Studied by 5 people
plate boundaries and the landforms that occur at them
what is distribution?
the spatial coverage of the hazard
what is frequency?
refers to the distribution of the hazard through time
what is magnitude?
assesses the size of the impact
what three main components is the interior of the earth divided into?
the crust, the mantle and the core
what is the core made of?
dense rocks containing iron and nickel alloys
what are the two parts of the core?
inner core and outer core
what is the mantle made of?
molten and semi-molten rock rich in iron and magnesium
what is the upper part of the mantle called?
asthenosphere
how is the asthenosphere different from the mantle above it?
the asthenosphere is more fluid and hotter whereas the mantle above it is more solid
what are the two types of crust?
oceanic and continental
which crust is thicker?
the continental
who developed the theory of plate tectonics?
Alfred Wegner
what is continental drift?
the continents were originally joined together as one landmass - after which they separated and slowly drifted along the ocean flows to their current locations.
what is sea-floor spreading and how did it help Wegner’s theory?
sea-floor spreading: new ocean crust was continually being created in zones in the middle of the ocean
the continents were moving but the creation of the crust was the driving force
what are convection currents?
heat within the earth powers the movement of the plates on its surface. at temps of over 1,000 the rocks in the mantle flow towards the surface - high pressure of rocks above the crust stop them melting in the heat. nearer the earth’s surface the rocks cool and move sideways, then as they loose the heat from the core they move downwards.
how do convection currents cause plates to move?
the sideways movement of rocks beneath the crust
what is ridge push/gravitational sliding?
at constructive boundaries, less dense, hot magma wells up and produces an ocean ridge above the ocean floor. as the rock gets older it cools and becomes denser. gravity acts on the older lithosphere, causing it to slide away from the ridge down the sloping asthenosphere below. due to the tearing apart of of the newly formed crust, shallow earthquakes form, indicating that there is some frictional resistance to the force.
what is slab pull?
at a destructive boundary, the oceanic plates are subducted under the overlying mantle. as the subducting plate is heavier it sinks into the mantle due to the downward gravitational force acting on it and pulls the plate down into the mantle.
what are the three types of plate margin?
conservative (transform), constructive (divergent) and destructive (convergent)
what happens to plates at constructive (divergent) margins?
plates move apart - earthquakes+volcanos
plates are pushed apart by magma rising from the mantle beneath the crust. as soon as the magma reaches the surface it solidifies, forming a new crust.
what happens to plates at destructive (convergent) margins?
plates collide
the oceanic plate subducts under the continental plate into the subduction zone.
what happens when two oceanic plates collide?
one is usually subducted underneath the other - same as normal, and a line of volcanic islands (island arc) appears.
what happens when two continental plates collide?
they collide and buckle up to form fold mountains
what happens to plates at conservative (transform) margins?
plates slide horizontally past each other
no volcanoes, earthquakes occur as the plates tend to stick, creating friction - then slip violently rather than sliding smoothly
what are ocean ridge systems?
underwater mountains and volcanoes formed at constructive margins
how are rift valleys formed at constructive margins?
New convection currents in mantle increase pressure on lithosphere – lithosphere stretches and warps upwards which creates large fractures
As convection crusts drag lithosphere, crust subsides along fractures leaving almost-vertical, stepped valley sides
Volcanic activity is found in many forms around the valley
As the plate continues to be separated and crust subsides further, oceanic water will eventually flood the valley
A new constructive margin forms, which starts to build a mid-ocean ridge and the ocean becomes larger as the sea floor spreads
e.g. east African rift valley
how do deep sea trenches form at destructive margins?
oceanic plate is forced under lighter continental plate in the process of subduction. the downwards displacement of oceanic plate forms a deep-sea trench that runs parallel to the plate boundary.
how do fold mountains form at destructive plate boundaries?
during the subduction process, sediments that have accumulated on the edge of the continental plate are deformed by folding and are then uplifted to form young fold mountains.
what are magma plumes:
magma plumes are formed when heating at the core/mantle boundary causes a plume of magma to rise through the mantle and eat into the plate above
how do magma plumes form hotspots?
when the lava breaks through to the surface, active volcanoes form above the hotspot. as a plate moves away from the hotspot, it takes with it the volcano that had formed. the volcanoes cool and subside and over millions of years, hotspot tracks form. the volcanoes get progressively older the further they move away from the hotspot. the oldest volcanoes are the smallest due to erosion.
what is Harry Hess’s evidence for sea-floor spreading?
Paleomagnetism is the study of rocks that show the magnetic fields of the Earth.
As new rock is formed and cools the magnetic grains within the rock align with the magnetic poles. Our poles (North and South) switch periodically.
