Tectonics Flashcards
what are intra plate earthquakes
occur in middle or interior of tectonic plates and are much rarer than boundary earthquakes
what is a volcano
a landform that develops around a weakness in the earths crust from which molten magma, volcanic rock and gases are ejected and extruded.
what are seismic hazards
generated when rock within 700km of the earths surface come under such stress that they break and become displaced.
where do most earthquakes occur
on plate boundaries
what % of earthquakes occur on pacific ring of fire
70%
name 3 places that earthquakes can occur
- ocean fracture zones eg mid Atlantic ridge
- continental fracture zones (across) activity found in mountain ranges eg Himalayas
- scattered earthquakes in continental interiors eg along old faults such as church stretten shropshire
where do volcanoes usually form
hot spots
how many volcanos are there in the world and how many erupt each year
500
-50 yearly
what is the size of a volcanic eruption determined by
amount of dissolved gases released
name some characteristics of continental crust
30-50km thick
-layer of granite, sedimentary and metamorphic rocks floating on mantle
name some characteristics of oceanic crust
6-8km thick
- thinner and composed of several layers
- top layer composed of basalt
what are intra plate earthquakes and how do they occur
- in the middle of the plate
- occur as over time stress builds and releases pressure in ancient faults causes to become active again
- however they dont occur in well defined patterns along plate margins and are harder to predict
what is a volcanic hotspot
- area in mantle where heat rises as hot thermal plume deep in the earth often called a magma plume.
- caused by heat and pressure melt rock of lithosphere
what is james huttons theory of the earth
- 1785
- realised erosion, deposition and uplift were connected and these processes operate continuously
- driven by earths internal heat
what other findings were concluded by james hutton
- after studing strata and rock deposits in various locations, he also concluded that the earth was much older than people believed at the time which was around 6000 years old
- came up with rock cycle (rocks transported to sea, buried, solidified and later lifted back to earth surface through tectonic processes.
what was Alfred Wegeners theory
continental drift
-suggested 200 million years ago supercontinent called pangea broke into pieces and parts moved away from one another and fragments we see today are pieces of that
how did wegener support his theory
by finding matching rock formations and similar fossils across continents
-concluded these were onced together
why was wegener ridiculed
because of religion
what theory did harry hess come up with
1962
- hypothesis of sea floor spreading
- theory of oceans basins which outlined a theory which later was known as seafloor spreading
- he discovered oceans were shallower in the middle
how did Hess conclude his theory
identified presence of mid-ocean ridges which were as high as 1.5km above flat sea floor and envisaged that oceans great from their centres with molten material/basalt oozing up from the earths mantle creating sea floor spreading each side
what did john tuzo wilson come up with the theory of
1963
- was sceptical of tectonic theory but then became a great supporter
- wondered why volcanoes occur 1000s of miles from plate boundaries and proposed hotspots which occur in middle of plate form volcanic island chains
what did tuzo wilson discover
-the idea of a 3rd plate boundary called transform faults or Conservative plate boundaries
what is the plate tectonic theory
lithosphere broken up into several major and minor plates
-these plates moved relative to each other over asthenosphere and a number of processes drive their movement
what is the asthenopshere
mantle below lithosphere made of semi-molten rock
what is Palaeomagnetism and how did it help confirm a theory?
study in 1950s confirmed sea floor spreading
- every 400,000 years the earths magnetic fields change direction and N and S swap
- when lava cools and becomes rock, minerals inside the rock line up w/ earths magnetic direction/ polarity at the time
what is mantle convection
- heat produced by radioactive decay in core heats lower mantle creating convection currents
- hot magma currents move within asthenosphere in circles causing plates to move
(this theory now less responsible for plate movement)
what is slab pull
newly formed oceanic crust at mid-ocean ridges becomes denser and thicker as it cools.
-causes it to sink into mantle under its own weight pulling the rest of the plate down with it
which is a more relevant theory of plate movement between mantle convection and slab pull
slab pull
what is subduction
-process of plate being destroyed when either two oceanic crusts or oceanic and continental plates move towards each other and the less dense slides under the other into the mantle where it melts into an area called the subduction zone
what is seafloor spreading
- process of a new crust pushing tectonic plates apart
- mid ocean ridges or underwater mountain ranges are formed when hot magma is forced up from the asthenosphere and hardens to form a new oceanic crust
how much do plates move a year
5-15 cm
name 3 types of plate boundary
convergent-destructive margins
divergent- constructive margins
conservative- transform margins
what is a divergent margin
two plates moving apart leading to formation of a new crust hense constructive.
