Here's why:
* Decompression melting: As plates move apart, the pressure on the underlying mantle rock decreases. This decrease in pressure leads to decompression melting, where the mantle rock melts to form magma.
* Mafic composition: The mantle rock is predominantly composed of mafic minerals (rich in magnesium and iron). This results in magma with a mafic composition, which solidifies to form basalt.
Other igneous rocks can also form at divergent boundaries, but basalt is the most common:
* Gabbro: A coarse-grained, intrusive igneous rock that is chemically equivalent to basalt. It forms when mafic magma cools and solidifies slowly beneath the Earth's surface.
* Andesite: An intermediate igneous rock that can be found in some areas of divergent boundaries, particularly where there's more interaction with continental crust.
In summary: Divergent boundaries are characterized by the formation of basalt due to decompression melting of mafic mantle rock.