Each time these switch the new rocks being formed at plate boundaries align in the opposite direction to the older rock. On the ocean floor either side of constructive plate boundaries, Geologists observed that there are symmetrical bands of rock with alternating bands of magnetic polarity.
when does an ocean ridge form?
when the diverging plates are under the ocean
As the plates move apart, magma rises up to fill the gap and this accumulates over time to become taller and wider
The Mid Atlantic Ridge is an example of an ocean ridge
what happens when two continental plates meet (a collision boundary)?
Two plates of similar density move towards each other
Neither is dense enough to subduct so the land is pushed upwards
This forms fold mountains such as the Himalayas
what are strato volcanoes?
Plate Boundary: Destructive (Convergent) Boundaries – where an oceanic plate subducts beneath a continental plate.
Formation Process: Subduction causes melting of the oceanic plate, forming magma that rises through the continental crust.
Magma Type: Andesitic or Rhyolitic magma – viscous, high silica content, traps gas, leading to explosive eruptions.
Eruption Style: Explosive, violent eruptions with pyroclastic flows, ash clouds, and lahars.
Example: Mount St. Helens (USA), Mount Pinatubo (Philippines), Mount Fuji (Japan).
what is shield volcanoes?
Plate Boundary: Constructive (Divergent) Boundaries – where tectonic plates move apart (e.g., mid-ocean ridges).
Formation Process: Magma rises from the mantle through fissures, creating broad, gently sloping volcanoes.
Magma Type: Basaltic magma – low silica, runny, allows gases to escape easily.
Eruption Style: Effusive eruptions, producing lava flows that spread over large areas.
Example: Mauna Loa and Kīlauea (Hawaii), Surtsey (Iceland).
how are island arcs formed?
destructive plate
forms at a destructive plate margins, where an oceanic plate is subducting beneath another
subducted plate meets in the mantle, creating new magma that then rises through cracks in the crust of the other plate and erupts at the surface
as the other plate is under water, the eruptions create an arch of volcanic islands that follow the plate boundary due to the movement of the plates, the arc will move away from the boundary over time
Japan is a classic island arc and has multiple active stratovolcanoes, representing the nature of a destructive boundary
what landforms form at constructive margins?
Rift valley
Mid oceanic ridges
Shield volcanoes
how does ridge push occur at constructive plate margins?
magma rises from the mantle to create new crust, forming a mid-ocean ridge.
magma cools and solidifies → new crust, elevated due to thermal expansion.
Gravity causes the newly formed crust to slide away from the ridge, down the sloping asthenosphere pushing the tectonic plates apart.
how does slab pull occur at constructive plate margins?
oceanic plate moves away from the ridge, it cools, becomes denser + reaches a subduction zone at a destructive plate margin.
denser oceanic plate begins to sink into the mantle under its own weight in a process called subduction.
The downward force exerted by the sinking plate pulls the rest of the plate along with it, helping to drive plate movement.
how do slab pull and ridge push work together at margins?
Ridge push initiates movement at the mid-ocean ridge by forcing plates apart.
As the oceanic plate moves further away, it cools and eventually becomes dense enough to be subducted at a destructive margin.
Slab pull takes over as the denser plate sinks, exerting a stronger force that continues to drive the motion of the plates.
what are some of the characteristics of shield volcanoes?
When volcano erupts the lava spreads for a while before cooling down
Easy to predict and monitor – not particularly explosive
Main hazards are lava flows, steam and ash but no pyroclastic flows or explosive eruptions – nature of volcano is dictated by plate margin
how do mid-oceanic ridges form?
Ridge push starts the movement by forcing plates apart at the mid-ocean ridge.
As the oceanic plate moves away and becomes denser, slab pull at a subduction zone takes over, pulling the plate further.
This cycle continues, ensuring the constant formation of new oceanic crust at mid-ocean ridges and its destruction at subduction zones.
what landforms form at destructive plate margins?
Deep ocean trench
Young fold mountains
Stratovolcano
Island arcs
what happens at destructive plate margins?
Main force driving plates is convection currents
Oceanic and continental plate are moving towards each other -> oceanic plate is denser and sinks between the continental plate
As it sinks, it grates past the other plate causing friction and huge earthquakes
The oceanic plate starts to melt in the subduction zone beneath the surface which generates new magma which is less dense
Melted magma from the plate then rises through gaps in the continental plate and erupts at the surface forming volcanoes
where are deep ocean trenches formed?
in subduction zones
how do young fold mountains form at destructive plate boundaries?
Continental plate crumbles and folds and is pushed up into fold mountains so ends up with a long mountain range
what are some of the characteristics of a strato volcano?
