Ultimately, the viscosity of a magma will determine the shape a volcano takes over continued eruptions. A magma's silica content will not only control the rock type that forms, it also controls the relative viscosity of the magma. Differences in a magma's viscosity will determine the shape a volcano takes.
Magmas differ in composition, which affects viscosity. Magma composition has a large effect on how a volcano erupts. Felsic lavas are more viscous and erupt explosively or do not erupt. Mafic lavas are less viscous and erupt effusively.
The properties of the magma inside a volcano affect how an eruption will play out. In particular, the viscosity of this molten rock is a major factor in influencing how hazardous an eruption could be for nearby communities.
Shield volcanoes Where a volcano produces low viscosity, runny lava, it spreads far from the source and forms a volcano with gentle slopes: a shield volcano. Most shield volcanoes are formed from fluid, basaltic lava flows. Mauna Kea and Mauna Loa are shield volcanoes.
How is the viscosity of magma related to its explosivity? The more viscous the magma, the more explosive the eruption.
CONTROLS ON EXPLOSIVITY The viscosity of the magma, however, is also an important factor in determining whether an eruption will be explosive or nonexplosive. A low-viscosity magma, like basalt, will allow the escaping gases to migrate rapidly through the magma and escape to the surface.
The low viscosity of the magma allows the lava to travel down slope on a gentle slope, but as it cools and its viscosity increases, its thickness builds up on the lower slopes giving a somewhat steeper lower slope. Most shield volcanoes have a roughly circular or oval shape in map view.
Explosive eruptions occur where cooler, more viscous magmas (such as andesite) reach the surface. Dissolved gases cannot escape as easily, so pressure may build up until gas explosions blast rock and lava fragments into the air! Lava flows are much more thick and sticky so do not flow downhill as easily.
Very viscous magmas are linked with more powerful explosions because they can block gas from escaping through vents, allowing pressure to build up inside the volcano's plumbing system. On the other hand, extrusion of more viscous magma results in slower-moving lava flows.
Part 2: Viscosity The higher viscosity fluid takes more force to stir. The higher the viscosity, the more force one has to exert on the straw to create a rising bubble in the fluid. The higher viscosity fluid also has bubbles that rise with more force and have more of an explosive effect upon reaching the surface.
Temperature, composition, and volatile (gas) content largely determine the viscosity of lava. Temperature: The hotter the lava, the lower the viscosity (the thinner it is). The cooler the lava, the higher the viscosity (the thicker it is).
The viscosity of a magma is affected by its silica content and temperature. The amount of silica influences the magma's viscosity in such a way that if the silica content is low the magma would have low viscosity whereas a high content of silica would make the magma more thick and sticky.