physical geography - plate tectonics

plate boundaries and the landforms that occur at them

what is distribution?

what is distribution?

the spatial coverage of the hazard

what is frequency?

refers to the distribution of the hazard through time

what is magnitude?

assesses the size of the impact

what three main components is the interior of the earth divided into?

the crust, the mantle and the core

what is the core made of?

dense rocks containing iron and nickel alloys

what are the two parts of the core?

inner core and outer core

what is the mantle made of?

molten and semi-molten rock rich in iron and magnesium

what is the upper part of the mantle called?

asthenosphere

how is the asthenosphere different from the mantle above it?

the asthenosphere is more fluid and hotter whereas the mantle above it is more solid

what are the two types of crust?

oceanic and continental

which crust is thicker?

the continental

who developed the theory of plate tectonics?

Alfred Wegner

what is continental drift?

the continents were originally joined together as one landmass - after which they separated and slowly drifted along the ocean flows to their current locations.

what is sea-floor spreading and how did it help Wegner’s theory?

sea-floor spreading: new ocean crust was continually being created in zones in the middle of the ocean

• the continents were moving but the creation of the crust was the driving force

what are convection currents?

heat within the earth powers the movement of the plates on its surface. at temps of over 1,000 the rocks in the mantle flow towards the surface - high pressure of rocks above the crust stop them melting in the heat. nearer the earth’s surface the rocks cool and move sideways, then as they loose the heat from the core they move downwards.

how do convection currents cause plates to move?

the sideways movement of rocks beneath the crust

what is ridge push/gravitational sliding?

at constructive boundaries, less dense, hot magma wells up and produces an ocean ridge above the ocean floor. as the rock gets older it cools and becomes denser. gravity acts on the older lithosphere, causing it to slide away from the ridge down the sloping asthenosphere below. due to the tearing apart of of the newly formed crust, shallow earthquakes form, indicating that there is some frictional resistance to the force.

what is slab pull?

at a destructive boundary, the oceanic plates are subducted under the overlying mantle. as the subducting plate is heavier it sinks into the mantle due to the downward gravitational force acting on it and pulls the plate down into the mantle.

what are the three types of plate margin?

conservative (transform), constructive (divergent) and destructive (convergent)

what happens to plates at constructive (divergent) margins?

plates move apart - earthquakes+volcanos

plates are pushed apart by magma rising from the mantle beneath the crust. as soon as the magma reaches the surface it solidifies, forming a new crust.

what happens to plates at destructive (convergent) margins?

plates collide

the oceanic plate subducts under the continental plate into the subduction zone.

what happens when two oceanic plates collide?

one is usually subducted underneath the other - same as normal, and a line of volcanic islands (island arc) appears.

what happens when two continental plates collide?

they collide and buckle up to form fold mountains

what happens to plates at conservative (transform) margins?

plates slide horizontally past each other

• no volcanoes, earthquakes occur as the plates tend to stick, creating friction - then slip violently rather than sliding smoothly

what are ocean ridge systems?

underwater mountains and volcanoes formed at constructive margins

how are rift valleys formed at constructive margins?

rising magma causes the whole region to be uplifted which creates weaknesses in the crust through which low viscosity basalts emerged and flooded the area. there is then a partial collapse of the crust and the development of a deep, steep-sided rift valley. the two sides of the valley move apart and are flooded with water.

how do deep sea trenches form at destructive margins?

oceanic plate is forced under lighter continental plate in the process of subduction. the downwards displacement of oceanic plate forms a deep-sea trench that runs parallel to the plate boundary.

how do fold mountains form at destructive plate boundaries?

during the subduction process, sediments that have accumulated on the edge of the continental plate are deformed by folding and are then uplifted to form young fold mountains.

what are magma plumes:

magma plumes are formed when heating at the core/mantle boundary causes a plume of magma to rise through the mantle and eat into the plate above

how do magma plumes form hotspots?

when the lava breaks through to the surface, active volcanoes form above the hotspot. as a plate moves away from the hotspot, it takes with it the volcano that had formed. the volcanoes cool and subside and over millions of years, hotspot tracks form. the volcanoes get progressively older the further they move away from the hotspot. the oldest volcanoes are the smallest due to erosion.

what is Harry Hess’s evidence for sea-floor spreading?

Paleomagnetism is the study of rocks that show the magnetic fields of the Earth.

As new rock is formed and cools the magnetic grains within the rock align with the magnetic poles. Our poles (North and South) switch periodically.

Each time these switch the new rocks being formed at plate boundaries align in the opposite direction to the older rock. On the ocean floor either side of constructive plate boundaries, Geologists observed that there are symmetrical bands of rock with alternating bands of magnetic polarity.