Key terms today…..

• Silica – Silicon dioxide Si0² a mix of silicon and oxygen the most common material on Earth.

• Viscosity – How ‘runny’ a material (lava) is. The higher the level of Silica, the more viscous (sticky) the lava is.

LO: To be able to describe and explain what happens at the two types of divergent plate boundary and be able to give a detailed example.

Plate Movement:

Why do the plates move?

https://www.youtube.com/watch?v=kwfNGatxUJI

Mid-ocean ridge• A mid-ocean ridge or mid-oceanic ridge is an underwater mountain range,

formed by plate tectonics.

• This uplifting of the ocean floor occurs when convection currents rise in the mantle beneath the oceanic crust and create magma where two tectonic plates meet at a divergent boundary.

• The mid-ocean ridges of the world are connected and form a single global mid-oceanic ridge system that is part of every ocean, making the mid-oceanic ridge system the longest mountain range in the world, with a total length of about 60,000 km.

• There are two processes, ridge-push and slab-pull, thought to be responsible for the spreading seen at mid-ocean ridges, and there is some uncertainty as to which is dominant.

LO: To be able to describe and explain what happens at the two types of divergent plate boundary and be able to give a detailed example.

Divergent Plate Boundaries: Oceanic.

Diagram and sequence of what happens:

clip

Mid Ocean Ridges and other features:• The longest continuous uplifted feature on the surface of the Earth – have a total

combined length of over 60,000km. In some places they rise 3,000m above the ocean floor.

• Their form appears to be influenced by the rate at which plates separate:• A slow rate (Mid Atlantic Ridge) produced a wide ridge with a deep central rift valley.

• An intermediate rate (Galapagos ridge in the Pacific) produces a less well marked rift.

• A rapid rate produces no rift and a smooth ridge. (East Pacific Rise)

• Volcanic activity also occurs along the ridge forming submarine volcanoes which sometimes rise above sea level e.g. Surtsey (an island to the South of Iceland).

• The volcanoes have gentle sides because of the low viscosity of basaltic lava. Eruptions are frequent but gentle.

• As new crust forms and spreads, transform faults occur at right angles to the plate boundary. The parts of the spreading plates on either side of these faults may move at different rates, leading to friction and shallow focus earthquakes.

Transform faults

• Transform faults are commonly found linking segments of mid-oceanic ridges or spreading centres.

• These mid-oceanic ridges are where new seafloor is constantly created through the upwelling of new basaltic magma.

• With new seafloor being pushed and pulled out, the older seafloor slowly slides away from the mid-oceanic ridges toward the continents.

• This separation between segments of the ridges causes portions of the seafloor to push past each other in opposing directions.

• This lateral movement of seafloors past each other is where transform faults are currently active.

It is estimated that 20 volcaniceruptions occur each year along earth's mid-ocean ridges and that every year 2.5 km2 (0.97 sq mi) of new seafloor is formed by this process. With a crustal thickness of 1 to 2 km (0.62 to 1.24 mi), this amounts to about 4 km3

(0.96 cu mi) of new ocean crust formed every year.

The mid-ocean ridges of the world are connected and form the Ocean Ridge, a single global mid-oceanic ridge system that is part of every ocean, making it the longestmountain range in the world. The continuous mountain range is 65,000 km (40,400 mi) long (several times longer than the Andes, the longest continental mountain range), and the total length of the oceanic ridge system is 80,000 km (49,700 mi) long.[

Rift Valley – East African Rift Valley

• Here continental plates are diverging.• The central blocks ‘sink’ creating valleys.• Often these infill with water to form lakes.• In time if they fall below sea level, a new sea can form (for example

the Red Sea).• The volcanoes here produce lava with very little silica, so it is of very

low viscosity.

LO: To be able to describe and explain what happens at the two types of divergent plate boundary and be able to give a detailed example.

Divergent Plate Boundaries: Oceanic and continental

For each plate boundary you need to be able to:• Draw a diagram/sequence of diagrams.• Describe what happens step by step (sequence)• Give explanations for what is happening.• Name and describe the features/landforms found at the plate

boundary.• Give 2 examples of the type of plate boundary including:

• The plates involved.• Names of the features found there.

Task

• Examine the evidence for sea floor spreading (9)