Plate Boundaries Flashcards
What is the oceanic crust?
- The plate that is made up of more dense rock.
- It sinks easily due to its density and is constantly created at boundaries
- It is ‘young/new rock’ and forms our ocean bed
What is the subduction zone?
Where one plate is forced downwards below another plate and grinds past it causing huge amounts of friction and heat
What is a convergent (destructive) plate boundary?
Where two plates are moving towards each other, one of which is continental crust and one is made of oceanic crust
What is the continental crust?
- A plate that is made up of less dense rock which means it doesn’t sink
- It is very old and makes up our land surfaces
What is a plate boundary?
Where two very large crustal plates meet on the earth’s surface
What are fold mountains?
Large mountain ranges formed by the collision and ‘folding’ of two plates as they continually push into each other
What is an ocean trench?
A particularly deep point of the ocean bed where the oceanic and continental crust have dragged each other downwards
What the differences between the continental and oceanic crust?
- Continental is 70km thick, oceanic is 7km
- Oceanic is 200 million years old, continental is 3.6 billion years
- Oceanic is made of denser basalt, continental is made of granite
What is an example of a convergent plate boundary?
Nazca and South American plates
- Convergent plate boundary
Denser Oceanic plate (Nazca) is forced below the continental plate
- Convergent plate boundary
Less dense Continental plate (South American plate) moves towards Oceanic plate
- Convergent plate boundary
Pressure builds up at the subduction zone - friction causes melting of oceanic plate and can cause earthquakes when friction is released
- Convergent plate boundary
Molten rock (magma) builds up in the chamber
- Convergent plate boundary
When the pressure builds up magma escapes through a composite volcano
- Convergent plate boundary
Lighter continental crust remains at the surface but crumples into fold mountains (Andes)
What are pyroclastic flows?
Fast-moving currents of hot gas and rock, which reaches speeds of up to 450 mph, and temperatures of about 1000°C
How do pyroclastic flows move?
They normally hug the ground and travel downhill, or spread laterally under gravity
What is a volcanic bomb?
A mass of molten rock larger than 64mm, formed when a volcano ejects viscious fragments of lava during an eruption
How do volcanic bombs move?
They are thrown many kilometres from an erupting vent, and become more aerodynamic further into their flight
What do lava explosions do when erupted?
They can erupt during either non-explosive or explosive lava fountains and destroy everything in their path
What are lahars?
They are when a snowy volcano erupts, melting the snow and ice and have the thickness of wet concrete and travel at 30 mph
How do lahars travel?
Down a volcano and mixes with mud and debris from forests and communities, which turns into a “Mudslide of Death”
What else can be formed at convergent plate boundaries?
Tsunamis which are caused by the sudden uplift of the oceanic plate which displaces huge amounts of water
- Conservative boundary
Earthquakes occur here because two plates are sliding past each other
- Conservative boundary
The plate can also be moving in the same direction at different speeds such as the Pacific plate which moves north at 6cm/year and the North American plate which also moves north at 2cm/year
- Conservative boundary
Plates become stuck and friction builds up
- Conservative boundary
Eventually the pressure builds up and is suddenly released into an earthquake
- Conservative boundary
No new crust is created or destroyed because the plates are not forced up or down, therefore there are no volcanoes
When has San Francisco suffered from an earthquake?
1906 and 1989
What were the impacts of the 1989 San Francisco earthquake?
- 63 people killed
- 3,757 people injured
- 2 mile section of freeway collapsed
- Damage cost $10 billion
What evidence is there for collision plate boundaries?
- Joining up of countries, e.g. India
- Rocks folding
- Collision boundary
90 million years ago, Indian and Eurasian plate were separated by the Sea of Tethys and rivers drained land around the ocean and deposited sediment at the ocean floor
- Collision boundary
Deposition meant pressure increased and created new sedimentary rock on the ocean floor
- Collision boundary
50 million years ago, the Indian plate slammed into the Eurasian plate driven by convection currents but because they had the same density as they are both continental they rised up together
- Collision boundary
As the two plates push together, the plates get jammed and lock together due to friction and this causes sudden earthquakes
- Collision boundary
As the fold mountains are young, they are still growing and expected to grow by 5000m but erosion and weathering will limit this
