Lecture 3- When continents collide notes 6pp PDF

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Lecture 3- When continents collide notes 6pp...

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2/28/21

Readings ERTH1000

When continents collide Physical Geology Chapter 10 - Plate tectonics

Understanding Earth Chapter 2- Plate tectonics

Observations leading to theories of continental drift, sea-floor spreading & ultimately, plate tectonics

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Dr Kevin Welsh School of Earth and Environmental Sciences, University of Queensland, Australia

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Questions without answers

How do we know plates move?

• Why are there mountains? • Why are there oceans? • How does sediment get buried/ why are rocks recycled? • Why are there volcanoes? • What causes rocks to be metamorphosed? • Why are rocks on land and in the ocean different?

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We can see it happening!

Pacific Plate = 10cm/7yr

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Early ideas

More early ideas

Abraham Ortelius (1596) - Dutch cartographer noted similar coast lines of continents

Alfred Wegener (1910) - Proposed idea of continental drift based on parallelism of coasts of South America and Africa

Lateral tearing away of America from Europe and Africa

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Alexander Du Toit (1937) “Our Wandering Continents”

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Cynognathus Distribution of certain fossils: •

Fo exampl ca. 300 Myrs old reptiles from South America and Africa are similar.

• This suggest that Africa/South America was together and drifted apart later.

Leaves of Glossopteris – a seed fern from India, the characteristic plant found in separate continents today which were all together before their break-up

1.2 m long Skull- up to 30 cm alone Lived during Triassic It was a synapsid Africa and South America And terrestrial…

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Prehistoric plant links to the past

Glossopteris

Family Proteaceae Sclerophyllous (hard leaves, short internodes)

Seed fern Permian (290-250Ma)

Low dispersal ability therefore, required land for migration

Australia, Antarctica, India, S. America, Africa Seed dispersal?

Exhibit in the Houston Museum of Natural Science, Houston, Texas, USA

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Matching rock sequences Distribution of certain climatically sensitive rocks

Similar links to an ancient continent

Glacial till We can determine the environment of deposition from the shape and size of the sediments We only get large continuous deposits of glacial till at high latitudes https://www.sandatlas.org/diamictite/

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But h o the c w could on move tinents ?

Distribution of climatically sensitive rocks (tillites/glaciers)

Rec ons truc ted

Pangea = All Land

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The Meteor

Marie Tharp

New York Times Harry Hess

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Cyprus ophiolite complex

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Geomagnetic reversals

We live in the Brunhes chron

Vine-Matthews-Morley hypothesis 1963

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Age of the sea floor

Why is old shown blue and young shown red?

Why is mean max. age of ocean only 200 Ma?

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The planet likes to recycle! Convergent plate boundary

Volcanoes – Ring of fire

Continental lithosphere

Oceanic lithosphere

Subduction

Destruction (recycling) of oceanic lithosphere at convergent boundaries (subduction zones)

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Types of plate boundary

A major breakthrough Arthur Holmes (1940’s) Continental drift could be related to mantle convection Proposed that ocean floor spread apart at the top of mantle convection cells. This (sea-floor spreading) moved the continents

The hypothesis was theoretical based on convection

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Wilson cycle

Allen and Allen, 2001

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Take home messages • Scientists have been puzzling over empirical evidence for many years showing a different organisation for the continents – – – –

Sedimentary sequences Glacial sequences Land animal and plant biogeography Fit of the continents

• Plate tectonics relies on a ductile asthenosphere (requires heat from the planet and a rigid lithosphere) • Evidence for mechanism: – – – –

Moving plates Earth quakes showing location of plate destruction New material produced at Mid Ocean Ridges Age of the ocean floor

• Three types of plate boundary: divergent, convergent and transform

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