2Colliding Plates Objectives Key Terms Explain how plate tectonics accounts for the movement of continentsCompare and contrast divergent, convergent and transform plate boundaries.Explain how convection currents inside Earth might be the cause of plate tectonicsKey Termsplate tectonics, lithosphere, divergent boundaryConvergent boundary, transform fault boundary
3Plate Tectonics Tectonic plates Think of the hypotheses of continental drift and sea-floor spreading as clues to a mystery.How can the two hypotheses be explained?In the 1960’s, geologists developed a new theory to explain the apparent movement of the continents.The theory of plate tectonics suggests that Earth’s crust and upper mantle are broken into sections called plates (that move).But, what are they made of and how do they move??
5We already know that the Earth’s crust is a layer of solid rock. The uppermost portion of the mantle is also solidTogether, these two areas are known as the lithosphere
6We also know that there is an area in the mantle that is less solid We also know that there is an area in the mantle that is less solid. Here, the material acts more like a putty; it’s a solid that can flow.This putty-like layer is called the asthenosphere
7Plates Divergent Boundaries The boundary between two tectonic plates that are moving away from each otherMagma is forced upward between the two plates, creating a new crust, thus making….Mid-ocean ridges are divergent boundaries
9Learning CheckThe theory that suggests that Earth’s crust and upper mantle are broken into sections called plates (that move) is known as…plate tectonicsTogether, the Earth’s crust is a layer of solid rock and the uppermost portion of the mantle is also solid are known asLithosphereThe boundary between two tectonic plates that are moving away from each other is known as…Divergent boundary
10Convergent Boundaries Crustal material can bedestroyed where 2 plates meet head on.This type of boundary is called a convergent boundaryWhat do you think happens when two plates containing continental crust collide?The two plates crumple forming mountain ranges!The Himalaya Mountains formed when the Indian Plate collided with the southern part of the Eurasia plate.
11HimalayasExample:India used to be an island, but about 15 million years ago it crashed into Asia (see map).As continental crust was pushing against continental crust the Himalayan mountain belt was pushed up.“Mountains” were also pushed down into the mantle as the normally 35 km thick crust is approximately 70 km thick in this region.Mt Everest is the highest altitude mountain on our planet standing 8,840 metres high. This means that below the surface at the foot of the mountain the crust is a further 61 km deep!!
12Convergent Boundaries There are three styles of convergent plate boundariesContinent-continent collisionContinent-oceanic crust collisionOcean-ocean collisionConvergent boundaries are where the plates move towards each other.There are three types of convergent boundary, each defined by what type of crust (continental or oceanic) is coming together.Therefore we can have: continent-continent collision, continent-oceanic crust collision or ocean-ocean collision….
14Manned or unmanned submersible vehicles (top right photo) have explored small parts of trenches discovering new species (like the fish photographed here) and amazing ecosystems.
15Transform Fault Boundaries A third type of boundary that is formed when two plates slide past one another in opposite directions.Think about the San Andreas Fault in California – This is where the North America and the Pacific plate slide past one another!Along this boundary, the Pacific plate moves NW relative to the North American plate at an average rate of 2 cm per year.
18Causes of Plate Tectonics How does the theory of plate tectonics explain the cause of plate movements?The driving force behind this movement is HEAT!A material that is hot is less dense than the same material that is coldThis is because the same mass takes up more volume (space) when the material is heated.This helps us explain the process of convection currents…
20Convection CurrentsC.C.’s within the Mantle cause the various plates in Earth’s lithosphere to move around.As the plates bump into each other, boundaries form!Plates move in different directions because there are many conv. cells within the Mantle!
21Learning Check!A type of boundary that is formed when two plates slide past one another in opposite directions is known asTransform Fault BoundaryBoundary where crustal material can be destroyed where 2 plates meet head on isConvergent BoundaryMountain RangesConvergentEarthquakesTransform Fault
24Continent-Continent Collision Forms mountains, e.g. European Alps, HimalayasWhen continental crust pushes against continental crust both sides of the convergent boundary have the same properties (think back to the description of continental crust: thick and buoyant). Neither side of the boundary wants to sink beneath the other side, and as a result the two plates push against each other and the crust buckles and cracks, pushing up (and down into the mantle) high mountain ranges. For example, the European Alps and Himalayas formed this way.
25Continent-Oceanic Crust Collision Called SUBDUCTIONAt a convergent boundary where continental crust pushes against oceanic crust, the oceanic crust which is thinner and more dense than the continental crust, sinks below the continental crust.This is called a Subduction Zone.The oceanic crust descends into the mantle at a rate of centimetres per year. This oceanic crust is called the “Subducting Slab” (see diagram).When the subducting slab reaches a depth of around 100 kilometres, it dehydrates and releases water into the overlying mantle wedge (Presenter: explain all of this using the diagram).The addition of water into the mantle wedge changes the melting point of the molten material there forming new melt which rises up into the overlying continental crust forming volcanoes.Subduction is a way of recycling the oceanic crust. Eventually the subducting slab sinks down into the mantle to be recycled. It is for this reason that the oceanic crust is much younger than the continental crust which is not recycled.
26SubductionOceanic lithosphere subducts underneath the continental lithosphereOceanic lithosphere heats and dehydrates as it subsidesThe melt rises forming volcanismE.g. The AndesThe Andes mountain range along the western edge of the South American continent is an example of a mountain belt formed by subduction.The continental crust of the South American plate has buckled under the compressional strain of converging with the Nasca and Antarctic plates. Additionally there are many volcanoes, the result of melting of the subducting slab and the production of new material that has risen through the crust to the surface.
27Ocean-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!When two oceanic plates converge, because they are dense, one runs over the top of the other causing it to sink into the mantle and a subduction zone is formed.The subducting plate is bent down into the mantle to form a deep depression in the seafloor called a trench.Trenches are the deepest parts of the ocean and remain largely unexplored.