Here's why:

* Viscosity: Viscosity is a measure of a fluid's resistance to flow. Think of honey versus water. Honey is very viscous, meaning it flows slowly. Water is less viscous, flowing more easily.

* Magma Composition: The composition of magma directly impacts its viscosity.

* Silicic Magma: Magmas rich in silica (SiO2) are very viscous. They flow slowly and tend to trap gases, making them prone to explosive eruptions.

* Mafic Magma: Magmas with lower silica content are less viscous. They flow more easily and are associated with effusive eruptions, where lava flows smoothly out of the volcano.

* Temperature: Hotter magma is less viscous and flows more easily. As magma cools, it becomes more viscous.

* Gas Content: Dissolved gases in magma can significantly decrease its viscosity. As the magma rises, the pressure decreases, allowing gases to expand and make the magma more fluid.

In summary:

* High viscosity magma = Slow, difficult flow = Explosive eruptions (like Mount St. Helens)

* Low viscosity magma = Easy flow = Effusive eruptions (like Kilauea in Hawaii)