Unit 2: Plate tectonics, volcanoes and earthquakes
The theory of plate tectonics
• It is es mated earth formed 4.5 billion years ago (old earth theory) at
same me as sun and other planets from orbi ng material, which
gradually cooled to form Earth.
• Spinning of earth caused densest par cles to move toward centre
forming core, where temperatures are ho est, least dense material
moved toward surface where it cooled to form crust.
• Evidence for this comes from interpreta on of how energy released
by earthquakes and nuclear weapons tes ng travels through planet
• Such events emit several forms of energy wave that help iden fy
changes in composi on
• The speed at which compression waves (P waves) and shear waves (S
waves) travel vary with density of material and shear waves will not pass trough liquid at all
• In 1909 a line called the Mohorovivic discon nuity (known as Moho) was discovered, It is a line of change where
rocks of crust meet those of mantle.
Core Mantle Crust
Diameter: 70000km 29000km thick Thinnest part of structure (av. 20km)
Composed of inner solid core and Contains 80% earth’s volume ranging from 60km in con nents to
outer liquid core Solid rock but near outer edge 5km beneath oceans
Inner core largely iron anItd along beneath crust it can behave like Oceanic crust composed of denser
with outer core help create Earth’s liquid and ow or deform as molten basal c rock while con nental
magne c eld rock/ magma- this layer known as lighter grani c
Composi on is known from asthenosphere Mdern theory suggests lithosphere is
calcula on of densi es, mel ng Above this upper mantle and crust not single unit but composed of 13
points and clues from meteorites form shell of solid rock termed huge separate slabs/ plates of
lithosphere con nental and oceanic material
Oceanic crust Con nental crust
Thickness 5-7km (thin) 10-60km (thick)
Rock 3-3.3 gm/m3 (denser) 2.7 gm/m3 (less dense)
density
Rock type Basal c SIMA (silica and Gratanic SIAL (silica and
magnesium) alumina)
Rock age <250 million years 1,000-35,0000 million
years
The theory of plate tectonics- the earth’s crust is composed of irregularly shaped sec ons (plates), some of which
carry con nent, while others form ocean oor. The plates move slowly by convec on currents in the asthenosphere
• It states that earth’s surface is made of rigid plates (13/14) that form lithosphere (crustal rock and top, most rigid
layer of mantle).
• Plates rest on mobile layer called asthenosphere which enables them to move 2-20cm per year.
• Some made of con nental crust, some oceanic and some both. Earth’s crust is dynamic with some parts being
destroyed, while elsewhere new crust is formed
,In 1912 Alfred Wegner put forward theory (con nental dri theory) that all con nents were once joined together in
1 supercon nent called Pangea (280 million years ago). Later this mass split up into the con nents and they dri ed
apart. The lack of an explaining mechanism prevented his ideas from being accepted at the me. More recent
research established presence of huge convec on currents, which explain this ‘dri ’.
Evidence for movement of plates and plate tectonics theory
1. Jigsaw t- relates to similarity of the coastlines of astern South America and West Africa.
The best t is matching at depth 1,000m below sea level- it is clear these coastlines t together almost perfectly but
have been pulling away from each other for a long period of me. Areas of gaps or overlaps can be explained by:
Coastal erosion and deposi on, rises in sea level (eusta c change) and changes in land level (isosta c change) since
separa on. This provides evidence of sea oor spreading in the Mid-Atlan c ridge along a construc ve plate margin.
2. Geological evidence- relates to the pa erns of rock forma ons across the world that reveals evidence of
plate tectonics and con nental dri .
• When geology of eastern S America and west Africa was mapped it revealed ancient rock outcrops (cratons) over
2,000 years old were con nuous from one to other, sugges ng the masses were once joined and have separated
slowly over millions of years.
• Observa ons of rock types and mountain ranges- fragments of old mountain belt (Caledonian fold mountain
belt) formed 450 million years ago are found in widely separated con nents- Greenland, Canada, Ireland,
England, Scotland and Scandinavia. When these land masses are reassembled the mountain belt forms a
con nuous linear feature showing the plates dri ed apart over millions of years due to sea- oor spreading in
Atlan c ocean (construc ve boundary)
• Fossil evidence- fossils of Triassic land rep le, Lystrosaurus has been found in modern day Antarc ca, India and
Africa and fossils of fern, Glossopteris have been found in ve con nents, sugges ng land masses were once
joined but have dri ed apart over millions of years and con rming plate tectonics theory. (Alterna ve is breeding
pairs of such dinosaurs swam between land masses over thousands of miles= implausible)
• Topography of oceans- rst accurate maps of ocean basins e.g. Atlan c created by sonar showed they had huge
underwater mountain ranges with deep central valleys, which formed con nuous mountain chains for 50,000km
around earth, called mid-ocean ridges e.g. Mid Atlan c ridge discovered in 1948. They form at construc ve plate
boundary, where magma rises to surface from asthenosphere, cools and hardens into layers of rock- further
evidence for tectonics theory speci cally the result of landforms at construc ve plate boundary where plates
pulled slowly apart (sea oor spreading)
• Age and pa ern of ocean basin geology- discovered a er 1950s that sediment of Atlan c sea was thin and rocks
on ocean bed much younger (millions or tens but not billions of years) poin ng to recent forma on. The age of
rock on Atlan c ocean oor increased away from mid ocean ridge in mirror pa ern. Therefore Atlan c ocean is
ge ng slowly wider (sea oor spreading) as plates pulled apart at 2-9cm/ year (construc ve plate boundary). The
oldest rocks (Jurassic) are now over 1000km east and west of Mid-Atlan c ridge towards west Africa and eastern
USA.
