How does a hawaiian volcano such as Kilauea differ from a volcano in the Cascades of North America such as mount saint helens?

Answer 1

Its has to do with the composition of the magma. Kilauea's magma has magma with a low-silica content. This means it has a low viscosity (is runny) and thus a low gas content. As a result it doesn't damage much force to cause it to erupt, and the low gas content means there is little, if any, explosive component to the eruptions. The lava, as it is called on the surface, can then flow downhill pretty easily. It hardens into a rock called basalt Mount St Helens contains magma with a high silica content. This gives it a very high viscosity, meaning it flows with great difficulty. The high viscosity means it can hold a lot of dissolved gasses. When this magma approaches the surface, the low pressure causes the gasses to form bubbles. Since the high viscosity has held this process back for quite some time, once it starts, it happens rapidly and violently. Thus, sudden expansion of gasses results in an explosion, turning the magma into a sort of spray that becomes ash while chunks of lava "foam" become pumice. When lava with a lower gas content does erupt, it is so viscous that it does easily flow downhill and instead builds into a crumbly structure called a lava dome. The rock formed by lava from Mount St Helens is called Dacite.