Types of plate margin
There are 2 types of crust:
Continental crust which is older, thicker and less dense than…
Oceanic crust which is younger, thinner and denser
There are 3 directions of movement between plates:
Converging: where 2 plates are moving towards each other
Diverging: where 2 plates are moving away from each other
Passive: where 2 plates are moving side by side
There are 6 different combinations of types of crust and directions of movement:
1.Destructive – where oceanic and continental crust converge
2.Destructive – where oceanic and oceanic crust converge
3.Destructive (Collision) – where continental and continental crust converge
4.Constructive – where 2 plates diverge under the ocean
5.Constructive – where continental crust is diverging
6.Conservative – where 2 plates move parallel to each other
Each of the 6 combinations above create different tectonic hazards and produce different landforms.
Description
• denser oceanic crust is subducted
• friction and heat cause partial melting of thecrust
• this magma is less dense than the mantleand rises / forces its way through fissures inthe lithosphere
Landforms
• deep ocean trenches such as the Peru-Chiletrench
• Fold mountains from rock scraped off thedescending plate and folding of thecontinental crust
Hazards
• Violent volcanoes (composite cone) – due toviscous acidic lavas which block the vents inthe volcanoes
• Powerful earthquakes
1. Destructive boundary - Continental / oceanic convergence
Example: South American Plate/Nazca Plate (The Andes)
Description
• 2 plates with oceanic crust collide
• the densest crust will besubducted
Landforms
• Deep ocean trenches e.g. theMarianas trench
• Island arcs as volcanoes rise outof the sea e.g. Caribbean,Philippines, Japan, Indonesia
Hazards
• Violent earthquakes andvolcanoes
2. Destructive boundary - Oceanic / oceanic convergence
Example: Pacific Plate/North American Plate(Aleutian Islands)
Description
• Subduction of oceanic crustbrings 2 continental massestogether
• Both have a similar density socan’t be subducted
• Sedimentary rocks scraped offthe old sea floor are compressedtogether to form young foldmountains
Landforms
• Young fold mountains such asthe Himalayas (created in the last40 million years) These are stillgrowing despite erosion
Hazards
• Earthquakes as powerful as atdestructive margins
3. Collision boundary - Continental / continental convergence
Example: Indian Plate/Eurasian Plate(The Himalayas)
Description
• plates are moving away from each othercausing the crust to weaken and hotmagma to force upwards creating a ridge
• in the centre of the ridge, the crust cansubside into to magma below creating avalley
• the fissures (splits) in the crust provide aroute for the more fluid lavas to escape ifthese continue submarine volcanoes form
Landforms
• Mid-ocean ridges
• Volcanic islands e.g. Iceland, Surtsey
Hazards
• Volcanoes that erupt with basic (runny)lava – more frequent & less violenteruptions
• Shallow focus earthquakes
4. Constructive boundary – sub-marine divergence
Example: Eurasian/North American (Surtsey)
Description
• occurs where spreading occurs beneath thecontinents
• the up-welling magma leads to fracturingand rifting
• central sections collapse to form rift valleys
• volcanoes form where magma forces itsway through fissures
Landforms
• Rift valleys such as East African Rift valley– these can widen and eventually get floodedby the sea e.g the Red Sea
• Volcanoes such as Mt Kilimanjaro & MtKenya
Hazards
• Volcanoes
• Shallow focus earthquakes
4. Rifting – continental divergence
Example: African Plate (Great Rift Valley)
Description
• plates are moving parallel to eachother, no collision or subductionoccurs
• movement is erratic as platesstick together, pressure builds upand is released in a suddenmovement
Landforms
• Fault lines such as the SanAndreas and Hayward faults
Hazards
• No volcanoes
• Earthquakes as powerful as atdestructive margins
4. Conservative boundary – passive movement
Example: North American Plate/Pacific Plate(San Andreas Fault)
