural similarities Paleoclimatic evidence
Matching mountain ranges Figure 2. 6
Wegner’s theory was that about 180 million years ago, Pangaea began to break up into separate continents. To back this theory up, he perserved remains and evidence from ancient animals and plants from South America, Africa, India, and Australia that were almost identical.
Continental drift and paleomagnetism Magnetized minerals in rocks Show the direction to Earth’s magnetic poles Provide a means of determining their latitude of origin
Continental drift and paleomagnetism Polar wandering The apparent movement of the magnetic poles illustrated in magnetized rocks indicates that the continents have moved Indicates Europe was much closer to the equator when coal-producing swamps existed
Continental drift and paleomagnetism Polar wandering Curves for North America and Europe have similar paths but are separated by about 24 of longitude n Differences between the paths can be reconciled if the continents are placed next to one another
Seafloor Spreading The other theory supporting plate tectonics emerged from the study of the ocean floor. Scientists were suprised to find that rocks taken from the ocean floor were much younger than those found on the continents. The youngest rocks were those nearest the underwater ridge system which is a series of mountains that extend around the world, stretching more than 64 thousand kilometers (40 thousand miles).
Paper conveyer belt show ; ) Pull 1 piece of paper with S/N/S/NNSNS apart
The theory of seafloor spreading suggests that plates move apart molten rock- hot substance (lava) from the mantle rises under the underwater ridge and breaks through a split at the top of the ridge The split is called a rift valley.
The rock then spreads out in both directions from the ridge as if it were on two huge conveyor belts. As the seafloor moves away from the ridge, it carries older rocks away. Seafloor spreading, along with the continental drift theory, became part of theory of plate tectonics.
As mentioned earlier, tectonic plates are always moving: n n n pulling away from each other crashing head-on or sliding past each other.
Pulling Away - Divergent Plate Boundaries Spreading centers – in the ocean or in a continent In the ocean plates spread at the Mid Atlantic Ridge On land spreading occurs at continental rift valleys
Continental rifting Figure 2. 21
The Crash! What happens when plates crash into each other depends on the types of plates involved. n Because continental crust is lighter than oceanic crust, continental plates ”float” higher. n Therefore, when an oceanic plate meets a continetnal plate, it slides under the lighter plate and down into the mantle.
n n The piece of oceanic rock melts when the edges get to a depth which is hot enough. A temperature hot enough to melt si about a thousand degrees!) This process is called subduction. Molten material produced in a subduction zone can rise to the earth’s surface and cause volcanic building, mountains, and islands.
Convergent plate boundaries Types of convergent boundaries Oceanic-continental convergence n n n Denser oceanic slab sinks into the asthenosphere Along the descending plate partial melting of mantle rock generates magma Resulting volcanic mountain chain is called a continental volcanic arc (Andes and Cascades)
Oceanic-continental convergence Figure 2. 22 A
Convergent plate boundaries Types of convergent boundaries Oceanic-oceanic convergence n n n When two oceanic slabs converge, one descends beneath the other Often forms volcanoes on the ocean floor If the volcanoes emerge as islands, a volcanic island arc is formed (Japan, Aleutian islands, Tonga islands)
Oceanic-oceanic convergence Figure 2. 22 B
Volcano and volcanic island arc One plate is subducted under another plate The plate being subducted melts The melted rock is hot and less dense therefore rises Melts its way through the overlaying plate until it tunnels through to the surface and spills out
Hot spots and mantle plumes Caused by rising plumes of mantle material Volcanoes can form over them (Hawaiian Island chain) Mantle plumes n n Long-lived structures Some originate at great depth, perhaps at the mantle-core boundary
The Hawaiian Islands Figure 2. 27
Convergent plate boundaries Types of convergent boundaries Continental-continental convergence n n n Continued subduction can bring two continents together Less dense, buoyant continental lithosphere does not subduct Resulting collision between two continental blocks produces mountains (Himalayas, Alps, Appalachians)
Continental-continental convergence Figure 2. 22 B
Converging. . . They Crash! And they’re both Continental Plates When both are continental plates, the plates push against each other, creating mountain ranges.
They Crash and are both continental plates! Earth’s highest mountain range, the Himalayas, was formed millions of years ago when the Indo-Australian Plate crashed into the Eurasian Plate. Even today, the Indo-Australian Plate continues to push against the Eurasian Plate at a rate of about 5 cm a year!
Transform fault boundaries Plates slide past one another and no new lithosphere is created or destroyed Transform faults Most join two segments of a mid-ocean ridge along breaks in the oceanic crust known as fracture zones A few (the San Andreas fault and the Alpine fault of New Zealand) cut through continental crust
They meet and slide past each other! Sometimes, instead of pulling away from each other or colliding with each other, plates slip or grind past each other along faults. This process is known as faulting.
These areas are likely to have an earthquake For example: Here, the San Andreas Fault lies on the boundary between two tectonic plates, the north American Plate and the Pacific Plate. The two plates are sliding past each other at a rate of 5 to 6 centimeters each year. This fault frequently plagues California with earthquakes.
Transform faults Figure 2. 24
Convergent boundary Divergent boundary Transform fault Pears and bananas
Plate motions - Quick Review Figure 2. 29
What drives plate motions Researchers agree that convective flow in the mantle is the basic driving force of plate tectonics Forces that drive plate motion Slab-pull Ridge-push Slab suction
Forces driving plate motions Figure 2. 30
Definitions: Trench: n a deep underwater valley formed when an oceanic plate is subducted under a continental plate. Diverging plates n plates moving away from each other Converging plates n plates moving and crashing into each other – volcanic island arc and mountains
Ocean Ridge n n Spreading center in the ocean Rift Valley – the center of the spreading center (Continent or Ocean) Subduction zone n One plate pushing below another