3. Magne c striping (paleomagne sm)- the discovery in 1950s of pa ern of normal and reversed polarity
magne sm recorded by rocks across the mid-coean ridges of construc ve plate boundaries. Provides more
evidence that plates are pulling apart in Atlan c ocean.
• Research conducted along mis-atlan c ridge shown when plates pulled apart and magma rose to form ridges and
volcanic islands, iron par cles in cooling magma polarised.
• Earth's magne c eld has reversed at known mes in history (171 reversals over 76 million years) so we can date
geological me period when rocks cooled
• Parallel bands (magne c stripes) of successively older rocks exist on either side of Mid-atlan c ridge. In 1962 age
of rocks was studies from Mid-atlan c ridge to N American coast, providing string evidence that ocean oor is
spreading spart along a line of upwelling magma
Other evidence for plate tectonics
Glacial deposits- formed during Permo-carboniferous glacia on (300 million years ago) are found in Antarc ca,
Africa, S America, India and Australia. If con nents haven’t moved, ice sheet must have extended from S pole to
equator- unlikely as UK at this me also close to equator and it has extensive coal and limestone deposits= evidence
con nents have moved away as large plates.
More evidence from glacial stria ons (scratches on bedrock made by blocks rock embedded in the ice as the glacier
moves). These show direc on of glacier and suggest ice owed from single central point.
, Distribu on of earthquakes and volcanoes- mapping these showed dis nc ve and repeated pa erns- narrow linear
bands, some mes through oceans of volcanic and earthquake ac vity e.g. ring of re, a line of volcanoes around
paci c basin including west coast S and N America, Japan and New Zealand. Also showed huge regions with li le/ no
ac vity. This suggests along these lines/ faults, tectonic ac vity is demonstrated and is viewing point for plate
movement and resultant landforms.
Why does plate tectonics theory happen?
Researchers, esp. Harry Hess (1960s) put forward that 2 processes are responsible for dynamic nature of crust: sea
oor spreading and subduc on
Sea oor speading- the crea on of new oceanic crust material at mid-ocean ridges that form a 50,000km long feature
in Paci c, Atlan c, Indian and Antarc c oceans.
Mid ocean ridges- weak zones on crust where ocean oor is being pulled apart along the ridge crest
Subduc on- process whereby denser oceanic crustal plates sink beneath lighter con nental oceanic or con nental
plates and get destroyed
• Hess reckoned crust was expanding along ocean ridges and Earth was not
ge ng bigger so crust must be shrinking elsewhere. (Mapping ocean basins
provided evidence for this).
• In some parts of ocean beds e.g. edge of Paci c there are long, narrow deep
trenches, associated with volcano and earthquake ac vity.
• It is here that crust spreading away from mid-ocean ridges is eventually
destroyed in a process of sinking/ mel ng called subduc on
What causes sea oor spreading and subduc on?
1. Sub-plate processes- convec on currents in the mantle
➢ Heat generated in earth’s core moves up through asthenosphere as convec on currents
➢ The currents diverge as they approach the lithosphere about 80km below surface and create fric on
➢ Lithospheric plates diverge under pressure along the ridge crest
➢ New magma from deep in asthenospheric mantle rises through these weaker plate zones and eventually erupts
along crest of these ridges, forming new ocean crust.
➢ This process has operated for millions of years forming ocean basins and slowly spreads the sea oor by just cm/
year
➢ Convec on currents drag plates towards others (converge) where denser plate will sink below lighter plate
causing, subduc on and destroying of plate as it melts a er being exposed to higher temp. and fric on in mantle
2. Plate processes- ridge push (gravita onal sliding)
➢ Beneath mid-ocean ridges there is upwelling of hot, low density mantle material and localised mel ng that
generated magma
➢ Newly created oceanic plate at mid-ocean ridge is hot and
low density compared to surrounding older oceanic plate.
This creates elevated mid-ocean ridge and slope between it
and far edge of oceanic plate
➢ Force of gravity causes higher, newer part of the ocean ridge
to ‘push’ on lower, older part. Sea oor spreads away from
ridge
Slab pull-
➢ Old, cool oceanic plates sink back down into mantle at
subduc on zones because as they cool become denser than
underlying asthenosphere
➢ The subduc ng, denser part of plate pulls younger, less dense part down as it sinks
➢ Gravita onal pull downwards accelerates the mo on
Plate- huge slab of solid rock that forms part of earth’s
crust (lithosphere). It moves because of convec on
currents, ridge push and slab pull processes. (see above)
Construc ve margins dominate oceans esp. Atlan c and
Indian Oceans. Destruc ve boundaries are more common
around Paci c ocean while collision zones less common
with 1 good example in central Asia, Himalayas.
Conserva ve are rare but can be found in California, SW
