PHYSICAL GEOGRAPHY
HAZARDOUS EARTH
, TOPIC ONE - WHAT IS THE EVIDENCE FOR CONTINENTAL DRIFT AND PLATE TECTONICS?
TOPIC ONE A - PLATE TECTONICS AND CONTINENTAL DRIFT
STRUCTURE OF THE EARTH
Crust, mantle, outer core, inner core
The Mantle
The upper mantle consists of three layers
● Lithosphere
- Rigid layer between the crust and the asthenosphere
- Its boundary with the asthenosphere is difficult to define precisely as it begins to melt and becomes
incorporated into the asthenosphere
- 0-100km
● Asthenosphere
- Semi molten and viscous layer capable of flowing slowly
- 100-350 km
● Mesosphere
- Lower mantle
- Hot but stronger due to high pressure. Less viscous than the asthenosphere
- 350-2900 km
The mantle-crust boundary is marked by the mohorovicic discontinuity (moho)
Density is 3.3kg/cubic metre at the moho and 5.6kg/cubic metre at the core
The core
Found about 2900 km beneath the surface of the earth
Radius of about 3485 km
The boundary between the mantle and the core is known as the Gutenberg discontinuity
Composed of iron and nickel, which remains solid at the centre due to intense pressure despite maintaining a temperature
of 2600 degrees celsius
Evidence for the structure of the earth
● Drilling through the earth’s surface has provided most of the information about the crustal layer
● Volcanoes and igneous rocks have provided evidence of the composition of the crust and the asthenosphere as
these rocks are erupted from these volcanoes
, ● The particular velocity at which a seismic wave passes through a layer will suggest what a layer's chemical
composition is likely to be.
● Meteorites are thought to be the remnants of the core and mantle of other planetary bodies from within the
solar system, all of which were formed at the same time and from the same material as the Earth. The earth’s
mantle and core is likely to be made from the same material
CONVECTION CURRENTS
Convection currents in the asthenosphere cause tectonic plates to move very slowly
Magma closer to the earth’s core heats and rises. Once it reaches the upper mantle, it cools down and sinks again
This process happens continuously and forms a rising heat current
As the current movers, it drags the tectonic plate that sits above it with it, causing the plate to move
Ridge push and slab pull are forces that occur because of convection currents
Ridge push
- Directly related to the part of convection that has rising mantle material
- As the mantle material rises and hits the underneath the crust, it pushes and separates underneath the crust.
Because there is a separating motion underneath the crust, the actual crust then moves in the same way
- Leads to the formation of mid ocean ridges. Because these mid ocean ridges are higher in elevation, gravity pulls
the surrounding rocks down and away from the ridge
Slab pull
- Occurs when the convection current is sinking
- As it sinks, it takes the older, more dense crust with it
- Due to the density and heaviness of the plate, gravity continues to pull on the rest of the plate, causing it to sink
further
- The more the plate subducts, the faster it pulls
CONTINENTAL DRIFT
The theory of continental drift is the gradual movement of the continents across the earth’s surface throughout geological
time
Alfred Wegner - proposed the theory of continental drift
- Suggested that all continents were originally joined together to form a single continent called ‘Pangea’ before
drifting apart into Eurasia in the north and Gondwana in the south
SEA FLOOR SPREADING AS EVIDENCE FOR CONTINENTAL DRIFT
Sea floor spreading occurs at mid ocean ridges and helps to explain how continents move
Oceanic plates diverge allowing basaltic magma to rise up, cool, and form new crust
Newer crust is found at mid ocean ridges and older rocks further away from it
Seas floor spreading is known to be caused by convection currents in the asthenosphere
Paleomagnetism
Confirms sea floor spreading
Involves the study of the history of changes in the earth’s magnetic field (polarity)
Every 400,000 years, the earth’s magnetic field switches polarity, causing the magnetic north and south poles to swap
As plates move apart, magma rises and cools on the ocean floor
- Particles of iron oxide (called magnetite) in the lava record the earth’s magnetic orientation at the time of cooling
When new magma rises and cools, the earth’s magnetic field would have switched, meaning this magma records the
opposite magnetic orientation to the magma previously
This results in a series of magnetic strata with rock aligned alternately to north and south poles
The striped pattern is mirrored on either side of the ridge, proving that the tectonic plates are slowly moving apart from
each other
, ANCIENT GLACIATIONS AS EVIDENCE FOR CONTINENTAL DRIFT
Significant evidence of glaciation from about 290 million years ago in Africa, Australia, South America, India and Antarctica
suggests these land masses were once linked and located close to the south pole
There are glacial deposits in continents that could not have been made at their current latitude such as the tropical
rainforests of Brazil, showing that they have moved into the position that they are now in.
FOSSIL RECORDS AS EVIDENCE FOR CONTINENTAL DRIFT
Permian fossils known as brachiopods found in limestone in India match those found in Australia, suggesting that at some
points these areas were joined together
Fossil remains of mesosaurus, a prehistoric reptile, are found in both southern Africa and South America
