Plate Tectonics (Option A) Flashcards
How many years old is the Earth? How many years do geologists know the most about?
The Earth is around 4600 million years old. Geologists know most about the last 570 million years (the Cambrian period) from dating rocks and fossil records.
Name the 3 main sections of the Earth
Core (inner + outer)
Mantle
Crust
Describe the inner core
- Centre of the earth
- Hottest part
- Solid
- Made of iron and nickel
- Temperatures up to 6000 degrees
- Like the engine room of Earth
Describe the outer core
- Layer surrounding inner core
- Liquid
- Made of iron and nickel
- Extremely hot with temperatures similar to inner core
Describe the mantle
- Widest section of earth
- Diameter of approx. 2900km
- Solid
- Asthenosphere (outer edge of mantle) can flow; convection currents operate here
Describe the lithosphere
- Above asthenosphere
- Upper mantle and crust form solid layer
- Divided into 8 major plates and several minor plates
What is the Moho?
The point where the rocks of the crust meet those of the mantle.
Named after line of change or discontinuity between the two layers.
Describe the crust
- Outer layer of earth
- Thin layer between 5-60km
- Solid layer upon which we live
Name the 2 types of crust
Continental + Oceanic
Describe oceanic crust
Thickness: thin (5-7km)
Rock Density: denser (3-3.3gm/m3)
Rock Type: basaltic SIMA (Silica + Magnesium)
Rock Age: less than 250 million years
Describe continental crust
Thickness: thick (10-60km)
Rock Density: less dense (2.7 gm/m3)
Rock Type: granitic SIAL (Silica + Alumina)
Rock Age: from 1000 million years and older
What are the 2 theories about the earth’s surface moving?
- Continental Drift : an old theory + suggested that the earth’s surface moved but couldn’t say why
- Plate Tectonics: a modern and more sufficient theory as it suggests a mechanism for the movement
TIMELINE : 1620
Francis Bacon noted ‘jigsaw’ fit of the continents e.g. South America & Africa
TIMELINE : 1912
Alfred Wegener proposed continental drift theory. Today’s map formed one land mass “Pangaea” then split into Laurasia and Gondwanaland, continuing to today’s earth
TIMELINE : 1948
Mid-Atlantic Ridge was discovered
TIMELINE : 1950s
Paleomagnetism discovered
TIMELINE : 1960
Harry Hess’ theory about the sea floor spreading
Why was Wegener’s theory not initially accepted and what evidence later on supported it?
He could not explain the theory
Not accepted until the 1950s when discoveries such as paleomagnetism provided strong support for continental drift
How long ago was Pangaea one large super continent?
225 million years ago
How is the way our world today explained?(i.e how have we moved away from Pangaea)
Wegener’s continental drift theory
What evidence did Wegener use for continental drift?
- the distribution of rock types and mountain chains
- fossils
- past climatic evidence
EVIDENCE of CD : distribution of rock types and mountain chains
The distribution makes more sense when the separate landmasses were placed together.
E.g. Caledonian epoch created long fold mountains across world
EVIDENCE of CD : fossils
Land based reptiles such as freshwater Mesosaurus have been found in South America and the south-west of Africa.
Glossopteris ferns found in similar aged rocks in South America, Africa, India and Antarctica. These are now separated by thousands of miles of water in which these animals and plants could not survive.
EVIDENCE of CD : past climatic evidence
These continents had in the past been covered and carved by huge ice sheets.
The direction and flow of such sheets seemed to only make sense when the continents were placed together.
EVIDENCE of CD : climatological evidence
Places including America, Svalbard and the U.K. all contain coal deposits of similar age that were formed in tropical conditions. They are no longer in tropical climate zones.
Who provided the theory of the sea floor spreading?
Harry Hess
Briefly describe Hess’ sea floor spreading theory.
He said there were mid-ocean ridges where the crust was cracked and through which material emerged to create new crust on the seabed.
Elsewhere continents were subducted and therefore crust was destroyed.
THIS EXPLAINED WHY THE EARTH WASN’T GETTING ANY BIGGER OR SMALLER.
Evidence for the sea floor spreading
- Existence of great submarine mountain chains and rift valleys at ocean ridges
- Younger rocks along mid-ocean ridges which become older with distance
- Bands of rock have alternating polarity, reflecting repeated reversal of earth’s magnetic field
- Submarine volcanoes show processes at work e.g. in Surtsey
Evidence of crust being destroyed
Crust must be being destroyed as the earth was not expanding overall.
Evidence used for subduction was deep ocean trenches.
Continents are pushed apart by ________ .
Ridge-push
Subduction of oceanic crust occurs by ______ .
Slab-pull
Describe paleomagnetism
- earth’s magnetic pole periodically flips
- iron within magma will orientate in the direction of the magnetic pole at the time of cooling
- moving out from mid ocean ridges there was a symmetrical strips pattern either side of the ridge showing alternating magnetic orientation within rocks
What did paleomagnetism lead to?
The revival of the continental drift hypothesis and its transformation into plate tectonics.
When was paleomagmetism discovered?
During the 1950s as ships with magnetometers were mapping the ocean floors, a particular pattern began to be revealed.
What are convection currents?
Plates that make up the lithosphere are driven by slow flows of molten magma in the asthenosphere called convection currents.
What can convection currents cause?
In some areas they can break through the crust causing volcanic activity.
What is plate tectonics?
The movement of the plates and the activity inside the earth
What do plate tectonics cause?
Earthquakes and volcanoes
What is the point where 2 plates meet called?
A plate boundary
Name the 3 types of plate boundary
- Constructive
- Destructive
- Conservative
What is a constructive plate boundary?
Plates are forced apart allowing new material to be formed by rising magma. Most common at ocean ridges.
What is a destructive plate boundary?
Plates are forced together causing one of the plates to be subducted or for both plates to buckle upwards at a collision margin.
What is a conservative plate boundary?
Plates slide past each other without crust being formed or destroyed.
