Types of plate marginThere 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 the crust

• this magma is less dense than the mantle and rises / forces its way through fissures in the lithosphere

Landforms

• deep ocean trenches such as the Peru-Chile trench

• Fold mountains from rock scraped off the descending plate and folding of the continental crust

Hazards

• Violent volcanoes (composite cone) – due to viscous acidic lavas which block the vents in the volcanoes

• Powerful earthquakes

1. Destructive boundary - Continental / oceanic convergence

Description

• 2 plates with oceanic crust collide

• the densest crust will be subducted

Landforms

• Deep ocean trenches e.g. the Marianas trench

• Island arcs as volcanoes rise out of the sea e.g. Caribbean, Philippines, Japan, Indonesia

Hazards

• Violent earthquakes and volcanoes

2. Destructive boundary - Oceanic / oceanic convergence

Description

• Subduction of oceanic crust brings 2 continental masses together

• Both have a similar density so can’t be subducted

• Sedimentary rocks scraped off the old sea floor are compressed together to form young fold mountains

Landforms

• Young fold mountains such as the Himalayas (created in the last 40 million years) These are still growing despite erosion

Hazards

• Earthquakes as powerful as at destructive margins

3. Collision boundary - Continental / continental convergence

Description

• plates are moving away from each other causing the crust to weaken and hot magma to force upwards creating a ridge

• in the centre of the ridge, the crust can subside into to magma below creating a valley

• the fissures (splits) in the crust provide a route for the more fluid lavas to escape if these 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 violent eruptions

• Shallow focus earthquakes

4. Constructive boundary – sub-marine divergence

Description

• occurs where spreading occurs beneath the continents

• the up-welling magma leads to fracturing and rifting

• central sections collapse to form rift valleys

• volcanoes form where magma forces its way through fissures

Landforms

• Rift valleys such as East African Rift valley – these can widen and eventually get flooded by the sea e.g the Red Sea

• Volcanoes such as Mt Kilimanjaro & Mt Kenya

Hazards

• Volcanoes

• Shallow focus earthquakes

4. Rifting – continental divergence

Description

• plates are moving parallel to each other, no collision or subduction occurs

• movement is erratic as plates stick together, pressure builds up and is released in a sudden movement

Landforms

• Fault lines such as the San Andreas and Hayward faults

Hazards

• No volcanoes

• Earthquakes as powerful as at destructive margins

4. Conservative boundary – passive movement