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Plate Tectonics

Map of the tectonic plates.

In 1912 the meteorologist Alfred

Wegener independently developed

what he called continental drift,

(expanded in his 1915 book The

Origin of Continents and Oceans).

He started the scientific debate that

became the theory of plate

tectonics 50 years later, in the early

1960s.

Text Antonio Snider-Pellegrini's Illustration of the closed and

opened Atlantic Ocean (1858).

The fit of Africa to South America was noted with early maps.

The speculation that continents might have 'drifted' was first

put forward by Abraham Ortelius in 1596.

Early in the 20th Century Alfred Wegener proposed

strongly, with good evidence (as follows), published

in 1912, that the continents had moved around

(Continental Drift). He found evidence that the

continents had been assembled as one, a

megacontinent, Pangaea (meaning "all lands").

Over time they have drifted apart into their current

distribution. He believed that Pangaea was intact

until the late Carboniferous period, about 300

million years ago.

Wegener was a geophysicist and meteorologist who

went on 4 Greenland Expeditions. He died in

Greenland in 1930 and was buried there. Mountain ranges match on either side of the Atlantic.

Wegener also showed geological similarities on both sides.

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Wegener used fossil evidence to demonstrate ancient links.

Fossils of the same land species are found in matching

continents, showing the land masses were once linked.

By about 1930 continental drift was not at all

favoured by most geo scientists, but Arthur Holmes

pioneered radiometric dating and later suggestions

on the age of the Earth as 3,000 million years.

He also supported Wegener’s Continental Drift

(against most current thinking), and published ideas

on mantle convection currents.

Part of his mantle convection model was the origin

of ideas on seafloor spreading.

Holmes published a widely used textbook

(Principles of Physical Geology), which some older,

(and some not so old), U3A members may

remember. Still on my bookshelves.

In the very early 1960s plate tectonics

became an accepted theory following

reassessment of the evidence produced in

support of Continental Drift, and some new

developments.

Evidence of ocean floor spreading was

becoming quite convincing.

Palaeomagnetism was showing strong

evidence of plate movements. The pattern of World earthquakes suggests plates with

boundaries (some very deep). Map of World Seismicity 1963-1955

Key shows depth of earthquake in kilometres.

At any point the magnetic field can be measured.

The measured magnetic field vector is divided into

declination (D), inclination (I) and intensity (F) The vertical inclination of the preserved magnetic direction can

give an estimate of the latitude at the time the rock was formed.

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The magnetic field signal locks in as the lava cools.

In sediments magnetic particles align with the magnetic

field as they settle at quiet river or ocean deposition sites.

Palaeo magnetism

studies made in Europe

suggested the magnetic

pole moved as shown.

But then N.American

studies gave different

positions.

Plate tectonics explains

this – if the continents

moved then the

apparent previous

position of the pole will

be moved.

An important step in developing understanding of plate

tectonics was the recognition of a big difference between

Continental Crust and Oceanic Crust.

Continental crust has a higher percentage of silica and

aluminium (and other chemicals) favouring formation of

feldspars & quartz.

Continental crust has a lower density, and typical igneous rocks

are granite (and rhyolite).

Oceanic crust has a higher percentage of magnesium and iron

(ferrous) – more “mafic” minerals. Hence oceanic crust is

denser, with typical igneous rocks being basalt and gabbro.

Typical oceanic/mafic igneous rocks are also darker in colour.

Oceanic Crust is much thinner than Continental Crust – and

also much younger (200–270my maximum, cf 3.5-4 billion yrs!).

Palaeomagnetic studies showed the polarity of the Earth’s

magnetic field reverses from time to time!

Sequences of magnetic rocks (e.g. A series of basalt lavas)

can record these magnetic reversals.

In the 1950s observations and ideas

began to show that there are mid-ocean

ridges where new basaltic crust was

being formed.

