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A Level Geography Tectonics > Topic 1 - Tectonic Processes and Hazards > Flashcards

Topic 1 - Tectonic Processes and Hazards Flashcards

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1
Q

what is the LITHOSPHERE

A

the MANTLE (hotter and denser than crust, temp and pressure increase with depth) and the upper part of the mantle (cooler and more rigid than deep mantle) = rigid layer of rock = LITHOSPHERE

tends to be thinnest under oceans, broken ip into moving plates containing continents/oceans

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2
Q

what is the ASTHENOSPHERE and what is it composed of and where is it located

A

below lithosphere there is a mobile zone called asthenosphere

composed of hot, semi-solid material, which can soften and flow after high temperature and pressure

lithosphere thought to ‘float’ or move about on slowly flowing asthenosphere

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3
Q

how is the core composed

A
  • core is twice as dense as mantle because of metallic (iron-nickel alloy) composition
  • made up of a 2,200 km thick liquid outer core and 1,250 km thick solid inner core
  • magnetic field created by liquid outer core spinning as earth rotates, also influences plate tectonics
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4
Q

continental crust:
thickness
age
density
composition

A

THICKNESS 30-70km
AGE over 1500 million years
DENSITY light (average 2.6)
COMPOSITION light in colour, numerous types, granite most common

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5
Q

oceanic crust
thickness
age
density
composition

A

THICKNESS 6-10km
AGE young (under 200 million years)
DENSITY heavier (average 3.0)
COMPOSITION dark in colour, few types, mainly basalt

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6
Q

how do we know what the earth is made of

A

geophysical surveys - seismic, gravity, magnetics, electrical, geodesy

ACQUISITION - land, air, sea, satellite
GEOLOGICAL SURVEYS - fieldwork, boreholes, mines

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7
Q

what is the Moho discontinuity

A
  • the boundary between the crust and the mantle
  • marks the lower limit of earths crust, average depth of 8km beneath ocean basins, 32km beneath continental surfaces
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8
Q

what happens at a discontinuity

A

used for a surface at which seismic waves change velocity.

one of these is the moho discontinuity, and seismic waves accelerate

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9
Q

what is CONTINENTAL DRIFT

A

the theory that the continental landmasses have changed position over time by Alfred Wegener

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10
Q

what is the theory of plate tectonics

A
  • earth’s lithosphere is comprised of large tectonic plates (thin planes of solid rock)
  • they float above molten asthenosphere
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11
Q

difference between oceanic and continental plates

A

CONTINENTAL plates = above sea level
OCEANIC plates = below sea level

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12
Q

what is a CONVERGENT boundary

A

(destructive boundary)
- area where two+ lithospheric plates collide
- one plate slides beneath the other (subduction)

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13
Q

where do convergent (destructive) boundaries occur

A

can occur between:

  • oceanic-oceanic lithosphere
  • oceanic-continental lithosphere
  • continental-continental lithosphere
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14
Q

where do convergent (destructive) boundaries occur

A

can occur between:

  • oceanic-oceanic lithosphere
  • oceanic-continental lithosphere
  • continental-continental lithosphere
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15
Q

what happens at the subduction zone

A

WADATI BENIOFF ZONE (subduction zone) is where earthquakes occur

these collisions can cause volcanism, earthquakes, orogenesis, destruction of lithosphere and deformation

geological features vary with crust type

(80% earthquakes at convergent boundary)

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16
Q

what is a DIVERGENT boundary

A

(constructive boundary)

  • two plates move away from each other
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17
Q

what can divergent boundaries produce

A
  • can produce rifts = rift valleys
  • form volcanic islands, occur when gaps are produced and magma rises to fill
  • produces earthquakes
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18
Q

where do divergent boundaries occur

A
  • most occur between oceanic plates and exist as mid-oceanic ridges
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19
Q

what is paleomagnetism

A

involves the study of the magnetic field by looking at the rocks formed from underwater eruptions

  • basalt (cool from high temp, high levels of magnetite) is susceptible to being magnetised but sedimentary rocks with small amounts of magnetite take on remnant magnetism
  • rocks can be dated and patterns of movement tracked and documented
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20
Q

how do fossils support continental drift

A
  • fossils of same species of extinct plants found in rocks same age but in different continents
  • eg. Mesosaurus found in Southern Africa and Eastern South America = they must have been joined
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21
Q

what is the importance of internal heating

A

provides energy for plate tectonic motion

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22
Q

what happened to influence the earths core temperature

A

earths core = 5500 degrees, as a result of :

