Physical Geography
Theme A: Plate Tectonics
Plate Tectonics: Margins and Landforms
Specification- demonstrate knowledge and understanding of the evidence for
and the theory of plate tectonics
Earth's three main sections:
Core- the inner core is solid the outer core is liquid
Mantle- thickest section, 2900 km thick
Crust- oceanic is thinner and more dense so heavier, and continental is thicker
and less dense so lighter
Lithosphere: consists of crust and upper part of the mantle, solid and brittle
Asthenosphere: upper part of mantle just below lithosphere, solid but flows,
convection currents occur here
The theory of plate tectonics:
● The modern theory of plate tectonics suggests that the Earth's lithosphere is
divided into 8 major plates and several minor plates
● It appears that while the rocks of the continents are often ancient, even billions
of years old, the ocean floors that form most of the earth’s outer skin are young
● The huge jigsaw pieces that carry these oceans and continents jostle and move in
three common patterns, in opposing directions: towards, away or past each
other
● Earth’s surface is made of rigid plates (13/14) that form lithosphere
● Plates rest on a mobile layer called asthenosphere which enables them to move
2-20cm per year
This theory is called Continental Drift (Jigsaw fit):
● Alfred Wagner proposed that the world’s continents had formerly been a single
landmass called this PANGEA
● Which over the past 200 million years had gradually broken up
● The evidence he gathered included the parallel nature of some continental
coastlines, along with other rock, fossil and past climate evidence
● Some coastlines fit together almost perfectly but have been pulling away from
each other for a long period of time and areas of gaps or overlaps can be
explained by coastal erosion and deposition, rises in sea level (eustatic change)
and changes in land level (isostatic change) since separation
● This provides evidence of seafloor spreading
,Evidence for theory of plate tectonics:
Distribution of rock types and the mountains:
● Relates to the patterns of rock formations across the world that reveals evidence
of plate tectonics and continental drift
● Rocks of similar age, type and structure are found in parts of the world separated
● This can be seen in fragments of the ancient Caledonian fold mountain belt
formed 450 million years ago are found in widely separated continents of
Greenland, Canada, Ireland, England, Scotland and Scandinavia
● Showing the plates drifted apart over millions of years due to sea-floor spreading
in the Atlantic ocean (constructive boundary) and would of originally been in the
middle of Pangea
● Geology of western Africa and eastern South America run parallel to each other.
Their coastlines and the edge of their offshore continental shelves would form a
neat junction if the South Atlantic did not exist
● Showing they were once joined together and have slowly separated over millions
of years
Fossil evidence:
● There are fossilised remains of the same animals and plants located in separated
continents
● Fossils of Triassic land reptile Lystrosaurus have been found in modern day
Antarctica, India and Africa which are now separate
● Fossils of fern Glossopteris have been found in five continents
● This suggests land masses were once joined and fossils would of formed one
band in Pangea but have drifted apart over millions of years, confirming plate
tectonics theory
Ice sheets:
● Glacial deposits formed 300 million years age are found in Antarctica, Africa, S
America, India and Australia
● If continents haven’t moved, ice sheet must have extended from S pole to
equator, which is unlikely as UK at this time was close to equator and it has
extensive coal and limestone deposit
● Evidence continents have moved away as large plates
● More evidence is from glacial striations, scratches on bedrock made as the
glacier moves. Showing the direction of the glacier and suggesting ice flowed
from a single central point
● It was clear from the landforms on their surface that these continents, including
tropical hot desert regions, had in the past been covered and carved by huge ice
sheets. The direction and flow of these sheets only made sense when these
continents were placed together
● Wegener's theory was not largely accepted as there was no proof of how
movement occurred. It took later evidence from seafloor to clarify the
mechanisms for continental drift
, Topography of the oceans:
● First accurate maps of ocean basins created by sonar showed they had huge
underwater mountain ranges with deep central valleys, which formed continuous
mountain chains for 50,000 km around earth, called mid-ocean ridges e.g. Mid
Atlantic ridge
● Rift valleys are similar to other rift valleys in continents that are pulling apart
showing the ocean floor has been pulled apart (sea floor spreading)
● They form at constructive plate boundaries
● Also, there are very deep ocean trenches of the coast of some continents e.g
South America which shows the sea floor was sinking down into the
asthenosphere
● Here new seafloor is being created at the ridges and then moving away from the
ridges the seafloor is subducting down in the asthenosphere at the trenches, the
seafloor is moving
The age and pattern of ocean basin geology:
● As you move out from the mid-Atlantic Ridge towards the continents
surrounding the Atlantic Ocean, the rocks get progressively older, following a
symmetrical pattern either side of the ridge
● Therefore the Atlantic ocean is getting slowly wider due to seafloor spreading as
the plates are pulled apart at constructive plate boundaries
● The oldest rocks (Jurassic) are now over 1000 km east and west of Mid-Atlantic
ridge towards west Africa and eastern USA. This consolidates the idea of the sea
floor spreading apart, moving away from the ridges and sinking down into the
asthenosphere at the trenches
Palaeomagnetism and Magnetic Striping:
● In the late 1950s, scientists had recently discovered that the Earth’s magnetic
pole periodically flips and reverses. Instead of being at the north as it is now, it
moves to the South Pole, and then back to the north, over and over again
● Research conducted along the Mid-Atlantic ridge showed when plates pulled
apart and magma rose to form ridges and volcanic islands, iron particles in
cooling magma polarised
● Earth's magnetic field has reversed at known times in history (171 reversals over
76 million years) so we can date geological time period when rocks cooled
● In 1962 the age of rocks was studied from Mid-Atlantic ridge to the N. American
coast which showed parallel bands (magnetic stripes) of successively older rocks
exist on either side of the Mid-Atlantic ridge
● Providing strong evidence that the ocean floor is spreading apart along a line of
upwelling magma
The distribution of earthquakes and volcanoes:
● Mapping the global distribution of active volcanoes and earthquake activity led
to patterns emerging. Both volcanic and seismic activity occurred in long narrow
