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If volcanoes get their magma from the crust below them, what can you say about the silica content of Earth's crust under the oceans?

The crust under the oceans would be low in silica. This is because the crust under the ocean is likely to be made of basalt. If low levels of silica are present then there are non-explosive volcanoes.  We know that basalt has low silica content and therefore basalt lave has a low viscosity. Since it has a low viscosity the lava can travel for long distances. Viscosity is a measure of a fluid’s resistance to flow. Low viscosity lava would be thinner and travel much faster. On the other hand, continental crust is made of granite rock. Granite has a high silica content, producing explosive volcanoes. This type of lava has a high viscosity so it flows much slower because it is thicker.


Low silica -> non-explosive volcanoes -> basaltic crust


High silica -> explosive volcanoes -> granitic crust

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