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The theory of plate tectonics

That the lithosphere is divided into 7 LARGER + SEVERAL SMALL PLATES

These plates are RIGID + FLOAT like rafts on the semi-molten mantle (ASTHENOSPHERE)
Moved by currents which form CONVECTION CELLS


What are plate tectonics

The study of the movement of these plates and their landforms / hazards as a result


What are convection currents

Heat causes magma to rise = diverges and pushes plates apart at OCEAN RIDGES

When they move/ descend, they pull the oceanic crust down into Subduction Zones - called Dragging


What’s ridge push / gravitational sliding

Occurs when the weight of an elevated ridge pushes an oceanic plate towards the subduction zone

Dun dun dun coming to eat you 🆘🆘‼️‼️


What’s slab pull

Occurs as the weight of the subduction plates pulls the traaaaaillinggg lithosphere into a subduction zone

Slow trailing death 💀💀☠️☠️☠️☠️


Scientists thoughts on plate movements

Consider slab pull as a MUCH STRONGER FACTOR than ridge push or mantle convection / convection currents


Alfred Wegener criticism to his theory

A meteorologist not a physicist so had no valid reasoning to why the continents moved

Thought the spinning of the earth + tidal forces were responsible for the MOVING CONTINENTS

Thought continents were moving through the earth’s crust like icebreakers plowing through ❄️❄️🚝🚝🚝

Plowing through oceanic crust would distort continents like you wouldn't believe

Tidal forces too weak to move continents
^^^ one strong enough to move continents would cause the Earth to stop rotatingin less than a year


What’s continental drift

The theory that continental landmasses have changed position over time

= Alfred wegener in 1912


Evidence for Wegener :
Continental fit

The “fit” of the continents is that similar to a jigsaw

West Africa + South E America have similar coastlines

Best fit is if the coastlines are matched at 1000m depth below the current sea level


Evidence for Wegener :

Fragments of old fold mountain belts are between 400-450 millions years ago were found on separate continents

Caledonian mountain belt pieces were found in Greenland, Iceland etc.

When the mountain belt is formed it’s continuous + as if it’s been joined at some point


Evidence for Wegener :

Fossils were found in South America and West Africa to suggest the continents were once together

Mesosaurus - freshwater crocodile like reptile in South East America & Southern Africa)

Lystrosaurus - reptile build like a big sturdy pig with fossils in India, Antarctica, South Africa

Species would evolve independently on separate continents - contradicting Darwin’s evolution theory


Evidence for Wegener:

Glacial deposits were formed 300 million years ago

Glacial scratches / striations - scratches on the bedrock made by blocks of rock embedded in the ice as the glaciers move & show glacial direction

^^^^ think rocks grinding on the river bed floor

Either hemisphere reassembles near the South Pole then the ice sheet adjusts to a more reasonable size (after the ice sheet extended potential from South Pole to Equator)


Wegener :

Pangaea definition

He developed his ideas on continent movement

- 300 million years ago the continents were a single landmass called PANGAEA

- next 100 million years this single landmass drifted apart it become 2

> Laurasia
> Gondwanaland


Wegener :

Laurasia + Gondwanaland


=In the Northern Hemisphere (N.America + Eurasia)
=In the Southern Hemisphere (Africa, Australasia, India, Antarctica, South America)


Developed Wegener theory :

The plate tectonic theory

The Earth’s crust is NOT a continuous skin but a SERIES OF RIGID CAPS AROUND 100km thick

Sea floor spreading as main evidence to support this idea = OCCURING IN THE ATLANTIC OCEAN


Sea Floor Spreading

Happens when plates move apart

Tectonic plates diverge and move apart
=magma rises up to fill the gap that has been created
=then cools to form a new crust

Over time, the new crust is dragged apart and more crust forms between it as the first has been dragged apart

* If this happens at a PLATE MARGIN UNDER THE SEA then the sea floor gets WIDER

* It creates a structure called mid-ocean ridges of high terrain and steep sloping barriers at either side of the margin/opening


Factors for the sea floor spreading idea

The age of the rocks on the sea-floor

*Near the Mid-Atlantic Ridge
basaltic rocks on the sea floor are younger
the continental shelves rocks at the edge of an ocean margin

=mirror each other - show they were once touching/joined

Polar Wandering

*the North and South Poles moving considerably during geological times
*the magnetism pattern relating to the alternative pattern found at north and south magnetism regions


What happened in the 1960s

The theory of plate tectonics was established. As evidence to believe this:

Ocean ridges


Sea-floor spreading