Lecture notes
Hazardous Earth Detailed Notes
- Module
- Hazardous Earth
- Institution
- OCR
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1.WHAT IS THE EVIDENCE FOR CONTINENTIAL DRIFT ANDPLATE TECTONICS
STRUCTURE OF THE EARTH
CONVECTION CURRENTS
- SLAB PULL = older, colder oceanic plates subduct as destructive plate margins,
because as they cool, they become denser than the underlying mantle. The cooler
sinking plate pulls the rest of the warmer plate along behind it.
- RIDGE PUSH = as the plates moves apart, magma rises at constructive plate
margins and cools, forming new crust. The new crust pushes the old crust either
side, further away.
Assess the role of convection currents within the mantle as a driver of plate movement.
6M
Convection currents occur within the molten rock in the mantle, acting as a conveyer
belt for tectonic plates. Slab pull is when the older, colder oceanic plates subduct as
destructive plate margins, because as they cool, they become denser than the
underlying mantle. The cooler sinking plate pulls the rest of the warmer plate along
behind it. Ridge push is when the plates move apart, magma rises at constructive plate
margins and cools, forming new crust. The new crust pushes the old crust either side,
further away.
EVIDENCE FOR SEA-FLOOR SPREADING
- Alfred Wegener – German scientist who first proposed that the continents had once
been a big landmass, Pangea and had drifted with time
- New rock erupted at the ridge displaces older rock so the sea-floor spreads away
from the ridge. The lateral movement of the new oceanic crust away from a mid-
ocean ridge results in a symmetrical pattern of geomagnetic reversals on either
, side of the ridge. This pattern of paleomagnetism is locked in igneous rocks formed
at the ridge.....................
AGE OF SEA-FLOOR ROCKS
- …..............The age of the sea floor rocks increases away from the mid-ocean
ridges. Volcanoes are carried away from the ridge where they originally were
formed, sometimes sinking below the sea’s surface and losing their capacity to
erupt as they are no longer above a suppl of rising lava.
PALEOMAGNITISM
- Magnesium in ancient rocks recording the direction and intensity of Earth’s
magnetic field at the time of the rock's formation
- As magma cools, iron minerals align themselves with the magnetic field, N or S,
and record direction and strength. The band on each side matched
- New rock fit with the N-S alignment
- Old rocks pointing to locations in N America, Asia and Pacific Ocean
EVIDENCE FROM ANCIENT GLACIATIONS
- Glacial evidence is present in rocks on continents that are now located close to the
equator
- Glacier scrapes were found on rocks in continents that are too warm to have
glaciers which means that these continents were likely to be closer to the south
pole before
FOSSIL RECORDS
, - Similar fossil reptiles exist either side of the South Atlantic in South America and
Southern Africa.
- Similar fossil shellfish records exist in Indian and Australian rocks
- The Glossopteris fauna is a plant fossil found in the South America, India, Australia,
Africa and Antartica
- Similar mountain ranges found in Europe and North America which were created
450M years ago
THE GLOBAL PATTERNS OF PLATES AND PLATE BOUNDARIES
- Most plate boundaries are divergent – Nazca plate and Cocos plate
- Convergent margins between the Pacific plate and Eurasian and the Fiji and
Philippines plates stretching across Japan and Malaysia
- Most convergent are located EuroAsian and North America – Philippines plate and
Indo-Australian plate
DIVERGENT (CONSTRUCTIVE) PLATE BUNDARY
- Moving away
- Volcanoes - Magma rising through the asthenosphere and forcing its way to the
surface
- Earthquakes – shallow focus and low magnitude
- Mid ocean ridges – long mountainous chains which remain hidden at an average
depth of 2.5km below the ocean surface. They are not continuous and are broken
up into segments by transform faults - vary in shape depending on the rate of
spreading which is determined by the amount of magma brought to the surface
- Small ocean rift valleys
- Black smokers – sea water seeps into rifts and heats up. As it rises towards the
surface it causes chemical changes in the basaltic ricks which leads to superheated
jets of water reemerging on the ocean floor
- Rift valleys – a steep side, flat based valley formed by the downward displacement
of a block of the earth’s surface
- Mid-Atlantic ridge
CONVERGENT PLATE BOUNDARY (O-O and C-O)
- Volcano - The denser oceanic plate is forced under the lighter continental one, into
the upper mantle. As the oceanic crust descends and starts to melt due to the
friction with the overlying continental crust. This melted plate is now magma which
will rise through the gaps because it is less dense than the surrounding
asthenosphere. Once it reaches the surface, this magma forms a volcano
- Lava = viscous created by complex, composite, explosive volcanoes.
- Fold mountains - formed at the edge of the continental plates as part of it are
uplifted during the subduction of oceanic plates. EXAMPLE – Andes between South
American plate and Nazca plate
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