  • extraterrestrial impacts
  • gravitational contraction of the earths interior
  • radioactive decay of unstable isotopes
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23
Q

what is radioactive decay and its importance

A
  • exothermic reaction (release heat)
  • eg. uranium 238 decays to lead 206
  • as it does this it tops up and generates heat in the earths interior
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24
Q

what is convection

A
  • CONVECTION causes movement
  • process by which hot, less dense material rises upwards away from heat source and are replaced by downward and lateral flowing colder, denser materials
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25
Q

how do rocks act in the creation of convection currents

A

the temperatures in asthenosphere and mesosphere are hot enough (>1300degrees) to cause rocks to become weak and pliable = convection currents

26
Q

how are convection currents generated

A
  • by the heat produced from radioactive decay in the core
  • currents linked to movement of plates leads to creation of new oceanic crust from mantle or destruction of existing crust
26
Q

what is ridge push

A

operates at divergent margin where plates pull apart from each other

the lifting and pulling apart of plates at ocean ridges/spreading zones

27
Q

what is slab pull

A

operates at the convergent margin where plates move together and denser plate pulled down into subduction zone

cool, dense crust sinking into earth in subduction zones pulling entire plate down with it

28
Q

what differs the pacific and atlantic ocean floors

A

pacific moves quicker than atlantic because it has both ridge push and slab pull acting on it while atlantic only has ridge push which it weaker

29
Q

how is tectonic activity created

A

Earth is internally heated through radioactive decay which releases heat in the form of convection currents, which on failing to escape the crust, form convection cells.
The mechanisms of ridge push and slab pull are then involved in the dynamic movement of plates, leading to tectonic activity

30
Q

how do plates move

A
  • plates of lithosphere moved around by hot convection currents (cc)
  • these large cc in asthenosphere transfer heat to surface, where plumes of less dense magma break apart plates at spreading centres = creation of divergent plate boundaries
  • plates move from spreading centres, cool, and higher density basalt rocks that make up ocean crust get consumed at ocean trenches/subduction zones
  • crust is recycled back into asthenosphere
31
Q

what is subduction

A
  • oceanic lithosphere subducts under continental lithosphere
  • oceanic lithosphere dehydrates as it subsides
  • the melt rising forms volcanoes
32
Q

what landform occurs at oceanic/continental convergence boundary + example

A
  • Ocean Nazca plate moving east at approx 12cm/year, converges with and subducts beneath continental South American plate moving westwards
  • The Andes is a chain of fold mountains rising to nearly 7000m above sea level, formed as continental crust been buckled and uplifted
33
Q

what landform occurs at oceanic/oceanic plate convergence

A
  • island arcs found in linear pattern though tend to be curved/arc-shaped
  • usually located offshore eg Mariana Islands
  • formed at destructive margins
  • subducted crust melts and low density molten material rises, aided by compressional forces from convergence to form chain of volcanic islands
  • where edge of subducting plate is curved in shape eg pacific plate islands form an arc
34
Q

what landform is found as oceanic crust subducts under continental crust

A
  • ocean trenches found in linear pattern located just offshore eg Peru-Chile trench off west coast of South America
  • some trenches eg Mariana trench in west pacific ocean are further offshore and are result of convergence of two areas of oceanic crust and subduction of more dense of the two
35
Q

what is a collision margin and what landform is created

A
  • convergence of two plates of continental crust
  • no subduction as both low density plates
  • intervenes oceanic sediments trapped between two converging plates are heaved upward = formation of major fold mountain ranges eg the Himalayas
  • no volcanic activity as no crust destroyed by subduction but earthquakes occur
36
Q

what happens at mid-atlantic ridge

A
  • divergence of eurasian and north american plates in mid-atlantic
  • ridge and rift system extends along mid-atlantic for about 10000km
  • created 60m years ago as Greenland and NW Scotland separated to form Atlantic Ocean
  • underwater volcanoes are along margin, sometimes appear above sea level as volcanic islands, largest being Iceland
37
Q

what is an ocean ridge

A
  • form a linear pattern, normally through middle of oceans eg mid-atlantic ridge
  • form at constructive margins where two oceanic plates are diverging
  • mid-oceanic position due to creation of oceans in which they now lie
  • eg. Galapagos Ridge moves at intermediate rate, East Pacific Rise moves at rapid rate
38
Q

what happens at East African Rift Valley

A
  • constructive margin in area of continental crust
  • east africa moving in northeasterly direction, diverging from main African plate which moves north
  • rift valley formed when sections of crust between parallel fault lines subside
  • east african rift valley consists of two parallel rift valleys extending 4000km from Mozambique to Red Sea
  • experiences volcanic activity suggesting crust has been weakened and thinned by tension resulting in magma escaping onto surface
  • thought to be embryonic constructive plate margins
39
Q

what is a conservative margin

A

where two plates move laterally past each other, no volcanic activity as no crust destroyed and no new crust formed by rising magma

