Tectonic movements

What is seafloor spread

Seafloor spread is where tectonic plates pull laterally a part at mid-ocean ridges, magma then rises to fill the gap forming new oceanic crust. This proves earth’s crust is continually being created. Leads to formation of ridges such as mid-Atlantic ridge.

Note:

older rock found further away from ridge.

At what rate does the crust grow a year

Around 2-16cm a year the same rate at which your fingernails grow

Why do continental plates subduct underneath the oceanic plates

Continental plates aren’t as dense as oceanic plate so does not sink into the asthenosphere

What is palaeomagnetism

1. Magma erupts on ocean floor at constructive plate boundaries as they are moving apart

2. Iron minerals with iron-rich basaltic rocks are aligned to the magnetic field of the earth and become polarised

3. Earth’s polarity reverses approximately every 400,000 years, which results in a series of magnetic stripes in alternating polarity within rocks

4. Pattern mirrored on both sides of ridge, suggesting sea floor is spreading away from boundary

How convection currents work

1. Earth’s core releases heat energy, heating up rocks in lower mantle which means they are less dense than the cooler rock above in the mantle

2. Warmer rock rises, whilst cooler rock in upper mantle begins to sink. Movement of rocks means convection currents formed within mantle

3. As mantle heats up from convection currents these currents warm the crust. The heat drives tectonic movement

OUTDATED though

What is ridge push

1. Newly formed crust is hotter and less dense than older oceanic crust so rises forming the elevated ridge

2. So height of mid ocean ridge is higher than surrounding ocean floor so gravitational force cause lithosphere to slide away from ridge towards subduction zone-this is called ridge push

3. As plates spread they get older and denser and sink slightly

What is slab suction

1. As oceanic plate sinks into mantle it drags surrounding mantle with it

2. This creates downward flow of mantle generating weak suction force which sucks overlying plates together

What is slab pull

The force at subduction zones where a cold dense oceanic plate sinks deep into mantle pulling rest of plate into lithosphere driving tectonic movement