Explain how the composition of magma affevt volcano eruptions?

The kind of magma that is found in the most explosive volcanoes, the composite volcanoes, is andesitic magma. It is magma that is quite high in silica content which makes it thick, sticky and gooey. Great lumps of this sticky magma cool at the top of the volcano to form a sealed cap. Magma beneath the cap builds up and pressure mounts  and eventually becomes too much to be contained and the magma violently erupts out from the top, blowing out the cap and shooting miles up into the air and in all directions.

The kind of magma found in gentle volcanoes such as shield  volcanoes is runny in consistency. It has very minimal amounts of silica and is not thick but loose. It therefore tends to trickle out of the top of the volcano rather than erupt or explode.

in cartography, a contour line (often just called a "contour") joins points of equal elevation (height) above a given level, such as mean sea level. a contour map is a map illustrated with contour lines, for example a topographic map, which thus shows valleys and hills, and the steepness or gentleness of slopes.

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