Section 1 - Plate tectonics Flashcards
What is the Earth’s core made of?
Inner core - Solid ball containing iron and nickel
Outer core - Semi molten and also contains iron and nickel
What is the difference between continental crust and oceanic crust?
Continental is thicker (30-70 km thick) and is less dense. Oceanic crust is thinner (6-10 km thick) and is more dense
How do tectonic plates move?
1) radioactive decay of some elements in the mantle and the core generates a lot of heat
2) When lower parts of the asthenosphere heat up they become less dense and slowly rise
3) As they move up towards the top of the asthenosphere they cool down, become more dense, and sink back down
4) These circular movements of semi-molten rock are called convection currents
5) Convection currents in the asthenosphere create drag on the base of the tectonic plates which causes them to move.
what is sea floor spreading and what can it create?
When the plates are dragged away from each other, magma rises in the gap formed to make new crust. When this happens at a plate margin under the sea the sea floor gets wider. It can create mid-ocean ridges which are ridges of higher terrain on either side of the margin.
What is the theory of continental drift?
1) In 1912 Alfred Wegener proposed the theory of continental drift.
2) He suggested that all the continents were once joined together as one super-continent called Pangaea, which drifted apart
3) He based his theory on geological evidence and fossil records, but couldn’t back it up with a mechanism that explained how the continents moved.
4) Over the years more evidence backed up his theory and it was developed further by scientist after these findings and it grew into the theory of plate tectonics
What is the evidence for the theory of plate tectonics?
1) Geology - areas of South America and Africa have rocks of the same age and composition, if you fit these continents together they match up
2) Fossil records - by fitting land masses together you can match up the distribution of some fossils, its very unlikely that these species migrated across thousands of miles of water, this suggests that these places were joined together when the organisms were alive
3) Living species - The same living organisms can be found on different continents and again, it’s unlikely that they migrated so far, so it suggests that the continents were once joined
4) Climatology - There’s evidence that past climates of some continents were similar despite being thousands of miles apart, this suggests that they were located together, and in a different place to where they are now. eg similar glacial deposits are found in Antarctica, Africa, South America and Australia suggesting that millions of years together they were joined together and located near the South Pole.
5) Palaeomagnetism - Palaeomagnetism is the study of the history of the Earth’s magnetic field. Once every 200,000 years the Earth’s polarity reverses. As magma erupts from mid-ocean ridges, magnetic minerals in the molten rock align themselves with the direction of the Earth’s magnetic field. This creates a striped pattern over years showing that the plates are moving away from each other
What is a mid-ocean ridge?
Where diverging plates are underwater, a mid ocean ridge forms, underwater volcanoes can erupt along mid ocean ridges and they can build up to be above sea level
What are rift valleys?
Where plates diverge beneath land, rising magma causes the continental crust to bulge and fracture, forming fault lines. As the plates move apart the crust between the faults drops down to form a rift valley
What happens at a destructive plate margin between an oceanic and a continental plate?
1) Where oceanic and continental plates converge, the more dense oceanic crust is forced under the less dense continental crust. This forms a deep sea trench.
2) Fold mountains also form where the plates meet. They’re formed by sediment that have accumulated on the continental crust, which are folded upwards along with the edge of the continental crust
3) The oceanic crust melts into magma and the magma then rises back up to the surface to form volcanoes
4) The pressure of the collision can cause an earthquake
What happens at a destructive plate margin between an oceanic and an oceanic plate?
The denser of the two is subducted and forms a deep sea trench and can trigger earthquakes
What happens at a destructive plate margin between a continental and a continental plate?
Neither is subducted so there aren’t any volcanoes but earthquakes can occur
What is intrusive volcanic activity?
Takes place beneath the Earth’s surface and includes the formation of large magma chambers and magma being forced into the crust
What is extrusive volcanic activity?
Takes place on the Earth’s surface, the major form of this activity is volcanic eruptions of lava and other materials
What are examples of minor types of extrusive volcanic activity?
Hot springs, geysers and boiling mud pools
How are batholiths formed?
When large chambers of magma cool underground they form domes of igneous rock called batholiths
How are dykes and sills formed?
Where the magma has flowed into gaps in the surrounding rock and cooled it forms vertical dykes and horizontal sills
