Heat rising and falling inside the
mantle creates convection currents
Convection currents are created by radioactive decay in the core
The point where two plates meet is called a plate boundary
Volcanos and earthquakes are most likely to occur on or near boundaries
Plate tectonics cause earthquake and volcanoes
They move in
different directions
History
Continental drift
Concept attributed
to Alfred Wegener
F.B Taylor
independently
came to the same
idea
Francis Bacon in
1620 first saw that the
continents fit like a
jigsaw
Supercontinent - Pangea
In Quaternary split to Laurasia in the north and Gondwanaland in the south
Theory rejected at first as
could not describe drifting
the continents
Evidence
Biology
Mesosaurus, a small Permian reptile, found only in SA and Brazil
A plant only existed in coal-forming times found only in India and Antarctica
Geology
Rocks of a similar age and type found only in SA and Brazil
Like Mesosaurus
Mountain ranges and fold belts all become consistent if fitted back into Pangean landmass
E.g. NW European mountains and the Appalachians of the USA
Climatology
Evidence of glaciation found in tropical Brazil and central India
Coal forms under warm, humid conditions, found in Antarctica
Limestone found in England could not have formed under current climate
Later evidence for sea spreading
Paleomagnetism
1950's
As molten lava cool on surface the
material, iron, within align themselves
with the magnetic pole
creating a record of the Earth's magnetic polarity
Symmetrical either side of the Mid Atlantic Ridge
1962 Maurice Ewing
discovered mountain range
extending the entire length of
the Atlantic sea floor
Rocks volcanic in
origin and young
He had discovered the
Mid Atlantic Ridge
Harry Hess discovered rocks became
increasingly younger towards the Ridge and
the sea floor was spreading
He confirmed the newest rocks
were still being formed in Iceland
However if new crust is being
created it had to be being destroyed
elsewhere as the world was not
expanding in size
Evidence found around the Pacific
Plate tectonics theory was
universally accepted
Theory of the plates constantly moving, driven by convection currents
Plate boundaries
Constuctive
plates move away, new crust formed
transform faults perpendicular
mainly between oceanic so underwater
rarely on land E.g. East Africa, Great African Rift Valley
It is said the Red Sea is the
beginning of a new ocean,
associated volcanoes include
Mount Kilimanjaro and Mount
Kenya
rift valleys initially, marked by ocean ridges
submarine volcanoes
may grow above sea level e.g. Ascension Island on the Mid-Atlantic RIdge
73% of lava
(magma after it
reaches the
surface) is found
at mid-ocean
ridges.
Destructive
2 plates move towards each other and one is subducted (forced below into the mantle)
Pacific ocean surrounded by them, perimeter is known as Pacific Ring of Fire
different plates converging means different features
OC vs CO = Oc dips, deep-sea trench forms, sediments pushed up to form fold mountains, heat generated by
friction helps to melt platemagma less dense then mantle so rises and forms vol, des margins magma acidic so may
cool before surface and form a Batholith at base of mountains
shallow Eqs occurs as plate initially stretched
and dipped. Deeper eqs arise from increased
friction.
At subduction zome where more eqs take place is known as the Benoiff zone
greater subduction angles gives shallower eqs
If occurs off shore Island Arcs may form E.g. Japan and WI
Oc vs Oc = either subducted
CO vs CO = very rare, Oc cannot sink so
crumple to form deep-rooted fold
mountains
mark site of the Earth's thickest crust E.g. the Indo-Australian Plate is
moving NE to the Eurasian Plate creating the Himalayas. Uplift is a
continuous process as is weathering and erosion.
Conservative
slide past eachother
boundary between two plates pronounced by transform faults
plates stick, friction builds, releases eqs
E.g. San Andreas Fault
North America and Pacific plates, both moves NW But pacific moves faster (6CM TO 10CM)
Mark the site of major landforms.
mountain-building, volcanoes and earthquakes are found here