Constructive Boundaries

• Plates separate at a Constructive Boundary.
• In constructive boundaries magma rises to the surface and cools and hardens to form rock.
• Newer rock is found closest to constructive boundaries, older rock is found further away.
• As magma continues to flow to the surface a mountain chain known as a mid - ocean ridge will form.
• The Mid – Atlantic Ridge is forming where the Eurasian plate is moving away from the North American plate.
• The crust is moving apart at a rate of 5 cm  per year.
• This mid – ocean ridge breaks through the ocean surface as volcanic islands eg. Iceland.
• One side of Iceland is on the North American Plate, the other on the Eurasian plate.
• Iceland is getting wider as the plates continue to move apart.
• A rift valley may form at a Constructive Boundary e.g. the East African Rift Valley.
• As the plates pull apart, the earth's crust is stretched, creating a valley with steep sides and a flat floor.
• Shield volcanoes also form at constructive boundaries.
• The lava at constructive boundaries has a low silica content and as a result is quite runny. It is known as Basic Lava.
• Basic lava can travel long distances and builds up to form gently sloping Shield volcanoes.
• Mount Hengill in Iceland is an example of a shield volcano.