14.1 Flashcards
what is the Mohorovic discontunuity?
boundary between the crust and mantle
lies at 35km below the continents and 10-15km below the ocean floor
aka moho line
what are the 3 primary concentic layers?
core, mantle and crust
asthenosphere
the layer that extends from 100km-300km down
semi-molten or viscous and can flow slowly
lithosphere
lies above asthenosphere
sandwiched between crust and asthenosphere
varies in thickness and boundaries with asthenosphere are difficult to defone adn it melts and inco-operates with asthenosphere
how do and where do convection currents exist?
asthenosphere
vast amounts of heat generated in the mantle
semi-molten asthenosphere flows carrying with it the solid lithosphere and crust
describe inner core
solid due to high pressure
2x denser than iron
describe mantle
lithosphere and asthenosphere
2900 km thick
thickness of crust
continental 35km
oceanic 5-10km
thickness of mantle
29000km
density of crust
continental 2.6-2.7 kg/m3
oceanic 3.0 kg/m3
density of mantle
3.3 kg/m3 at moho
5.6 kg/m3 at core
mineral composition of crust
continental - granitic, silicon, aluminium = SIAL
oceanic - balsatic, silicon and magnesium = SIMA
mineral composition of mantle
rich in iron and magnesium
name different types of seismic waves
primary (P)
secondary (S)
Love
Rayleigh
what cant S waves do?
pass through water
why are seismic waves helpful?
help determine different layers of earth
alexander von humboldt
came up with jigsaw theory in 1801, he believed that large chunks of continental land mass, used to fit together
alfred wegener
1912
theory of continental drift
in the carboniferous period the was one large continent called pangea which then broke off into smaller continents eg laurasia and gondwanaland
theory not accepted until mid 1900s
geological evidence to support wegeners theory
the fit of the continents such as south america and africa on either side of the atlantic
contemporaneous glaciation in south africa australia south america india and antartica suggests that they were one connected
mountain chains and some rock sequences on either side of oceans show great similarity eg NE canada and N scotland
biological evidence to suppport wegeners theory
similar fossil brachiopods found in australian and indian limestones
similar fossil repitles found in south america and south africa eg mesosaurus
fossils form rocks younger than the carboniferous period in places such as australia and india showing fewer similarities, suggesting that the followed different evolutionary patterns
what are 4 pieces of modern evidence to support wegeners theory
ocean floor mapping
magnetic stripping and polar reversals
sea floor spreading
concentration of earthquakes
ocean floor mapping
most geological processes occuring on land are linked to dynamics of ocean
after ww1 echo sounding devices began to measure ocean depth my recording time it took for sound signal to return to device
discovered roughness of ocean floor and mid atlantic ridge
magnetic stripping and polar reversals
1950s scientists discovered odd magnetic variations discovered odd magnetic variations across the ocean floor
alternating magnetic variations running parallel to the mid atlantic ridge alternating N to S, proved that as magma rises and the earths north and south poles switch every 30 years and land moves apart
seafloor spreading
mid ocean ridges mark a structurally weak zome where the ocean floor was ripped in two lengthways and magma erupts through the weak zones
at centre of ridge rocks are young and they get older as they move further away
youngest rocks have present day polarity
stripes alternate in magnetic polarity
concentration of earthquakes
1920s several prominent earthquake zones identifies parallel to trenches
zones became known as Wadati and Benioff
establishment of WWSSN allowed seismologists to map zones precisely
reveals plate boundaries
divergent plate boundary
plates are moving apart
magmas rises through asthenosphere due to convection currents moving in opposite directions due to intense heat
produces mid ocean ridges at 2.5km below the surface
ridges break not continuous and break into intervals and segments
forms pillow lavas
forms volcanic islands
causes chemical changes in balsatic rocks as dea water seeps into rifts and is superheated
how are pillow lavas formed?
magma erupting onto sea bed cools quickly, forming rounded mounds
how are volcanic islands formed?
mid ocean ridges are produced by overlying rocks being forced up into a dome as magma rises up to the surface
the rigid lithoshere is placed under great stress and fractures along parallel faults
eg Iceland
rifting away from mid-oceanic ridges
rift zomes are not confined to the ocean floor
they also occur on land and explain how continents break up
eg rift stretching from Red Sea towards Turkey
sometimes where the crust has been stretched faulting is caused and a sunken valley known as a graben in created
rift widens and magma erupts as the surface
if magma continues to well up, a new spreading boundary will form
what are the three different types of convergent boundaries?
oceanic continental
continental continental
oceanic oceanic
oceanic continental convergent boundary
denser oceanic plate is forced under the continental plate
subduction causes the deepening of the ocean at the plate boundary and forms an ocean trench - long narrow depressions with depths of 6000-11000m
normally asymetric
layers of sediment and sedimentary rock develop on oceanic plates adjacent to continents
rocks crumple and drag across continental plate
forming mountain chains such as the Andes
oceanic plate subducted at 30-70 degrees
fractures in Benioff zone
oceanic plate melts and rises to surface as plutons of magma creating further uplift of fold mountains
oceanic oceanic convergent boundary
slightly denser one subducts creating a trench
as magma rises, forming chains of volcanic islands eg North American plate subducted underneath Carribean plate forms antilles
eg Mariana trench is partly formed from this
continental continental convergent boundary
little if any subductions takes place as htey have simalar densities
eg collision of african and eurasian plates created the alps
conservative plate boundary
plates slide past each other in a shearing motion
no volcanic activity
frictional resistance to movement along the palte boundaries often causes a build up of pressure
causes rocks to fracture releasing a lot of energy as earthquakes
doesnt produce landforms
tears landscape
if rocks are exposed at surface movement between strata may be visible
drainage modified as river courses are deflected by movements
eg california san andreas fault
what are the two classifications for volcanoes?
shape
type of eruption
lava plateaux/ fissure eruptions
shield volcanoes may erupt along lines of fissures rather than a central vent spilling liquid lava in successive layers
form broad plateaux such as the Colombia plateaux - often cut by deep canyons that expose the layers of rock - Grand Canyon
extensive lava flow are basaltic in nature so they can they can flow great distances
flat and featureless
macrofeature
basic/shield volcano
3-4 miles wide and 1500-2000m tall
eg Mauna Loa
built up slowly by accreation of thousands of flows of highly fluid basaltic
low silica so spreads over wide areas
cools as thins forming gently dipping sheets
lava erupts form rift vents along fractures that develops on the flanks of the cone
magma has low gas content and is low in silica allowing it to flow over wide distances
acid/ dome volcanoes
craggy, steep sloped convex sides covered with rock debris
typically found near large composite volcanoes
made of layers of lava
formed by repeated violent eruptions and slow moving lava flows which gives layered structure
magma is made of higher silica and high gas pressure making magma slow moving and explosive
eg Puy region of central france
ash-cinder cones
formed from ash, cinders and volcanic bombs ejected from crater
sides are steep and symmetrical
eg Patricutun, Mexico
may occur as single volcanoes or secondary volcanoes on the sides of stratovolcanoes or shield volcanoes
tephra ejected
lava cools and builds up around the vent forming crater
build up over time
composite volcano/ stratovolcano
tall and conical
built up from many layers of hardened lava, tephra, pumice and volcanic ash
steep profiles
periodically explosive and quiet eruptions
lava typically cools before spreading due to high viscocity
layered structure built up of sequential outpourings of eruptive materials
increase in temp causes dome to expand while its outer lava cools, causes newly hardened surface to splinter causing loose debris to fall from its sides
eg Mt Et na
calderas
bowl shaped
