Constructive: continental-oceanic

• Occurs above rising convection currents where plates are moving away from each other. Overlying plate is stretched thin, breaks and pull apart. Magma rises as is less dense and turns molten due to change in pressure. It finds weakness and faults in the thinned overlying plate and breaks through. This erupt basaltic magma and it hits the cold water to form pillow lava. The basalt rock is porous so ocean water can seep through but when the rock is still hot in the middle, the water becomes super heated creating black smokers. These precipitate out materials and heavy metals such as sulphur.
• these eruptions build up over time and a mid-oceanic ridge is formed. the steepness of the ridges slops depends on the rate of spread-slow=steep. the rift in the middle of the ridge is again prominent and deep where it’s slow. example the Mid-Atlantic Ridge is spreading at 2-3cm/yr. the East Pacific Rise is the fastest at 11cm/yr and has a gentle gradient with a shallow rift. the crust either side of the ridge is older the further away from the ridge you go.
• underwater sea-mounts are created. eventually these will grow so they break the surface as a volcanic island. the ridge is not completely linear and perpendicular transform faults connect the ridge due to tension all stresses.
• landforms: it cools quickly forming basaltic rock known as ‘pillow lava’ and a mid-ocean ridge. if the plates are slow moving away you will get a high ridge with steep slopes. newest rock is at the ridge itself and it gets older the further away you go from the ridge. you will find black smokers with heavy metals settled around it.