Even more surprising, there were

symmetrical bands of normal and

reversed polarity rocks parallel to the

mid ocean ridges e.g. the Mid Atlantic

ridge. Reversals of magnetic polarity through time produce

symmetrical bands of magnetic basaltic rock with alternating

polarity parallel to a spreading mid ocean ridge.

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This diagram shows

how bands of rock

showing magnetic

reversals have

developed.

Dating of the rocks on the floor

of the Atlantic Ocean further

supports the idea of formation of

new ocean crust over time.

S

The tectonic plates of the world were mapped in the second half

of the 20th century. Very like the seismicity map!

World Seismicity 1963-1955 Key shows depth in kilometres.

The rigid lithosphere moves over the asthenosphere which

is hotter and capable of plastic flow.

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Now we recognise three types of plate boundary (at top).

Transform Plate Boundary, Divergent Plate Boundary

and Convergent Plate Boundary

Diagram of an oceanic-continental convergent plate margin.

The denser oceanic crust subducts beneath the lighter

continental crust. Friction with the subducting oceanic crust

causes heating and volcanic activity. (W coast S.America)

Continental-continental convergent margin. One plate may

subduct under the other, or there may simply be a lot of

crumpling. In either case a mountain range results. e.g. The

Indian Plate being thrust under the Eurasian Plate, creating the

Himalayas and the Tibetan Plateau beyond (also e.g. the Alps).

Oceanic crust subducting under oceanic crust will produce a

sub ocean trench, and an island arc of active volcanic islands.

There are several examples in the Pacific. The water in the

subducting crust (ocean sediments) both lowers the melting

point of the rock, and gives especially explosive volcanic activity.

A transform fault or transform boundary, (also known

as conservative plate boundary since these faults neither

create nor destroy lithosphere) is a type of fault whose

relative motion is mainly horizontal, left or right handed parallel

to the fault.

Transform faults are commonly found linking segments of mid-

oceanic ridges or spreading centres. The movement does not

increase the distance between the two ridges because new

crust is being created.

Transform faults are also found at continental margins e.g.

the well known San Andreas Fault on the USA Pacific coast.

The North American Plate moving SSE slides past the

Pacific Plate moving NNW. It runs for around 800 miles

(1300km).

Another example of a transform fault is the Alpine Fault that

runs almost the entire length of New Zealand's South Island.

It forms a transform boundary between the Pacific Plate and

the Indo-Australian Plate.

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View of the San Andreas Fault,one of the few transform

faults exposed on land.

Age of the Crust under the main oceans. Reds are youngest (0 to

33 million years), then yellow (48-56my), green (68-120my), pale

blue (132-148my) & dark blue(150 to 180my).

Plate motion based on Global Positioning System (GPS) satellite data from NASA JPL. The vectors show direction and magnitude of motion.

About 250 million years ago the land masses of the world were

all assembled together – a supercontinent (named Pangea).

The breakup of

Pangea

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Formation of supercontinents

seems to have been cyclical.

There is evidence of at least 3

previous supercontinents.

There is general agreement that

there were earlier supercontinents,

but the detail gets less certain

further back in time.

Continents and continental crust has a long and

complex history (about 3.5 billion years), and

complex structures in the older parts. There have

been many episodes of crumpling and of parts

being thrust over other parts. Continental crust is

less dense (2.7 g/cm3) but on average much

thicker (25 to 70 km).

Oceanic crust is much younger (270my maximum

and most is less than 180 my). Oceanic crust

develops at ocean floor spreading sites. Oceanic

crust is denser (2.9 g/cm3) and on average much

thinner (7-10km).

Plate tectonics explains a lot about the evolution of the

present world and explains many large scale geological

features.

We can understand how the highest parts of Everest, the

Alps etc can be sediments that were once beneath the

oceans.

We can understand why the World’s main mountain belts are

where they are.

It explains the distribution of many of the World’s volcanoes.

And also explains many other features.

World Seismicity 1963-1955 Key shows depth in kilometres.

Thank you for your patient attention.