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Plate tectonics and
associated hazards…read more

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The theory of plate tectonics was published in 1912 by a German called Alfred Wegner. His theory stated
that about 30 million years ago, a super continent (called a Pangaea) was what the Earths land consisted
of. He then went on to explain how the Pangaea split into two large continents, Laurasia in the north and
Gondwanaland in the south.
The continents that appear on the Earth today formed through further splitting of the two large
Evidence to fit the continental drift theory includes:
· The way South American and Africa fit (on world map)
· Evidence of the glaciation period 290 million years ago as deposits from it are found in South America,
Antarctica and India. Must have formed together then moved apart through continental drift.
· Striations have been found on rocks in Brazil and in West Africa which could be due to the same
· Rock sequences in northern Scotland agree with those found in eastern Canada, so they must have
been laid down under the same conditions in the same location at one point.
Evidence found was also biological:
· Fossil brachiopods found in Indian limestone are also found in fossils in Australia
· Fossil remains of a reptile `mesosaurus' are found in South America and Southern Africa. It's unlikely
that the reptile developed in both areas or that it migrated across the Atlantic.
· The fossilised remains of a plant which existed when coal was being formed have been found in India
and Antarctica only.…read more

Slide 3

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At first his theories did not explain how continental drift actually happened, but after the 1940's, more
evidence began to accumulate to prove his theory could be correct. When the mid-Atlantic ridge was
discovered and studied, sea floor spreading was found to be occurring. The evidence for this was the
alternating polarity of the rocks from the oceans crust. Iron particles in lava erupted on the ocean floor are
aligned with the Earths magnetic field. As the lavas solidify, the particles provide a permanent record of
the Earths polarity at the time of eruption (palaeomagnetism). As the Earths polarity reverses about every
400,000 years it has resulted in a set of magnetic `stripes' with rocks aligned to the north and south poles.
The striped pattern, which is mirrored exactly on either side of a mid-Atlantic oceanic ridge suggests that
the ocean crust is slowly spreading away from this boundary. Sea floor spreading implies that the Earth
must be getting bigger. This cannot be true as plates would be being destroyed if the Earth was expanding,
so the theory of continental drift seemed more realistic.…read more

Slide 4

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Plate tectonics is a scientific theory that describes the large scale motions of the Earth's lithosphere.
The theory builds on the concepts of continental drift, developed during the first decades of the
20th century, when the concepts of seafloor spreading were developed in the late 1950s and early
The lithosphere is broken up into tectonic plates. In the case of the Earth, there are currently seven
major and many minor plates. The plates ride on the asthenosphere. These plates move in relation
to one another at one of three types of plate boundaries: convergent, divergent or
conservative boundaries. Earthquakes, volcanoes/ volcanic activity, mountains and ocean trenches
are all formations which can occur along these plate boundaries.
The tectonic plates are dived into two categories thick continental and thin oceanic depending on
the material they are made from. Below the lithosphere is the semi molten asthenosphere on
which the plates float and move. Continental crust is 30-70km thick and consists of mainly granite.
It is more dense than oceanic crust which means it is able to float, whereas the oceanic crust is only
6-10km thick and mainly composed basalt and a lot more dense and so more easily subducted.
Rising and falling convection currents in the mantle cause very slow movements in the plates
roughly a few inches per year. The source of the heat driving the convection currents comes from
residual heat from the Earth's formative processes, pressure from gravity, and from radioactive
decay of elements. Convection currents inside Earth provide the mechanism for plate motion.…read more

Slide 5

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Convergent Plate Boundaries
Also known as destructive plates
A convergent plate boundary is where two plates move towards each other.
There are 3 types of convergent situations
Oceanic plate moves towards Oceanic plate
Oceanic plate moves towards Continental plate
Continental plate moves towards Continental plate
Oceanic vs. Oceanic, when this movement occurs when the two plates meet one plate
is forced under the other and the process involved with subduction begins. Landforms
created from this are ocean trenches and island arcs. An example of this is where the
Pacific Plate is being subducted beneath the smaller Philippines plate.
Oceanic vs. Continental, when this movement occurs the more dense oceanic plate is
subducted beneath the Continental plate due to the continental plate being more
buoyant and light. Landforms associated with this movement are ocean trenches, fold
mountains (due to the uplift on the edge of the continental plate), can trigger major
earthquakes and composite volcanoes (which are viscous, andesitic lava) an example of
this is the Nazca plate sinking under the South American plate forming the Andes or the
Juan de Fuca plate sinking under the North American plate to form the Rockies.…read more

Slide 6

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Continental vs. Continental, when this movement occurs due to both plates being
buoyant and having a low density neither plate will be subducted rather they will collide
and the plates will be forced up as there is no subduction there is no volcanic activity
however the impact can trigger earthquakes. Landforms associated with this movement
are fold mountains a famous located example of this is the Himalayan mountains which
occurred when the Indo-Australian plate collided with the Eurasian Plate.
Oceanic vs Oceanic Oceanic vs Continental Continental vs Continental…read more

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