-in oceans this divergence forms mid ocean ridges and on continents forms rift valleys
example of a divergent plate boundary w/ oceanic crust
- mid atlantic ridge
2. 3km long
describe the tectonic hazards at mid ocean ridges
-earthquakes- shallow focus as less than 70km into crust and occur frequently but pose little threat to humans as they occur underwater
-volcanoes- submarine volacnoes can occur which grow above sea level to create a new island eg) iceland which is on mid atlantic ridge
however these eruptions are less explosive and more effusive especially underwater
what does effusive mean
-eruption where lava flows steadily from volcano
what is a rift valley and how is it formed
- when continental plates move apart at divergent plate margins
- the crust breaks and stretches into sets of parallel cracks called faults
- the land between the fault collapses forming steep sided rift valleys
example of rift valley
east african rift valley, divergent plate boundary with continental crust
describe the tectonic hazards at rift valleys
earthquakes-similar to midocean ridges, shallow but low magnitude
no volcanoes
what is a locked fault
CONVERGENT
in a subduction zone as plates move together they can get stuck due to frictional forces
-these faults may store strain for long periods and pressure will build and is eventually released in a large magnitude earthquake
what is the benioff zone
area of seismicity corresponding with slab being thrust downwards in subduction zone
why do earthquakes occur at destructive plate boundaries
-in sybduction zone where two plates join together and the moe dense plate slides under the thicker less dense plate forming a locked fault where there is a build up of frictional resistance and stress which can be released as an earthquake in the benioff zone
how do deep ocean trenches form
when oceanic crust subducts underneath continental cust it forms deep ocean trenches, continental plate is pushed up to form fold mountains chain
describe the tectonic hazards at convergent plate boundaries on oceanic meets continental
-explosive volcanoes are generated here as magma created from melting oceanic plate pushes up through the faults in the continental plate
why is nepal prone to earthquakes and how big was the 2015 Earthquake
sits on continental meets contental plate boundary between eurasian and india plates and is a conservative boundary where two continental plates collide
-2015 7.8 mag earthquake
what is the pacific ring of fire
oceanic pacific plate meets many plate boundaries causing hundreds of earthquakes
where can deep ocean trenches form
convergent
what is the difference between oceanic meets continental and oceanic meets convergent plate boundary
- when oceanic meets continental plate boundary the oceanic plate subducts as it is thinner and forms deep ocean trenches and the continental plate is pushed up into fold mountains
- when oceanic meets oceanic they converge and the faster more dense plate would subduct and mountains that are formed, are formed by continuous eruptions of magma from asthensosphere
when two continental plates meet what is the margin called
collision margin because neither plate is subducted or destroyed but is push upwards into fold mountains
-because both plates are similar density and less dense than asthenosphere beneath
may be some subduction inevitably caused by compressed denser sediments
example of fold mountains
himalayas
describe what a conservative plate margin is
aka transform fault-
two plates are sliding past each other resulting in a major break in the crust between them as they move
-the break is called a fault and on large scale is transform fault
give example of transform fault and how big was the last earthquake here
san andreas
1906- 8.3
movement to the left at a conservative plate boundary is called
sinstral
movement to the right at a conservative plate boundary is called
dextral
describe tectonic activity at conservative plate boundaries
- no volanoes as neither plate subducts so no magma is formed similarly to continental meets continental
- shallower powerful earthquakes as large build up of stress due to frictional resistance plates stick as they move past one another causing stress and pressure to build
is there a benioff zone at conservative plate boundary
no
why could next earthquake at san andreas be big
100 years since last one so a lot of stress will have built up and because it is shallow focus it will have large impact
why are volcanoes more likely along some plate margins than others
common where one plate subducts such as oceanic and continental where the oceanic plate subducts and melts to form magma which can rise to the surface forming volcanoes.
