Plate Tectonics Flashcards
Continental drift
Theory proposed by Alfred Wegener 1912. That continents were once connected to each other as a landmass called Pangaea.
What are the 2 main observations that Wegener’s conclusions were based on?
Continents fit together like a jigsaw, prehistoric fossils found on countries far away from each other.
Why didn’t many scientists first agree with Wegener’s theory?
There was no way to measure if the continents were moving and there was no known force to move such large landmasses.
Seafloor spreading
Theory proposed by Harry Hess 1962 to explain mid ocean ridges and the great global rift system. That new rocky crust was being formed at the edges of ridges and spreading outwards, crust subjecting and forming ocean trenches.
What are the 3 pieces of evidence that supported the theory of seafloor spreading?
Magnetic striping, age of the sea floor, sediment thickness
What is magnetic striping?
Patterns of strips of rocks with alternating magnetism parallel to mid ocean ridges with patterns symmetrical on each side of the ridge.
How did magnetic striping support Hess’ theory?
They concluded that there were great cracks in the crust and that magma rose up to fill the cracks and form new crust, which was added equally on each side of the ridge.
How did the age of the sea floor support Hess’ theory?
The further the rocks were from the ridges the older they were, suggesting new crust was being formed at the ridges and spreading out.
Rocks on the seafloor were much younger than rocks found on the continents
How did sediment thickness support Hess’ theory?
The crust further from the mid ocean ridges was thicker and ad gathered more sediment, suggesting it was older
What is each section of the crust called?
A tectonic plate
What are all the layers of the earth?
Crust (outermost, solid layer), lithosphere (crust and upper mantle together), asthenosphere (semi solid ‘plastic layer’), core (dense, many iron and nickel) outer core (liquid, flows), inner core (solid)
What are the two theories for how plates move?
- Plates are dragged as hot magma rises up and foes under the plates, creating convection currents, the friction between the plates and flowing magma causes them to move.
- The force of gravity in either slab pull or ridge push.
What is slab pull?
Since the ocean crust in denser at subduction zones, gravity pulls the plate downwards at the subduction zone and away from mid ocean ridges.
What is ridge push?
New crust forms at the ridges above the old crust and gravity pulls the new crust downwards, pushing older crust underneath and squeezes plates sideways.
What was the believed process that continents split up by and how did it work?
Rifting, the crust cracked and subsided, allowing in water from the ocean. Then seafloor spreading occurred. The continents moved along with the ocean floor that was between them.
What are the 2 types of crust? compare and contrast
Oceanic thinner and denser (contains heavier elements) , continental thicker and less dense with lighter elements
What are the 3 types of plate boundaries?
Diverging, converging and transform
What are diverging boundaries?
Plates that are moving in opposite directions, away from each other. They leave a rift between them,magma rises into the rift and solidifies as it cools and forms new crust. (constructive boundaries)
What are converging boundaries?
Plates that are moving towards each other and colliding, rock is destroyed (destructive boundaries). Form mountains, tenches, and chains of islands.
How are fold mountains and volcanoes formed?
When oceanic crust collides with continental crust and the oceanic crusts subducts, the continental crust becomes distorted and folds, forming fold mountains and volcanoes.
How are trenches formed?
Where an oceanic plate subducts underneath a continental plate, a deep depression in the crust is formed called a trench
How are high mountain systems formed?
When 2 continental plates collide, both are pushed upwards because they have similar densities
How are island arcs formed?
When 2 oceanic plates collide, the faster moving plate subducts and forms a trench. The descending plate melts to form magma which rises up to the surface creating a chain of volcanic islands.
What is a transform boundary?
Two plates are sliding parallel to each other in opposite directions. Usually travel slowly then slip quickly past each other with force. Causes earthquakes, fold mountains and fault lines
What are fault lines?
Cracks in the rock at transform boundaries, usually not one continuous crack but many parallel to each other.
What is a volcano?
A place where extremely hot material from inside the Earth erupts at its surface.
What are examples of the material that erupts from a volcano?
gas, ash, lava, lumps of solid rock
When do volcanoes form?
When magma is formed at weak spots in Earth’s crust. It is pushed upwards wit great force into the volcano
Why are most volcanoes at plate boundaries?
Because they cause weaknesses in the crust and generate heat to melt rock
How do diverging plate boundaries weaken the crust and form volcanoes?
They thin the crust and lower the force on the rock in the asthenosphere and they melt and rise through weaknesses.
What are hot spots?
Isolated weak spots in the crust not at plate boundaries where a lot of hot magma is being created
What are the 2 possible origins of the magma formed at hot spots?
Heat escaping in narrow plumes from the mantle and stays in a fixed place.
Magma created where crust is flexing as tectonic plates move.
What is an earthquake?
A rapid movement of the ground in a wave motion due to the movement of tectonic plates.
How do earthquakes occur?
When the tectonic plates overcome the force of friction and the plates suddenly move.
What are seismic waves?
The waves in Earth caused by earthquakes
What are the 3 main types of seismic waves?
Primary waves, secondary waves and surface waves
What are primary waves (p-waves)?
Longitude waves that travel fast back and forth through the earths crust
What are secondary waves (s-waves)?
Transverse waves that travel slightly slower than p-waves up and down through earth
What are surface waves?
The slowest waves that travel through the surface and can cause the most destruction
Where do the strongest and most earthquakes occur?
At converging boundaries
What is the focus of an earthquake?
The place underneath the surface where it starts
What is the epicentre of an earthquake?
The place on the surface directly above the focus
How is the severity of an earthquake measured?
Using the richter scale- each successive number is 10x more severe, or moment magnitude scale
What are the 2 main reasons that Australia is such a stable continent?
The lithosphere is thicker than any other continent, resistant to melting and fracture
Not near plate boundaries were the crust is weakened
