Plate Tectonics Flashcards
what is the asthenosphere?
the upper layer of the earth’s mantle, below the lithosphere, in which there is relatively low resistance to plastic flow and convection is thought to occur
what is the lithosphere?
the rigid outer part of the earth, consisting of the crust and upper mantle.
what is the earths structure?
inner core, outer core, mantle and crust
what is the plate tectonic theory and who invented it?
that the lithosphere is divided into a couple of dozen “plates” that move around across the earth’s surface relative to each other. Alfred Wegner 1912
what are convection currents?
zones of hotter, more liquid magma exhibit motion in the asthenosphere that causes the crustal plates to move. Pulling apart the crust at ocean ridges and rift zones, and pulling oceanic crust down at subduction zones.
what is the evidence for Alfred Wegners theory?
continental fit: South America and Africa
geological evidence: Brazil and South Africa, same rock type and age
climate: Antarctica, North America & UK have similar coal deposits formed in same tropic zones
biological: Similar plants and animals found in coal deposits in India and Antarctica.
what is paleomagnetism?
The study of earth’s past magnetic field in rocks, sediment and other materials, certain minerals in rocks lock in the direction of the magnetic field. Provides evidence for the past behaviour of the earth’s magnetic field and past locations of tectonic plates.
what is seafloor spreading?
at mid-ocean ridges, new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor spreading helps explain continental drift in the theory of plate tectonics. Harry Hess 1948
what is continental drift?
the gradual movement of the continents across the earth’s surface through geological time.
differences between continental and oceanic crust
Oceanic: Newer, most less than 200 million years old (continetal near 1500 years old) Denser Can be subducted Can be renewed and destroyed Mainly basaltic not very thick
destructive plate margin
The oceanic plate is forced under the lighter continental plate. Friction causes melting of the oceanic plate and may trigger earthquakes. where volcanoes occur.
constructive plate margin
Plates move apart. Volcanoes are formed as magma wells up to fill the gap, and eventually new crust is formed. An example of a constructive plate boundary is the mid-Atlantic Ridge.
conservative plate margin
Plates slide past each other in opposite directions, or in the same direction but at different speeds. Friction is eventually overcome and the plates slip past in a sudden movement.
how are fold mountains formed?
- Geosynclines are huge depressions found naturally on the ocean floor.
- Rivers deposit sediment in these geosynclines.
- Sediments are compressed and turned to sedimentary rocks like limestone.
- Plates are forced together at destructive margins.
- Sedimentary layers are forced upwards into fold mountains
how do ocean trenches form?
Form the deepest parts of the Earth
Occur at destructive subduction margins
As oceanic plate is subducted beneath the continental plate creates deep ocean trenches.
Mariana Trench, create by the subduction of the Pacific plate beneath the Mariana plate, oceanic-oceanic.
how do island arcs form?
As a lithospheric slab is subducted, slab melts when the edges meet hottest depth. Hot, remelted material from the subducting slab rises and leaks into the crust, forming a series of volcanoes. These volcanoes can make a chain of islands called an “island arc”.
how do hot spots form?
Plumes of superheated, less dense magma rise from deep within the asthenosphere up into the lithosphere and form basaltic shield volcanoes on the ocean floor.
shield volcano erupts, build up over time to form an island.
how are rift valleys formed?
When 2 continental plates move apart, central sections then drop down, this creates large parallel depressions. As this process continues the valley gets wider and wider until it becomes a large basin that fills with sediment from the rift walls and the surrounding area.
how do ocean ridges form?
Underwater mountain range. This uplifting of the ocean floor occurs when convection currents rise in the mantle beneath the oceanic crust and create magma where two tectonic plates meet at a constructive boundary.
what are the 3 different types of lava and their properties?
Basaltic (basic) lava: originates from the upward movement of mantle material. Most common around spreading ridges, hot spots and rifts. RUNNY
Andesitic (intermediate) lava: found at destructive plate margins where crust is being destroyed. SEMI-VISCOUS. Eruptive & volatile.
Rhyolitic (acid) lava: found at destructive and collision margins. VISCOUS. Eruptive & highly volatile.
