1. Understanding Plate
Tectonics
A2

2. The History
• Alfred Wegener 1915
• First came up with the theory that the
earth’s plates were slowly moving in
relation to each other (Continental Drift)
• It was not until the 1960’s and Harry Hess
who proposed that it was caused by sea
floor spreading at mid ocean ridges
• This was where the modern history of
plate tectonics begins

3. Why do we have tectonic Plates
• The earth has 7 large plates
• 7 smaller ones
• And dozens of micro plates
• The earth’s surface is broken up into
plates because the heat generated in the
earth's core by radioactive decay drives
convection currents in the outer core and
mantle.
• These slowly tear the earth apart and are
responsible for movement
• This movement will continue as long as
heat is generated at the core

4. All Major and Minor Plates

5. How do the move?
• Plates come in two types Oceanic and
continental
• Oceanic Plates
– Are young, thin and dense.
– They are formed on the sea floor spreading at
constructive plate boundary ridges and
destroyed at Subduction zones
– Most are under 150 million years old and made
of dense Basalt (6-8km thick)

6. Oceanic Plates

7. Continental Plates
• Are ancient thick and less dense
• They are over 4 billion yrs old and are not
being formed today
• They are made of less dense Granite and
are 30-60km thick

8. Convection
• At the rising limbs of convection cells, heat from the
core moves towards the earth’s surface then spreads
to either side.
• It is this spreading motion that splits the plates and
drags them apart.
• Plates move on a layer between the upper mantle and
lithosphere. The asthenosphere acts as a type of
lubrication
• The plate motion depends however on two factors:
– The weight of the cold plates at the subduction zones pulling
the plate downwards
– Gravitational sliding force between ‘High’ and ‘Low’
Trenches

9. Convection Currents
Layer Physical State
Lithosphere Solid, rigid
Earth’s Crust)
Asthenosphere Solid, plastic,
(partially molten
approx 4%)
Mantle Solid, plastic
Outer core Liquid
Inner Core Solid (Iron and
Nickel)

10. Convection Currents

11. Hazards
• Most occur where the plates meet.
Boundary Hazards
• There are rare ‘Intra-plate earthquakes’
and ‘Mid-plate volcanic hotspots’
• There many different tectonic ‘settings’
which produce various hazards

12. Settings
SETTING MOTION HAZARDS EXAMPLE
CONSTRUCTIVE 2 Oceanic plates moving Basaltic volcanoes and Mid-Atlantic ridge
PLATE BOUNDARY apart minor , shallow (Iceland) mostly
earthquakes submerged)
2 continental plates Basaltic splatter cone African Rift Valley and
moving apart volcanoes Mnt. Niyragongo
DESTRUCTIVE PLATE 2 oceanic plates in Island arc explosive Soufrierre Hills on
BOUNDARY collision Andesite eruptions and Montserrat
EQ’s
2 continental plates in Major, shallow Himalayan orogenic belt
collision earthquakes along thrust
faults
Oceanic and continental Explosive, andesitic Andes mountain chain
plate collision eruptions and major EQ
TRANSFORM Plates sliding past one Major shallow San Andreas fault
BOUNDARY another earthquakes
HOTSPOTS Oceanic Basaltic shield Hawaiian Island chain
volcanoes, minor
earthquakes
Continental Colossal Rhyolitic mega- Yellowstone
eruptions ‘supervolcano’ USA

13. Research Task
• What caused and started the following
earthquakes:
• San Francisco 1906
• Great Kanto (Tokyo) 1923
• Chile 1960
• Mexico City 1985
• Izmit Turkey 1999
• Kashmir 2005

14. Volcanoes
• Why aren’t they all the same?
• We have discussed the reasons for why
not all volcanoes are the same
– Basaltic – basic magma
– Andesitic – Intermediate
– Rhyolitic – Acidic magma

15. Basaltic
• Very hot iron rich silica poor
• Low gas content and very hot runny lava
(Melted ice cream)
• Can erupt almost continuously
• Not very explosive

16. Andesitic
• In the middle between rhyolitic and
basaltic
• Sticky can take decades or centuries
between eruptions
• Can be very explosive

17. Rhyolitic
• High silica content low temperature and
high gas content therefore combustible
• Erupt rarely
• Can be devastating

18. The Richter Scale
• Developed in 1935 to measure magnitude
of earthquakes
• Today the Moment Magnitude Scale MMS
id more commonly used and is very similar
• Most earthquakes over 6.5 on the Richter
scale generate interest as at this
magnitude they will cause some, if not
significant damage
• However this does not tell the whole story

19. Earthquake Depth
• Shallow surfaced EQ’s (<70km)
intermediate (70-300km) Deep focused
(>300km)
• Shallow ones are the most destructive as
less energy is lost travelling to the surface
• Especially important in the Benioff Zone of
subducting plates

20. The Benioff Zone
• This is the active seismic zone on a
subduction plate
• In a subduction zone the earthquake foci
normally plots along a dipping plane at an
angle of 33 to 60 degrees and this plane is
called a Benioff zone
• It is named after Hugo Benioff, a US
seismologist who first described this
feature
• The Benioff zone extends to a depth of
about 700 km

21. Physical Nature of the Ground
• If the ground consists of loose sediment then liquefaction
can occur
• In high mountain areas such as the Himalayan fold
mountains landslides can have devastating effect.

22. Finally
• After looking at these your research should
take into account the ‘Settings’ of the
events as this will help you understand the
impacts that it has on humans

23. References
• Leeds University Lecture Notes