Lecture 4 - Plate Tectonics and the Rock Cycle Flashcards
What did Francis Bacon observe?
The coastlines on either side of the Atlantic fit nearly perfectly together.
This happened around 400 years ago.
What did Alfred Wegener observe?
The fit of the coastlines on either side of the Atlantic was even better when the submerged margins of the continents were considered.
What was the latest supercontinent and single ocean called?
Supercontinent - Pangea.
Single Ocean - Panthalassa.
What were the northern and southern parts of Pangea called?
Northern - Laurasia
Southern - Gondwanaland.
How do fossils give evidence for continental drift?
The fossils of the same species occurred across more than one continent but no where else in the world. This shows that they were once connected.
How do sediments and geological features give evidence for continental drift?
Similar sediments and sediments formed under the same climatic conditions fit together when Pangea is reconstructed.
Ice scours formed by retreating glaciers extend across continents.
Coal forms in tropical swampy regions.
Ice rafted boulders are huge rocks that have been moved by an ice sheet.
Why was Wegener’s theory not originally believed?
There was a lack of evidence about how the continents were moving.
This would have caused the seafloor to be deformed and the general consensus was that the seafloor was flat and smooth.
What research proved the continental drift theory?
Mountain chains and ridges were observed running down the centre of the oceans.
The oceans had systematic changes in depth.
The presence/absence of volcanic rock and sediment.
Palaeomagnetism.
Seismicity.
How do mountain chains and ridges provide evidence for continental drift?
They provide a mechanism for the ‘pushing apart’ of land masses through the creation of new seafloor.
Sediment thickness and age increase with distance away from the ridge.
Heat flow is greatest closer to mid-ocean ridges where new ocean floor is erupted and cooling. This shows new rocks are forming.
How does palaeomagnetism give evidence for continental drift?
Magnetic surveys showed a striped seafloor. The patterns on either side of the ridge were symmetrical which shows the seafloor is spreading at an equal rate on either side of the ridge.
How does seismicity provide evidence for continental drift?
Earthquakes were found to happen spatially and the following areas had the most earthquakes:
- Areas along ocean ridges.
- In deep ocean trenches along continental margins.
- Along large faults that cut across ocean basins.
Where do shallow earthquakes occur?
Along mid ocean ridges, continental margins and along faults.
Where do deep earthquakes occur?
Along continental margins where there are deep ocean trenches.
What did the Plate Tectonic Theory propose?
The Earth’s surface is made up of rigid plates that can move as they sit upon viscous mantle material - the asthenosphere.
What forces move tectonic plates?
Convection currents.
Slab pull.
Ridge push.
What happened when Pangea split apart?
The tectonic plates moved away from each other and caused a rift valley to be formed.
These valleys filled up with water and this was the start of an ocean basin.
New crust is created at divergent plate boundaries. This means that crust must be destroyed elsewhere. This happens at convergent plate boundaries.
What are the three types of plate boundary?
Transform - Tectonic plates are sliding past each other.
Divergent - Tectonic plates are moving away from each other.
Convergent - Tectonic plates are moving towards each other.
What processes occur at divergent plate boundaries?
The upwelling asthenosphere stretches and causes the crust to thin. This eventually splits into two and molten rock forms new crust here.
Oceanic crusts moving apart form a mid-ocean ridge whereas continental plates moving apart form a rift valley.
Mid-ocean ridges form hydrothermal vents which are where seawater enters the ridge system, reacts with the ocean crust and is expelled back into the ocean.
What are the three types of convergent boundaries?
Continental-continental
Continental-oceanic
Oceanic-oceanic
What happens at oceanic-continental boundaries?
The oceanic crust subducts underneath the continental crust as the oceanic crust is denser. Water is drawn into the subduction zone and this lowers the melting point of the rock. Friction generates heat until magmatism occurs and this is forced through cracks and a volcano is formed.
What are two examples of oceanic-continental boundaries?
The Juan de Fuca plate colliding with the North American plate. This is happening in the Cascade Mountain Range in Oregon and caused Mt St Helens to erupt.
The Andes formed as a result of oceanic-continental boundaries. Subduction started occurring 140 million years ago, and magmatism started 10 million years after this. Within 50 million years, subduction was well established and the Andes mountain range was formed.
What happens at oceanic-oceanic boundaries?
The colder, older and denser crust is subducted underneath the newer crust. This causes volcanoes to form on the younger plate and due to the movement of the plates, a chain of volcanoes is formed. This is called an island arc.
What is an example of an oceanic-oceanic boundary?
The Aleutian Islands.
They are found in the North Pacific Ocean, just off the coast of Alaska.
What happens at continental-continental boundaries?
As continental crust is not very dense, it cannot be subducted and so piles up forming a fold mountain. They tend to lead to deep earthquakes.
This led to the formation of the Himalayas.
