3.1. Plate Tectonics Flashcards
Why is the Earth’s crust is broken up into moving plates?
- Convection currents in the mantle
- Heat from the core heats the mantle which makes the magma rise
- This current cools down as it comes closer to the surface of the Earth
- As a result it moves in a horizontal direction along the bottom of the crust
- When the current cools down more, the convection current descends and go towards the core
Oceanic Crust
- Denser, newer and thicker than continental
- Made of dark basaltic rocks
Continental Crust
- Colder, thicker and less dense than continental
- Made of light-colored granite rocks
Mantle
Zone of molten Silicates and other minerals. Molten so it moves, the source of this is the Earth’s intense heat which sets up convection currents in the mantle
Outer Core
- Made of liquid Iron and nickel
- Source of decay due to radioactive elements
- under slightly less pressure
Inner Core
- Made of solid iron and nickel
- Solid, despite temperatures of 3700 degrees celsius because of the intense pressure there.
Divergent Boundaries
- Occurs where 2 plates move away from each other
- Rifting occurs due to the plates moving apart.
Examples of Divergent Boundaries
- oceanic + oceanic - Mid-Atlantic Ridge - American plates are moving away from the Eurasian and African plates (all oceanic crust sea-floor spreading)
- continental + continental - East African Rift Valley
Convergent Boundaries
- Occurs where 2 plates converge (oceanic + continental or oceanic + oceanic)
Examples of Convergent Boundaries
- oceanic + continental - Nazca plate (oceanic) and South American plate (continental) creating the Andes mountains
- oceanic + oceanic - Japan
Collision Boundaries
- Occurs where 2 plates of continental crust converge
Examples of Collision Boundaries
Indian plate collides with Eurasian plate (both continental crust) forming the Himalayas
Conservative Boundaries
- Occurs where 2 plates move parallel to each other, land is neither created or destroyed
Examples of Conservative Boundaries
North American plate and Pacific plate - The San Andreas fault
Earth Structure
Crust Lithosphere Asthenosphere Mantle Outer core Inner core
Lithosphere
the rigid outer part of the earth, consisting of the crust and upper mantle.
Asthenosphere
Semi-molten and what tectonic plates float on
Thickness of continental crust
35-70km average
Thickness of oceanic crust
6-10km average
Nature of continental crust
light - silica and aluminium
Nature of oceanic crust
dark - mainly basalt
Mechanisms of plate movement
Ridge push, Convection drag, Slab pull
Ridge Push
Intrusion of magma into spreading ocean ridges pushes plates apart
Convection Drag
Convection currents in the mantle drags the crust
Slab Pull
Cold, denser oceanic lithosphere sinks due to gravity, and drags the rest of the plate with it
Sea-floor Spreading
- There is upwelling of magma as it pushes up through the Earth’s mantle and breaks through the crust at the mid-ocean ridge, resulting in doming of the surface
- The magma hardens to form new ocean crust
- The new ocean crust splits apart the old crust and pushes both halves away from the ridge
- The old crust gradually moves further away from the mid-ocean ridge, and the ocean floor gets wider
- The rock furthest away from the ridge is the most heavy, and so pulls the rest of the plate with it - slab pull
Subduction
- Sinking of a plate beneath another
- Oceanic plates are heavier than continental plates. Zones usually dip at angles 30-70 but older, heavier plates dip more steeply.
Fold mountains
- When plates move towards each other they are forced up as they collide
- The lighter plate buckles up and folds
Ocean ridges
- The longest linear, uplifted features of the Earth’s surface
- Magma rises as the plates move away from each other and cool when it reaches the surface
- Convection currents cause the uplifting of ocean floors
Ocean Trench
- Long, narrow depressions in the ocean floor with depths of over 6000m and up to 11,000m
- When plates converge, denser plates are pushed beneath the lighter plate and deep into the mantle
Volcanic island arcs
- A series of volcanic islands
- When lithosphere is subducted the slab melts when the edges reach a depth which is sufficiently hot. Hot, remelted material from the subducting slab rises and leaks into the crust, forming a series of volcanoes.
Processes at Convergent Boundaries
- two plates converge: an oceanic plate and continental plate and there is subduction of the oceanic plate under the continental plate
- at the point of subduction, an ocean trench is created on the ocean floor
- the descending ocean plate is heated by friction created at the subduction zone and melts due to heat and increasing pressure in the mantle
- the newly formed magma is less dense than the surrounding material and therefore rises through weaknesses towards the surface - where it rises through cracks in the crust
Processes at Collision boundaries
- 2 plates of continental crust move together - both plates are rigid, low density and no subduction occurs
- Forces of collision causes the crust to deform and any ocean sediments between the two plates are forced up
- This results in Orogenesis as the folding and faulting creates fold mountains
- No volcanoes due to the absence of a magma source due to lack of subduction