Icelandic eruption
lava flows gently from a long fissure
Hawaiian eruption
lava is emitted gently from a vent
Strombolian eruption
small but very frequent eruptions occur
Vulcanian eruption
more violent but less frequent
Peleean eruption
violent eruption accompanied by pyroclastic flow
Plinian eruption
large amounts of lava and pyroclastics are erupted
extrusive landforms
Molten rock that reaches the surface;
volcanoes, hot springs, boiling mud, geysers
intrusive landforms
molten rock that is injected into the crust, sills, dikes, batholith
describe a batolith formation
A large, deep seated mass of magma that cools very slowly to form coarse grained rock. Can be several hundred of kilometres wide (e.g. Cornubian Batholith, Cornwall).
describe a sill formation
A horizontal intrusion along the bedding planes with vertical cooling cracks. Cools rapidly, contracts and cracks (e.g. Whin Sill, North Yorkshire)
describe a dike formation
A vertical intrusion with horizontal cooling cracks that cuts across the bedding planes. Cools rapidly and cracks (e.g. Kildonan, Arran in Scotland)
describe how geysers form
an intermittent turbulent discharge of superheated water ejected from deep within the crust (geyser means ‘to gush’ in Icelandic).
describe how boiling mud forms
where hot water on its way up mixes with muds near the surface, a bubbling, boiling mud pool may form.
describe how hot springs occur
dissolved salts from the ejected, superheated water are laid down in spectacular calcium carbonate deposits.
major extrusive landforms
Volcanoes are major extrusive landforms. Classification of volcanoes can be based on:
shape (morphology) of volcano and its vent,
type of eruption,
frequency of eruption.
fissures eruptions
An elongated split allowing basaltic lava to spill out, found at constructive margins, ocean ridges and rifts. Persistent, gentle eruptions (e.g. Heimaey, Iceland).
shield volcano
Gently sloping cone of basaltic lava, found at diverging margins and hot spots. Predictable, gentle eruptions (Mauna Loa, Hawaii)
composite volcanoes
Made of layers of ash and andesitic lava from subsequent eruptions, found at converging destructive margins. Explosive, unpredictable. Pryoclastic eruptions (Mount Etna and Vesuvius, Italy)
acid or dome volcanoes
Steep-sided cone formed from viscous, rhyolitic lava found at converging continental crust. Lava cannot travel far, solidifies in vents producing explosive, unpredictable eruptions (Puy de Domes, France)
calderas
A cauldron shaped summit formed by gases building up beneath a blocked vent and a subsequent catastrophic eruption that blows apart the crater. Caldera can fill with water – forming a crater lake (Lake Nyos, Cameron). Destructive margins. Explosive, unpredictable.
what is the focus?
the point within the crust where the pressure is released
what is the epicentre?
the place on the earths surface directly above the focus
how do earthquakes occur?
Earthquakes occur where a build up of pressure within the Earth’s crust is suddenly released and the ground shakes violently.
what are P waves
primary waves; fastest waves, shake the earth backwards and forwards, move through solids and liquids
what are S waves
secondary waves; slower, move more in a sideways motion, (shaking the Earth at right angles to the direction of travel) they cannot move through liquids but do more damage than P waves
what are surface waves?
travel nearer to the surface, more slower than p or s waves, but more destructive. they include; L wave, move sideways and Rayleigh waves, move up and down.
what is the Richter scale?
(ML) a logarithmic scale with each unit representing a 10 fold increase in strength/magnitude
what is the Mercalli scale?
12 point scale, measuring the effects of a seismic event
what is the moment magnitude scale?
(MW) to measure the size of earthquakes in terms of the energy released. developed to succeed the Richter scale, more accurate for quakes above 3.5.
what is liquefaction?
Extreme shaking reduces pore water pressure which reduces the adhesion and shear strength of soil so it fails more easily.
what are soil volcanoes?
Extreme shaking can also cause groundwater to come to the surface, bringing liquefied soil with it.
what is the seismic distribution
- Earthquakes are not evenly distributed but tend to occur in broad
- Most powerful earthquakes are related to destructive plate boundaries, subduction
what is tsunami genesis
The first stage in the creation of a tsunami train, tsunamis are most commonly created by earthquakes. Genesis commonly follows this path:
- Fault uplifted
- Displacement of sea bed
- Ocean water displaced
- Waves radiate from epicentre in both directions
- Water shallows near coast, waves slow and increase in height
- Waves continue to hit coast for hours,
what is tsunami run up
the approaching wave and the temporary rise in local sea level. Depends on:
- cause of wave, eruption or earthquake
- distance from epicentre
- water depth
- friction and topography of sea bed
- coastline orientation
what is tsunami land fall?
the moment of contact inundation can last hours, depth and destruction will depend on:
- land uses
- population density
- any warning given
- the physical nature off the coast & orientation of coastline
- Natural ecosystem capacity to ‘buffer’ (i.e. wetland, mangroves will absorb wave energy and water volume)
what is tsunami drawback
before a tsunami waves strikes, the ocean is sucked back. fish are often left stranded and huge areas of the shore are revealed.
tsunami wave anatomy
In the open ocean tsunami waves are only 1 to 2 metres high. As the waves approach land friction with the sea bed slows the waves and their height increases. They may be up to 20 metres high.
