Plate boundaries

Question 1

O-O divergent (Mid oceanic ridge)

Answer 1

● When two oceanic plates diverge, magma from the mantle rises through fractures in the crust to the Earth’s surface and fills the gaps caused by the divergent plates
● The lava cools and solidifies to form basaltic rocks, making new oceanic crusts.
● An underwater mountain chain known as the mid-oceanic ridge is formed at this plate boundary
● At the centre of the ridge is a rift valley, which is the active zone
● Magma will fill up the gaps in this central rift valley, forming volcanoes
● After repeated eruptions, the lava will accumulate, and eventually break into the surface above the ocean to form islands
● Earthquakes also occur here due to the movement of plates

Eg. Mid-Atlantic ridge

Question 2

C-C divergent (Rift valley)

Answer 2

When two continental plates diverge, they are stretched. This causes tension and eventually cracks/fractures on the continental crust
● As plates continue to diverge, fault lines are formed on continental crust
● A central block of land will be lowered along the fault lines, forming a rift valley with steep sides, leaving blocks of land left standing at the side known as block mountains
● Magma from mantle rises through the cracks to the Earth’s surface to form volcanoes.

Eg. East African Rift Valley System (aka Great Rift Valley)

Question 3

O-O convergent (oceanic trench)

Answer 3

● When two oceanic plates converge, the denser plate subducts under the less dense plate. This area is called the subduction zone.
● A depression on the seafloor is created. This is the oceanic trench.
● The subducting plate melts due to friction and high temperatures in
the mantle. Magma is formed.
● Magma will then rise through cracks at the plate boundary to form
volcanoes. (volcanic islands)
● Earthquakes will also occur due to the build up and release of pressure from the convergent plate movement

Eg. Mariana Trench

Question 4

C-C convergent (fold mountain)

Answer 4

● When two continental plates collide, both plates are too buoyant, and thus resists subduction
● Instead, compressional forces will cause both plates to compress against one another
● This will cause the rocks on the plates to buckle and fold along fractures in the crust, thus forming fold mountains
● Earthquakes are very common due to the build up of a high amount of pressure. No volcanoes are formed (no subduction)

Eg. Himalaya Mountain range

Question 5

O-C convergent (ocean trench+ fold mountain)

Answer 5

● When an oceanic plate collides with a continental plate, the denser oceanic plate subducts, forming an oceanic trench.
● The pressure builds up on the continental plate, causing rocks on the plate to buckle and fold, forming fold mountains
● The subducting plate melts due to friction and high temperatures in the mantle. Magma is formed.
● Magma will then rise through cracks at the plate boundary to form volcanoes. (volcanic islands)
● Earthquakes are very common due to the build up of a high amount of pressure.

Eg Peru-Chile trench + Andes fold mountain