Which magma is more viscous

Felsic magma is viscous and does not flow easily. Most felsic magma will stay deeper in the crust and will cool to form igneous intrusive rocks such as granite and granodiorite. If felsic magma rises into a magma chamber, it may be too viscous to move and so it gets stuck. Dissolved gases become trapped by thick magma and the magma chamber begins to build pressure.

The type of magma in the chamber determines the type of volcanic eruption. A large explosive eruption creates even more devastation than the force of the atom bomb dropped on Nagasaki at the end of World War II in which more than 40, people died. A large explosive volcanic eruption is 10, times as powerful.

Felsic magmas erupt explosively because of hot, gas-rich magma churning within its chamber. The pressure becomes so great that the magma eventually breaks the seal and explodes, just like when a cork is released from a bottle of champagne.

Magma, rock, and ash burst upward in an enormous explosion creating volcanic ash called tephra. That is why it is so dangerous to inhale the air following an eruption.

Pyroclastic flows knock down everything in their path. The temperature inside a pyroclastic flow may be as high as 1,oC 1, degrees F. Prior to the Mount St. Helens eruption in , the Lassen Peak eruption on May 22, , was the most recent Cascades eruption. A column of ash and gas shot 30, feet into the air.

This triggered a high-speed pyroclastic flow, which melted snow and created a volcanic mudflow known as a lahar. Lassen Peak currently has geothermal activity and could erupt explosively again. Shasta, the other active volcano in California, erupts every to years.

An eruption would most likely create a large pyroclastic flow, and probably a lahar. Although silica-rich magmas have lower temperatures than silica-poor magmas, a high degree structure gives them higher viscosity.

Rises in temperature and in water content break down the structure of magmas and decrease their viscosity. Therefore, a water-rich magma is more fluid than a dry magma of the same composition. Many melts are rich in basic oxides such as MgO and FeO and relatively low in silica.

The silicate structures that develop favour the crystallisation of minerals of the olivine and pyroxene groups i. Conversely, silica-rich melts favour the crystallisation of silicates such as feldspars, micas and quartz i. Many lavas are vesicular have cavities , indicating former gas bubbles which escaped from within the magma as it erupted at the surface. Other gases that bubble out at the surface include nitrogen, oxygen, argon, carbon dioxide, boron, carbon monoxide, methane, hydrogen, nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid, sulfur dioxide and sulfur trioxide.

In some lava lakes, gases come out as invisible, colourless and odourless gas bursts and may kill many people and animals. The most common igneous rocks by far are granites and basalts. As magma erupted at the surface is difficult to sample, the solid rocks that form after cooling provide us with the most information on the chemistry of magmas.

These rocks are made up of:. The most abundant volatile in magma is water H 2 O , followed typically by carbon dioxide CO 2 , and then by sulphur dioxide SO 2. The general relationship between the SiO 2 content of magma and the amount of volatiles is shown in Figure 4.

Differences in viscosity and volatile level have significant implications for the nature of volcanic eruptions. When magma is deep beneath the surface and under high pressure from the surrounding rocks, the gases remain dissolved. As magma approaches the surface, the pressure exerted on it decreases. Gas bubbles start to form, and the more gas there is in the magma, the more bubbles form. If the gas content is low or the magma is runny enough for gases to rise up through it and escape to surface, the pressure will not become excessive.

Assuming that it can break through to the surface, the magma will flow out relatively gently. An eruption that involves a steady non-violent flow of magma is called effusive. A good analogy for a magma chamber in the upper crust is a plastic bottle of pop on the supermarket shelf. Go to a supermarket and pick one up off the shelf something not too dark.