Detailed and comprehensive notes on unit 1 topic 1 of Pearson Edexcel AS Level Geography (tectonic processes and hazards). Covers: structure of the earth, plate tectonic theory, plate boundaries, volcanoes, earthquakes, tsunamis. Case studies include: Eyjafjallajökull eruption (2010), Haiti earthq...
Structure of the Earth
The Core
● Found at the centre of the earth.
● Consists of the inner and outer core.
The Inner Core
● Found at the very centre of the earth.
● The hottest part (~6000°C).
● Solid and mostly consists of iron.
The Outer Core
● Semi-molten.
● Mostly consists of liquid iron and
nickel.
● Temperature ranges from 4500°C-
6000°C.
The Mantle
● Surrounds the core and is the widest layer
making up the earth.
● Upper part is solid but below the rock is semi-molten - forming the asthenosphere, which tectonic plates
move over.
● This is because the mantle has a geothermal gradient:
● There is a steady increase of temperature with depth as the lower mantle is in contact with the outer core.
● Rocks found in the upper mantle are cooler and brittle compared with those in the lower mantle which are
much hotter and plastic (not fully molten though).
● Rocks in the upper mantle are brittle enough to break under stress and produce earthquakes.
● However rocks in the lower mantle are plastic and instead begin to flow when subjected to forces and the
high pressure and heat.
The Crust
● Forms the outer shell of the earth.
● Two types: oceanic and continental.
Between them, the crust and upper mantle make up the lithosphere – the solid layer from which tectonic plates are
formed.
The Crust
Continental Crust
● Ranges from 25-70km in thickness (thicker than oceanic) and is underlain by about 80km of the uppermost
mantle layer. This is made from course grained igneous rock peridotite.
● Rock types – Granite/Diorite
● Older rock – some continental crust is estimated to be 4 billion years old.
Oceanic Crust
● Ranges from 6-11 km in thickness (thinner than continental) and is underlain by about 40-100km of the
uppermost mantle layer.
● Much denser rock
● Rock types – Gabbro/Basalt
● “Recycled” more frequently – most no more than 250 million years old.
,Timeline of Plate Tectonic Theory Proposals
1596
● Flemish mapmaker Abraham Ortelius noted that the coastlines of the continents appeared to fit together.
● He suggested that they were once joined together.
1638
● Danish scientist Nicolaus Steno proposed the Law of Superposition; each layer of rock is older than the layer above it.
1785
● Scottish farmer and naturalist James Hutton used Steno’s Law and compared rocks.
● He found that granite formed from the cooling of molten rock, not precipitation out of water as was believed at the time
and that granite must be younger than surrounding rock.
1912
● German meteorologist and geophysicist Alfred Wegener proposed that the continents were once joined together in a
supercontinent called Pangea.
● He believed that parts moved apart due to a phenomenon he called ‘continental displacement’ (now continental drift).
● His theory was rejected by most until the 1950s and 60s as he could not explain the driving force behind continental drift
1929
● British geologist Arthur Holmes proposed that convection in the mantle drove continental drift.
● His ideas were not taken seriously at the time but his hypothesis later gained support.
1950s
● Oceanographic vessels mapping the ocean floor led to the discovery of mid-ocean ridges.
1960
● American geophysicist Harry H. Hess developed the idea that oceanic crust forms along mid-ocean ridges and spread
out laterally from there.
● The following year geophysicist Robert S. Dietz named the phenomenon seafloor spreading.
● Their work played an integral role in the development on the modern theory of plate tectonics.
1963
● British geologists Frederick J. Vine and Drummond H. Matthews postulated that new crust would have magnetisation
aligned with the Earth’s geomagnetic field.
● They noted that this would appear over time as bands of crust showing alternating patterns of magnetic polarity.
Mid 1960s
● A global network of sensors recording earthquake activity showed that volcanic and earthquake activity almost
exclusively occurred at the edges of tectonic plates.
1968
● A ship called the Glomar Challenger explored the mid-ocean ridge between South America and Africa.
● Cores obtained from drilling revealed that rocks closer to the mid-ocean ridges were younger than those further away.
1968
● Dan McKenzie, Professor of Geophysics at the University of Cambridge, proposed the Theory of Plate Tectonics.
● He had mathematical evidence to back his theory up.
Mid 1970s
● Scientists created 3D images of Earth’s interiors using evidence from earthquakes which helped them to investigate and
model the dynamic processes in the deep interior of Earth.
, Plate Tectonic Theory
Mantle Convection
● Heat produced by the decay of radioactive elements in the Earth’s core heats the lower mantle – creating convection
currents.
● These hot, liquid magma currents are thought to move in circles in the upper mantle (asthenosphere) – thus causing the
plates to move.
● Less accepted as the main argument for plate movement.
1929 - British geologist Arthur Holmes proposed that convection in the mantle drove continental drift. His ideas were not taken
seriously at the time but his hypothesis later gained support.
Slab Pull
● Newly formed oceanic crust at mid- ocean ridges becomes denser and thicker as it cools.
● This causes it to sink into the mantle under its own weight, pulling the rest of the plate further down with it.
● It is also thought to be driven by subduction, where the ocean plate is pulled into the mantle.
● This pressure then further pulls down the rest of the plate.
● Increasingly seen as a major driving force for plate movement.
In 1975 Forsyth and Uyeda used the inverse theory method to show that, of the many forces likely to be driving plate motion,
slab pull was the strongest.
Subduction
● As new crust is being created in one place, its being destroyed in another – by subduction.
● As two plates move towards each other, one slides under the other into the mantle – where it melts in an area known as a
subduction zone.
● Since the Earth was not increasing in size despite Hess’ theory of “Sea Floor Spreading” and new crust being created, it
was surmised that somewhere the crust must be destroyed.
● Hess noted that the deepest parts of the oceans were very close to continental margins in the Pacific with Ocean Trenches
extending down to depths of over 11 km in the case of the Marianas Trench off the coast of Japan.
● It was in these deep ocean trenches that Hess put forward that the ocean floor was destroyed and recycled.
● This is where the first ideas of subduction come into play and we can see the theory developing into the ideas that we
have today.
● Part of the plate must be being forced back down into the mantle and being destroyed.
Seafloor Spreading (Ridge Push)
● In the middle of many oceans are huge mid-ocean ridges, or underwater mountain ranges.
● These are formed when hot magma is forced up from the asthenosphere and hardens, forming new oceanic crust.
● This new crust pushes the tectonic plates apart in a process called seafloor spreading.
● In 1962 Harry Hess published his work ‘The History of Ocean Basins‘ in which he put forward his theory “Sea Floor
Spreading”.
● Hess envisioned that the youngest and newest rocks were in the centre of the Atlantic Ocean and that oldest rocks around
the edges by the USA/Carribean.
● He also suggested that the Atlantic Ocean could be widening by up to 5cm a year.
● However he recognised that the implication of his theory was that the earth must be increasing in size as all this new
crust was created. More evidence was needed.
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