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Our transcription: Particular types of magma are associated with specific types of plate boundaries. Plate Tectonic Theory explains why we have magmas on the sea floor, which are very different from magmas that form on continents. The magmas on the sea floor are enriched in iron and magnesium tending to be on the basaltic side of the scale with higher temperature minerals in accordance with the Bowen's Reaction Series. In subduction zones, relatively cool, wet rocks on the sea floor and on a sea floor lithosphere are subducted under continents. Because of the presence of so much water, melting begins at a low temperature; thus a partial melting takes place. We tend to get rocks that are higher in silica associated with subduction zones hence andesite volcanoes in South America rather than the basaltic type rocks in the mid-oceanic ridges.
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