Why is it that magmas in continental coastal range volcanoes are of intermediate composition?
Igneous Rock Compositions:
Igneous rocks are formed when magma rises to the surface, cools and solidifies. They can be categorized by the amount of silicate minerals. Where the rocks originate determines the amount of silicate minerals contained within the rock. Intermediate magmas generally originate along coastlines but also in places where the mantle material directly intrudes on continental crust such as hot spots.
Answer and Explanation:
Igneous rocks that are composed of silicate minerals are divided into three general groups: mafic, intermediate, and felsic. Mafic rocks contain the least amount of silica, felsic contains the most, while intermediates contain less than felsic but more than mafic.
Coastal volcanoes are intermediate in composition as they form in the subduction zone where continental and oceanic crust converge together. As dense oceanic crust is subducted under less dense continental crust, the oceanic crust partially melts. As the crust melt, it becomes less dense and rises up through the continental crust until it settles within the continent, forming a magma chamber. The magma within this chamber (mafic in composition) heats the surrounding continental crust (felsic in composition) mixing the two types together forming intermediate composition magma. Volcanoes can form above these magma chambers, and their resulting eruptions will be intermediate in composition.
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from Earth Science 101: Earth ScienceChapter 7 / Lesson 4