Plate Tectonics Flashcards
(21 cards)
Plates
7 major plates, 8 minor plates and some micro plates
Plates aren’t always separate, they are sometimes stuck together
Some plates are therefore continental and oceanic e.g. North American
Nature of earth’s plates
Earth’s plates often surround continents, such as the Eurasian, African and South American plate
Some plates are purely oceanic, like the Pacific plate, while others are mixed
Some plates move away from each other creating a constructive margin such as in the mid-Atlantic
Some plates move together such as the Indo-Australian plate creating a destructive plate boundary
Some plates move parallel, creating fault lines such as the San Andreas fault
Continental crust
100km thick
Made up of old rock (over 1500 years) and has a lower density than oceanic crust
SIAL - silicon and aluminium
Oceanic crust
10km thick
Made up of younger, denser rock
SIMA - silica and magnesium
Island arcs
Systems formed when oceanic lithosphere is subducted beneath the oceanic or continental lithosphere
They are tectonically active belts of intense seismic activity containing a chain or arc of active volcanoes
Deep-sea trenches are formed on the oceanward side of these arcs
Fore-arc region
A bulge of around 500m high occurring about 120-150km from the trench
Comprises the trench, subduction complex and fore-arc basin
Subduction complex is made of slices of trench and oceanic crust scraped off downgoing slab by leading edge of overriding plate
Backthrusting occurs when accretionary wedge comes into contact with fore-arc basin
Sedimentary arc
Comprises of coralline and volcaniclastic sediments underlain by volcanic rocks older than those found in volcanic arc
As the cooler descending plate extends further into the asthenosphere the position of extrusive igneous activity moves backwards
Island arc and remnant arc enclose a marginal sea behind island arc
Marginal seas are generally 200-600k in width
Benioff zone
A flat zone of earthquakes along a line within the rock structure
The earthquakes can be caused by an abrupt slip along the subduction zone
Shallow earthquakes tend to be the most hazardous as there isno chance for seismic waves to disappear
Volcanic arcs
Include all volcanically active belts located above a subduction zone
True island arcs include only those separated from the land by a stretch of water e.g. Caribbean
Composite / stratovolcanoes
Form at destructive plate boundaries and are found on island arcs
20km wide and 2.5km high
Steep slopes
Alternating layers of lava and ash
Lava has high gas pressure and is explosive
Shield volcanoes
180km wide and 9km wide
No ash but lots of lava
Made of layers of solidified lava
Formed by frequent gentle eruptions of thin, runny basaltic lava
Form at hotspots and constructive plate boundaries and are found on island chains
Constructive plate boundary
(oceanic and oceanic)
Oceanic crust is created along mid-ocean ridges with chains of underwater volcanoes and mid-ocean ridges
As the plates pull apart, magma is produced
This is added to the edges of the plates as an igneous rock (gabbro)
Plates pulled apart by convection currents and depressions in asthenosphere
Earthquakes occur when plates move over asthenosphere
Ridge push - forces plates downhill due to gravity
Seafloor spreading occurs
e.g. East Pacific Rise, Mid Atlantic Ridge
Destructive plate boundary
(oceanic and continental)
Older, denser oceanic plate is forced under the lighter continental plate
Oceanic plate melts due to high pressure and temperature, forming magma
Deep oceanic trenches form at the subduction point
Fold mountains form as the continental plate crumples
Volcanoes form as rising magma creates volcanoes along the boundary
Earthquakes occur as friction between the plates causes seismic activity
Slab pull
e.g. Andes, Peru-Chile Trench
Conservative plate boundary
(continental and continental)
Two plates move either the same direction at different speeds or opposite directions alongside each other
Friction causes both plates to be rocked and so stress builds up over time due to resistance to movement
Earthquakes occur when this stress is suddenly released
e.g. San Andreas Fault, Alpine Fault, North Anatolian Faults
Collision plate boundary
(continental and continental)
Plates push against each other, forcing the crust to fold, forming fold mountains
Earthquakes may occur due to high pressure build up
e.g. Himalayas, Alps
Constructive plate boundary
(continental and continental)
Convection currents cause the plates to move apart
Fractures form leading to rift valleys forming
Magma rises to fill the gap and becomes part of the crust after cooling
Volcanic activity may occur on the rift and shallow earthquakes occur due to movement apart
When the two split apart, a new oceanic crust is formed around lakes
Ridge push
e.g. Great African Rift Valley
Destructive plate boundary
(oceanic and oceanic)
The older, denser oceanic plate is subducted underneath the less dense one, forming ocean trenches
Volcanic islands are formed by the magma created from the melting of the subducted plate as a result of high temperature and pressure
Earthquakes form due to friction between the two plates during subduction
Slab pull
e.g. Phillipines (volcanic islands), Mariana Trench (ocean trench)
Seafloor spreading
The geologic process that occurs at the boundary between 2 plates where molten material within the earth pushes its way up, causing the plates to move away from each other
Ridge push
Where gravity pushes oceanic lithosphere away from mid-ocean ridges, causing plates to move apart
Slab pull
Where the cooling and subsequent densifying of a subducting tectonic plate produces a downward force along the rest of the plate
Hotspot
Volcano denudes extinct volcanic islands reducing them to seamounts over time
When strings of volcanic islands are created by a hotspot, these are called island chains
These are different from volcanic arcs which form at destructive plate boundaries
e.g. Hawaii, Aleutian Islands
