The movement of tectonic plates causes volcanic eruptions.
The san andreas is a "transform fault" that marks the boundary between two tectonic plates --  the north american and the pacific.
The movement of two things that come together in san andreas causes volcanic eruptions.
What causes volcanic eruptions?

[1] a hemisphere is tilted away from the sun
[2] high-mass stars collapsing
[3] tectonic plates being pushed together
[4] Two things found in Florida
[5] Two things that come together in San Andreas
[6] Two things found in Washington DC
[7] Sudden shaking of the ground.
[8] Two things found in Manhattan