-less likely at C meets C as neither subducts to form magma and also same at conservative
how do plates behave at conservative plate margins
slide past one another
what % of earthquakes are along plate boundaries
95%
what is an earthquake
energy is released as seismic waves which cause the ground to shake and is released from inside earths crust along a fault
what is the hypocentre
point inside the crust where pressure is released
aka focus
what is the epicentre
directly above hypocentre on top of earths crust
what are P waves
+characteristics
- aka Primary or body waves which travel through earths body
- fastest and first to reach surface (8km/sec)
- travel through solids and liquids
- move in backwards and forwards motion (longitudinal)
- only damaging in most powerful earthquakes
what are L waves
aka Love waves
- they only travel on earths surface
- slowest and last to arrive
- move in side to side motion and are larger and cause MOST DAMAGE
what are S waves
- secondary waves which are also body waves as they go through earths body
- slower than P (4km/sec)
- only through solids
- transverse waves in up and down motion 90 degrees to direction of travel
- more damage than P
how is severity of earthquake determined in terms of seismic waves
- amplitude and frequency
- ground may be displaced horizontally/ vertically or slanted obliquely during earthquake depending on strength of individual waves
how do we predict earthquakes
currently no method to accurately predict when or where earthquake will hit but we can forecast where it is likely to happen with current understanding of how plate boundaries work
-research focuses on foreshocks which can suggest major earthquake is going to happen
what human factors can determine effects of earthquake
- population density
- level of development
- effectiveness of emergency response teams
- impact of secondary hazards
- how prepared they are education
what are primary effects of an earthquake
happen as direct result of earthquake
what are secondary effects of an earthquake
secondary effect of earthquakes happen as result of primary effects and often cause more damage
examples of primary effects
- ground shaking causes buildings roads infrastructure to collapse
- crustal fracturing- when earths crust cracks due to energy released
examples of secondary effects
- landslides or avalanches
- tsunamis
- liquefaction
how do landslides or avalanches occur due to earthquakes
-slopes fail as shaking places stress on them resulting in landslides, rock slides mudslides and avalanches
how does liquefaction occur
- when surface rock lose strength and become more liquid than solid.
- the subsoil loses its ability to support buildings and infrastructure foundations so they sinks or tilts
how do aftershocks occur
occur in area of original earthquake and are a result of the earth settling down or readjusting along part of the fault that slipped originally
- can occur weeks/months/years after quake
- larger earthquakes have larger more frequent aftershocks
- capable of causing additional damage and stopping recovery efforts
example of large aftershock
2011 6.3 in christchurch after 2010 quake and actually caused more damage than orginal and loss of life
when was the loma prieta earthquake
1989
what was the main reason for damage in lloma prieta
rock type- softer rock amplifies the effects of the earthquake
- the marina district was built on soft sandy soil which amplified ground shaking and liquified causing buildings to collapse
- the area of the freeway built on bedrock did not collapse
what kind of plate boundary did the loma prieta earthquake occur on
conservative
what is a volcano
most next to plate margins, some on hotspots
area in earths crust where lava gas and ash erupt
-get bigger as more eruptions occur
over last 300 years how many have died due to volcanic hazards
260,000
primary hazards of volcanos
- lava flows
- pyroclastic flows
- tephra and ash falls
- gas eruptions
what are lava flows
how hot??
- streams of lava erupt onto earths surface, destroy everything in path but not generally life threatening as its slow moving
- 1170 degrees and take years to cool
what are pyroclastic flows
how fast flow?
how hot?