40
Q

what occurs at conservative margins

A

shallow focus earthquakes of varying frequency and magnitude occur
low magnitude, high frequency events also occur when pressure along margin is easily released

41
Q

what is an example of conservative margin

A
  • movements of crust occur along fault lines like San Andreas fault
  • pacific plate moving NW approx 6cm/year while North American plate only moving at 1cm/year
  • low key landforms but earthquakes common and can be devastating
42
Q

what is a hot spot

A
  • fed by a region deep within Earth’s mantle from which heat rises through process of convection
  • rock melts at base of lithosphere because of heat, where brittle, upper portion of mantle meets Earth’s crust, magma formed pushes through cracks to create volcanoes
43
Q

where do hot spots occur

A
  • doesnt occur at tectonic plates, occurs at abnormally hot centres known as mantle plumes
44
Q

what is a diapir

A

as the plume head reaches lithosphere, spreads into mushroom shape reaching 500-1000km diameter, this is a diapir

45
Q

what is a seamount

A

volcanic activity at hot spots can create submarine mountains - seamounts

estimated they make up 28.8m sqkm, can rise hundreds/thousands metres from seafloor

46
Q

what is the importance of the hawaiian islands

A
  • chain of volcanic islands lying over stable hot spot
  • pacific plate been moving for 70m years and succession of volcanic islands and underwater volcanoes formed over that time
  • lava flows, tephra (airborne lava fragments) , volcanic gases, explosive eruptions, ground cracks and settling happens at Hawaiian volcanoes
47
Q

distribution of volcanoes

A
  • along ocean ridges where plates move apart (mid atlantic ridge)
  • associated with continental rift valleys (mt kenya)
  • along subduction zones (pacific ring of fire)
  • over hot spots (hawaiian islands)
48
Q

distribution of earthquakes

A
  • along plate boundaries, most powerful being associated with convergent margins
  • some occuring away from plate boundaries and are associated with reactivation of old faults
49
Q

what is the mercalli scale

A
  • used for measuring intensity of an earthquake based on observation of effects
  • effect of earthquake on Earth’s surface, humans, objects of nature and man made structures on scale of I to XII with this being the worst
50
Q

what is the richter scale

A
  • developed as mathematical device to compare size of earthquakes determined by logarithm of the amplitude of waves recorded by seismographs
  • magnitude expressed as whole numbers and decimals eg 5.3 moderate earthquake, 6.3 strong, each level is 10x stronger
51
Q

what is the volcanic explosivity index (VEI)

A
  • explosive volcanic eruption can be compared, can be used for historic and current volcanoes
  • volume of pyroclastic material ejected by volcano (ash, tephra, pyroclastic flows)
  • height of eruption column and duration of eruption also considered
52
Q

how is VEI measured

A
  • begins at 0 for eruptions producing less than 0.0001 cubic km of ejecta
  • eruptions at VEI 1 produce between 0.0001 and 0.001 cubic km of ejecta and at VEI 2 produce between 0.01 and 0.1 cubic km
  • each step = explosivity increase of 10x, two steps = 100x increase
53
Q

how is magnitude measured

A
  • mercalli and richter scales
  • vei measures magnitude of earthquakes
  • usually higher the magnitude, worse effects
54
Q

what is the influence of predictability

A
  • volcanoes generally predicted with high success and more lives saved than earthquakes
  • earthquakes extremely unpredictable and more lives lost
55
Q

relationship between frequency and magnitude

A

usually higher magnitude = lower frequency

56
Q

how does duration relate to effects

A
  • longer event lasts = worse effects
57
Q

how does speed of onset relate to effects

A
  • faster a tectonic event strikes area, worse effects are gonna be due to lack of prep and response time
  • mitigation prep can help reduce effects, developed countries suffer
58
Q

how does areal extent relate to effects

A
  • volcanoes often have much further reaching effects than locality
59
Q

what is the dregg model

A

shows disaster in the middle of two circles
first circle showing hazardous natural event (landslide, hurricane, earthquake)
second circle showing vulnerable population

A Level Geography Tectonics (7 decks)

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