Erebus exemplifies a family of volcanoes with an alkalic chemical composition, with lavas relatively rich in sodium, potassium and other elements including rare Earth's elements, while being relatively poor in silica.
Alkalic volcanoes are very different from volcanoes such as in the Cascade Range extending from northern California through British Columbia to Alaska. The Cascades are found in a place where Earth's tectonic plates are pushing toward each other, with the crust of the ocean forced below the crust of the continent. As that ocean crust sinks into the Earth and partially melts, the water in the rocks becomes part of the melt and is the dominant "volatile," or molecule that easily exsolves, or bubbles out of a solution like fizz out of a carbonated drink.
That evolving magma rises into and through the crust, but typically does not make it to the surface because, as the pressure from the overlying crust diminishes with ascent, the water flashes out, sometimes explosively as in the case of Mount St Helens in 1980 or Mount Lassen in 1912. The remaining magma stalls and freezes in place, typically at a depth of around three miles (five kilometers).