1) "Composite volcanoes have alternate periods of quiet eruption and explosive eruption."

"Composite volcanoes eject viscous lava and pyroclastic materials alternately."

These 2 quotes are both from reliable sources. But I don't understand how viscous lava flows can be quiet, and not explosive. I thought that only fluid (not viscous) lava can produce quiet eruptions...right?

2) "Pyroclastic materials are produced by (viscous) felsic lava and they can flow as fast as hundreds of km/h"

Why? If they are produced by (viscous) felsic lava, they should be very sticky, how can they flow that fast? (even faster than (fluid) mafic lava flow...)

kingwinner, pyro clastic is a couple of Greek words slung together by geologists to explain the formation of the rock. Interpreted, it means pyros (= fire) and klastos = (broken) i.e. broken by fire. There are two types of rhyolitic eruption. The first is where the magma is very high in silica content but low in gas, so it is extruded slowly, like toothpaste, and builds up volcanoes which are flat topped, steep-sided, and prone to landslides.

The second type of the rhyolitic eruption is extremely violent. This is the same sort of silica rich magma, but much higher in gas content. This makes the event much more dangerous, because the dense magma holds onto the gas much more tenaciously, and releases it in a tremendous eruption, only when sufficient pressure has been built up. This means also that vast quantities of magma build up in the chamber, and are released all of a sudden and all at once. So you're looking at fantastically violent eruptions that release vast amounts of ash very quickly.

Of course having all that immense pressure built up means that the rock is blasted into very small fragment (hence broken by fire). The pulverised rock is then superheated to hundreds of degrees, and a nuee ardente (French for firey cloud) develops. The first stage of the eruption is a gigantic fire column that can reach 60 miles into the air, before collapsing under its own weight.

The second stage is the development of a base surge, where to material from the fire-column starts flowing rapidly out over the countryside, wiping out everything in its path. Very often, the enormous calderas of rhyolitic volcanoes are occupied by big lakes, so when the magma bursts out it combines with huge amounts of steam, which acts as a carrier for the rock particles. Hence they can travel great distances.

kingwinner, pyro clastic is a couple of Greek words slung together by geologists to explain the formation of the rock. Interpreted, it means pyros (= fire) and klastos = (broken) i.e. broken by fire. There are two types of rhyolitic eruption. The first is where the magma is very high in silica content but low in gas, so it is extruded slowly, like toothpaste, and builds up volcanoes which are flat topped, steep-sided, and prone to landslides.

The second type of the rhyolitic eruption is extremely violent. This is the same sort of silica rich magma, but much higher in gas content. This makes the event much more dangerous, because the dense magma holds onto the gas much more tenaciously, and releases it in a tremendous eruption, only when sufficient pressure has been built up. This means also that vast quantities of magma build up in the chamber, and are released all of a sudden and all at once. So you're looking at fantastically violent eruptions that release vast amounts of ash very quickly.

Of course having all that immense pressure built up means that the rock is blasted into very small fragment (hence broken by fire). The pulverised rock is then superheated to hundreds of degrees, and a nuee ardente (French for firey cloud) develops. The first stage of the eruption is a gigantic fire column that can reach 60 miles into the air, before collapsing under its own weight.

The second stage is the development of a base surge, where to material from the fire-column starts flowing rapidly out over the countryside, wiping out everything in its path. Very often, the enormous calderas of rhyolitic volcanoes are occupied by big lakes, so when the magma bursts out it combines with huge amounts of steam, which acts as a carrier for the rock particles. Hence they can travel great distances.

So the QUIET eruptions of the composite volcanoes are meaning the flowing of viscous lava with low gas content! :)

The 2 types of rhyolitic eruptions, related to composite volcano, can be occuring in the SAME composite volcano, right? i.e. alternate periods of these 2 types of eruptions -- eruptions of viscous lava with high, or low gas content.

Pyroclastic materials are ejected from ciner cone or composite volcanoes during an explosive eruption. The thing that puzzles me is that both volcanoes have highly viscous, sticky felsic magma, but the pyroclastic materials they form always travel at hundreds of km/h, that's very fast and runny...how can sticky magma produce such a speed of pyroclastic flows? Sticky lava tends to flow slowly, right?

Under the extreme conditions of a pyroclastic eruption, the material flows out quickly because it's suspended in superheated gas and steam. Gravity does the rest. Vast areas of land can be affect by these events, thousands of square miles.