- mixture of hot dense rock, lava, ash and gases ejected
- destroys everything in paths
- much faster than lava flows as they are 100km/h
- 700 degrees
what are tephra and ash falls
- tephra is pieces of volcanic ash and rock that blast into the air during eruptions
- large pieces tend to land near volcano when they can injure/kill and damage structures
- smaller pieces can travel 1000km’s
- ash can be very disruptive and covers everything causing poor visability and slippery roads. roof may collapse under weight and engines can be clogged and stop working
what are gas eruption
- magma contains dissolved gases that are released into atmosphere during eruptions
- water vapour 80%, C02 and sulphur dioxide GHGs
- can travel 1000km’s
- some gases can be potentially hazardous to humans and livestock
what are secondary hazards of volcanic eruptions
- Lahars
- Jokulhlaup
what are lahars
- masses of rock that travel quickly down the sides of the volcano
- very in size and speed can we 100m’s wide and travel 10m/s so too fast to outrun
- A lahar is a mudflow composed mainly of volcanic debris and water. They are formed when rainwater washes away the volcanic ash on a volcano slope. Sometimes caused lahars when crater lakes leak. Lahars are among the biggest risks of a volcanic eruption
what are jokulhlaup
- aka glacial outburst floods
- heat of volcano melts snow and ice in glacier causing heavy sudden floods
- suddenly release large amounts of water, rock and gravel and ice that can be extremely dangerous and flood land and structures
how do scientists predict eruptions
- more accuracy than earthquakes
- use equipment such as GPS and satalite based radar placed on and around a volcano to monitor the signs:
- small earthquakes which caused by magma rising+breaking rock which can be detected by seismograms
- changes to surface volcano swelling due to magma
- changes in tilt of volcanoes moving magma inside can change slope or tilt
when was the eyjafjallajokull eruption
march to april 2010
how high did the plume of very fine volcanic ash of icelands volcano go
33,000 ft
which spread over west europe
what were the secondary impacts of the iceland eruption
jokulhlaups
-warm 17 degree water
what were the social impacts of the eyjafjallajokull eruption
- ash contaminated local water supplies
- evacuating due to lava melting glacier causing flooding
- people had to were face masks and goggles due to thick ash
- many roads shut
- stranded people who couldnt board flights
how many evacuated in iceland eruption
700
how many passengers unable to board flights
10 million
economic impacts of iceland eruption
- commercial flights cancelled
- airlines lost lot of money
- european economy lost money
- many different european transport companies such as trains able to benefit
how many more passengers did eurostar see
50,000
how many commercial flights cancelled
100,000
how much did airlines lose in iceland eruption
$1.7 billion
how much did europe economy lose in iceland eruption
$5 billion
what is a tsunami
series of larger than normal waves usually caused by underwater earthquakes but some caused by underwater landslides or asteroid strikes
-energy released during quake causes sea floor to uplift, displacing water collumn
what is a water collumn
an area of seawater from surface to sea floor
are ships able to detect tsunami waves
its harder as the wave is smaller out to sea as there is small wave height (1m high out to sea but 30m inland) and long wavelength of around 150-250m
how can some energy of the tsunami be lost when nearing coast
sea floor irregularities
what does the nature of tsunami waves depend on
- cause of wave eg eruption of earthquake
- distance travelled from source as energy is lost as they travel
- water depth over route affects energy loss through friction
- offshore topography and coastline orientation
why is wave higher closer to shore
-wave energy is concentrated into a smaller volume of water
name some characteristics of a tsunami
- fast up to 50mph
- when wave crest reaches shore it first produces vacuum effect by sucking water out to sea exposing a large amount of sea floor which is a warning sign
- linked to plate boundaries eg ring of fire
how can you predict tsunamis
- cant predict before occur as underwater earthquakes dont effect us
- can notice early warning signs before it reaches the coast and threatens lives
how can you monitor tsunamis
seismic sensors to detect sub-marine earthquakes
DART Deep ocean Assessment and Reporting of Tsunamis
Computer modelling
what is DART
seabed sensors and surface buoys to monitor changes in sea level and pressure
- when wave detected system sends information via satellite to tsunami warning stations
- stations analyse data to estimate size and direction
pros of monitoring tsunamis w/ computer modelling
- can save lives
- info reviewed regularly due to multiple sensors and sea surface buoys
cons of computer modelling
- if equipment faulty will cause inaccurate data
- if shallow earthquake equipment damaged
where did the 2004 indian ocean tsunami badly hit first and how large was the earthquake that caused it and how much did it displace the ocean floor
Sumatra after 15 minutes
-estimated 9.0 magnitude earthquake heaved ocean floor up 15m sending out shock waves
how many people died in indian ocean tsunami
300,000
where did the 2004 indian ocean tsunami hit
- sumatra
- malaysia
- thailand
- sri lanka
- india
- maldives
- seychelles
- madagacar
- somalia
- kenya
- south africa