✅HAZARDS 3.1.5.2 - Plate Tectonics Flashcards
Describe the crust:
The thin layer of the outer shell that we live on, 5-10km thick beneath oceans and 70km thick beneath continents, lithosphere
Describe oceanic plates:
An occasionally broken layer of basaltic rocks known as sima - silicon and magnesium
Describe continental plates:
Bodies of mainly granite rocks, known as sial - silicon and aluminium
What is the lithosphere?
The crust and upper mantle, where tectonic plates are formed
How thick is the mantle?
2900km
What is the asthenosphere?
A layer of soft, plastic like rock that carries the lithosphere
How hot is the core?
5000 degrees C
What is gravitational sliding?
The movement of tectonic plates under the influence of gravity
What is ridge push?
Gravity acting on the weight of the lithosphere near the ridge pushes the older part of the plate in front.
What is slab pull?
The lithosphere sinks into the mantle under its own weight following subduction, helping to ‘pull’ the rest of the plate with it.
What events and landforms occur at Constructive boundaries?
Volcanoes
Mid ocean ridges
Rift valleys
What events and landforms occur at Destructive boundaries?
Powerful earthquakes
Volcanoes
Fold Mountains
Ocean trenches
What events and landforms occur at Collision boundaries?
Powerful earthquakes
Fold Mountains
What events and landforms occur at Conservative boundaries?
Powerful earthquakes
What is jigsaw fit evidence for tectonic theory?
Similarity in outlines of West Africa and South America as well as other continental areas. Best fit at 1000m below sea level.
What is geological fit evidence for tectonic theory?
Ancient rock outcrops from South America and West Africa from over 2000 million years ago were continuous
What is Tectonic (orogenic) fit evidence for tectonic theory?
Fragments of the Caledonian mountain belt are found in Scotland, England, Greenland, Canada and Scandinavia
What is glacial deposit evidence for tectonic theory?
300 million year old deposits found in Africa, Australia, Antarctica, South America and India suggest ancient ice sheet
What is fossil evidence for tectonic theory?
Bands of identical fossils lie across continents, particularly of organisms which could not have travelled.
What happens at a conservative boundary?
Two plates do not directly collide but slide past one another
What happens at a constructive boundary?
2 plates are moving apart, leaving a gap for magma to rise up through. Volcanoes form but don’t erupt with force and earthquakes occur
Where are rift valleys common?
Where 3 plates meet at a junction
What happens at destructive boundaries?
Dense oceanic plate descends beneath less dense continental plate. Oceanic plate melted due to friction forming magma
What happens at collision boundaries?
Two plates of similar densities move together, causing the material between them to buckle and rise up
Where do rift valleys form?
On constructive boundaries
How do rift valleys form?
Magma rises and plates move apart
Over a magma chamber, crack and faults appear
Blocks of crust descend into mantle, creating steep sided valleys
Central plateaus sink in the middle, forming lakes
Mountains often form on the sides
What is a Benioff zone?
The further the rock descends, the hotter it gets. Together with the heat from friction begins to melt the plate to magma
What boundaries do ocean trenches form at?
Destructive
What are convention currents in the asthenosphere
Movement of hotter material in the mantle rising (causes magma to rise)
What is Plate Tectonic Theory
Idea the Earth’s surface is split into plates which glide over the Mantle (asthenosphere)
Destructive Plate boundary Example: what is the Ring of Fire
Surrounds Pacific
40,000 km
Many intense active volcanoes+ earthquakes
Krakatoa 1883 eruption, 2010 Japan EQ
Destructive Plate boundary Example: what is the Alpide Belt
Includes the Indian Plate, Eurasian Plate. Orogenic (mountains formed, e.g Himalayas)
15,000 km
Constructive Plate Boundary: Mid Atlantic Ridge
The largest mountain chain in the world 16000km, is underwater.
South American and African Plates
Slab Pull
Ridge Push
The upwelling of hot material creates a buoyancy effect which creates an ocean ridge. Here the plate experiences the force of gravity, pushing the plates
Rift Valley Example
What boundary do island arcs form at
Destructive
Oceanic and oceanic plates
How do island arcs form
Denser oceanic plate subducts
In the benioff zone, magma upwelling occurs
Volcanic island form
What is an example of an island arc
Pacific plate subducts under the Philippine plate
Line of volcanic islands including Guam appear
What is a plume
a hot column of magma which rises from deep within in the Earth
Why does not all volcanic activity occur at a plate boundary
Due to tectonic plate movement away from a hotspot where a volcanic island can form
What is a hotspot
A hot spot is an area on Earth over a mantle plume or an area under the crust where magma is hotter than surrounding magma. The magma plume causes melting and thinning of the crust and volcanic activity due to the localised heating between the crust and mantle
How do island chains form
Theory:
Active volcanoes form from mantle plume above hotspot
Tectonic plate moves taking with it the volcano, hotspot remains stationary
Another volcano is formed over the hotspot
Process continues until a chain is formed
Islands furthest from hotspot are oldest
Example of an island chain
Hawaiian islands
Youngest island is submarine volcano The Loihi Seamount, which will eventually emerge into a new island in the chain
Has active shield volcanoes like Mauna Loa
6000km long
shows movement of tectonic plates (the plate moves approx. 10 cm a year)
What is a sill
if magma moves along horizontal bedding planes, it cools to form sills
Often in composite volcanoes
Lava can erupt out of - causing volcano eruptions to be unpredictable - not sure where lava will come from
What is a dyke
If the magma is intruded along vertical joints, it forms dykes