Lava created by melting plate so it is a low silica content and has higher viscosity – it plugs the vent of the volcano, pressure from lava beneath leads to plug being blown out in an explosive eruption
Is much more unpredictable – may have some small tremors
Made up of layers of ash and lava but have a classic cone shape due to layers
Pyroclastic flows and nuee ardentes and lava bombs ash and emissions of sulphur dioxide and carbon dioxide, lahars/mudflows - when pyroclastic flows are mixed with water
Lahars are dangerous as very very fast e.g. mt pinatubo coincided with typhoon
how can volcanic activity differ?
contextual factors
e.g. iclandic erupted the glacier due to its explosivity potential which created lahars which would not be expected
how does an island arc form?
slab pull, is the main driver of subduction and island arc formation.
As the plate sinks, it melts due to increased heat and pressure, forming magma that rises through the overlying plate.
This magma erupts at the surface, creating a chain of volcanic islands known as an island arc (e.g., the Mariana Islands in the Pacific).
→ One oceanic plate subducts beneath another, forming a deep-ocean trench.
The sinking plate pulls the rest of the plate along with it (slab pull).
The subducting plate melts, and magma rises to the surface, forming an island arc.
what are collision zones?
two continental plates
how do collision zones form fold mountains?
Two continental plates move towards each other – neither sinks and both plates crumple and fold upwards
Due to the crumpling of massive pieces of rock, big earthquakes are common at these plate boundaries
Rivers on land + things sinking in the ocean creates layers of sediment on the bottom of the ocean
As these plates get closer the sediments on the bottom of the ocean start to crumple and fold upwards
Eventually when the two plates meet they do the same but they have the layers of sedimentary rock on top so all folds simultaneously to create young fold mountains
e.g. Himalayas
what happens at conservative plate margins?
two tectonic plates are sliding past each other – sometimes in opposite directions, sometimes in the same direction
As these slabs of rock slide past each other they create friction
Bits of the rocky surface will get stuck and tension builds
When the rocks free
e.g. san andreas fault in LA
what is the hot spot/magma plume landform?
island chains
how are island chains formed?
A hot spot on the core of the earth will cause a magma plume to appear in the mantle
Where this magma plume encounters a weaker/thinner part of the lithosphere it will break through the crust and erupt as a shield volcano
Plate it has broken through will be moving, and so as it moves the magma plume will 'punch holes' in the crust and create a chain of volcanic islands – an island chain
e.g. Hawii - youngest island still have active volcanoes e.g. Kilauea and Mauna Loa -> as you get further away from the active volcano, the islands will be older as they were created longer ago
Will also undergo erosion by the sea and so are likely to be smaller
how can volcanic activity be measured?
UV cameras – looking for heat
Scanning DOAS – measure ground deformation, when lava gathers in magma chamber it makes the ground swell e.g. tilt meters and satellite imaging which suggests the volcano may errupt and pressure is building
Spectrometer – measures sulfur dixoide -> indicates when it will erupt as emissions increase significantly BUT does not show what type of erosion, could just be gas
Seismometers to measure tremors – when they increase in intensity and frequency
how is monitoring volcanoes used?
for prediction and evacuation
USGS – monitor every active volcano in the world -> means economic development and income level does not effect this e.g. mt pinatubo - Philippines had lots of American air bases, 2 weeks before eruption they evacuated, evacuated around 15,000 people from surrounding area
However, 847 people died -> but indegenous farmers so could be due to fatalism
what is an issue with volcanoes?
However, difficult to prevent -> try to erect barriers to stop lava flows and spraying lava with sea water to encourage solidification
how can earthquakes be predicted to an extent?
Good at mapping and measuring them – USGS -> monitors every fault line in the world
Historic records -> mapping of EQ's and movement of fault lines using satellite technology
what are some ways in which buildings can be made more earthquake secure?
Birdcage interlocking steel frame
Outer panels attached to steel structure
Roads to provide quick access for emergency services
Fire-resistant materials
Foundations sunk into bedrock avoiding clay – can be retrofitted
Rubber shock-absorbers to absorb earth tremors
Computer controlled weights on roof to reduce movement
what are some examples of using materials in LICS to retrofit buildings against earthquakes?
Haiti: light roofs and small windows
sheet-metal roofs on wooden trusses are more resilient
regularly spaced openings create fewer weak spots in walls
peru: reinforced walls
natural materials such as bamboo or eucalyptus act as reinforcing rods + walls have been fitted with mesh wire to prevent collapse
indonesia: shock absorbers
tires filled with stones or sand and fastened between floor and foundation can serve as cheap-ground motion absorbers for many types of buildings
what are some adaptation measures against earthquakes?
Earthquake drills
Public information campaigns
Emergency earthquake kits '
Early warnings + alerts (More for tsumani's)
Building codes in law that are monitored for compliance
Organised, dedicated emergency services e.g. search and rescue
Dedicated government department for disaster management -> LA, have dedicated hazard management part of state government which coordinates response
Codes for practice for public response – e.g. trains stops immediately
Note 
Flashcard (42)