What causes the volcanism at spreading ridges? What will be an ...

[xroflmao]

What causes the volcanism at spreading ridges? What will be an ideal response?

Question 2

Briefly explain how it is possible for a single magma to yield rocks of felsic, intermediate, and mafic
  composition. What will be an ideal response?

[essyface1]

Answer to Question 1

Beneath spreading ridges, the temperature of the mantle rock locally exceeds the melting temperature,
at least in part because the pressure decreases as the plates separate at the ridge. Water is also present
in the mantle rock, which decreases the melting temperature beneath the spreading ridge since water
works with heat to break down the chemical bonds in minerals.

Answer to Question 2

There are three ways for a magma to yield rocks of vastly different composition. With crystal settling,
the first crystals to form, the ferromagnesian minerals, settle to the bottom of the magma chamber,
where they can cool to form a mafic rock. The remaining liquid is more intermediate and, if the
process continues, more felsic than the original magma. At any point, these liquids can cool to become
an intermediate or felsic rock. Assimilation adds the country rock to a magma. If a magma is mafic, it
can melt more felsic rock and change its composition from mafic to intermediate to felsic. Magma
mixing can combine magmas of different composition. If a mafic magma and a felsic magma mix,
they will create a magma of intermediate composition. These three magmas can lead to three different
rock types.