[MUSIC] [MUSIC] Now I mentioned the word earlier, pyroclastic debris. Pyroclastic debris Is a general term that includes volcanic ash, plus other kinds of fragments. We've seen that some of these fragments are little balls, or pellets of pumice. But, in some cases, they're other kinds of fragments, as well. For example, we're looking at here is a handful of what geologist refer to as lapilli. These are marble shaped sometimes pea shaped fragments of solid rock. They can include pellets that are formed by the freezing of little clots of lava that are ejected during a fountain of lava at an effusive eruption. Sometimes they're composed of little snowballs of ash that form when the ashes erupted into a cloud that contains a lot of moisture. And the ash sticks together because of the moisture and forms these little balls that then become large enough to fall down. But the term lapilli is used in either case because it's basically a size designation. It refers to pea- to marble-sized fragments of solid volcanic rock. Another kind of piece of pyroclastic debris is a volcanic bomb. Volcanic bombs form when a still molten blob of lava gets ejected into the air and as it falls, it streamlines and develops a fairly smooth surface. A third type of pyroclastic debris is sometimes called simply blocks. And these are more irregular, non-streamlined chunks of either frozen lava or chunks of preexisting volcanic debris that's gotten ejected during the volcanic eruption, then collects on the surface of the volcano. Now let's broaden our discussion of the fragmental material from volcanoes a little broader. We've seen that there's pyroclastic debris and that's usually restricted to stuff that's erupted directly from the volcano. But sometimes that debris mixes with water and then flows somewhere else. Now in some cases that water comes from the rainfall, the heavy rainfall, that sometimes is triggered, when ash enters a moist atmosphere. Sometimes it comes from the melting of glaciers, at the top of tall volcanoes or the melting of snow, at the top of tall volcanoes. In any case, when ash and fine grain debris mixes with water, it forms a muddy like slurry that then rushes down the side of the volcano. This muddy like slurry is called a lahar and when it finally slows and accumulates, it will develop another type of volcanic debris. It's a little different from pyroclastic debris because it's been transported a little bit with the aid of water. Sometimes, there's even more water or the lahar then gets picked up by streams and carried away. Or ash and other debris that settle down the side of a volcano, gets picked up and carried away by rivers. That stuff will be sorted by the action of water, because water will tend to carry the finer stuff further, and leave the coarser stuff behind. But, in any case, it will deposit sediment that's Is composed almost entirely of volcanic debris. So all this stuff together stream transported sediment, lahars and pyroclastic debris together are referred to sometimes as volcanoclastic debris. And it is in fact a major component of what builds up volcanoes in many environments. [MUSIC]