Block and ash flows are a kind of pyroclastic density current. "Pyroclastic flow" is a kind of catchall term, but there are more specific ones that better describe the makeup of one of these currents. "Block and ash flow" implies that the flow is composed of blocks (either of denser lava or pumice or both) and ash; "pumice flow" means that the contents are mainly pumice and ash, "ash flow" that there are few blocks and mostly ash in the current.
|Faint reverse grading in a block and ash flow deposit at Old Road Bay|
|Pointing out matrix in a deposit on the side of the Belham River Valley|
Debris avalanches form from older volcanic material that's unstable enough to collapse, either because of erosion or alteration or both. They form "hummocky" deposits that are very obvious in aerial photos, but in cross section they're also fairly easy to identify.
Lahars form when water mixes with volcanic material and flows downslope. These can have the consistency of soup to concrete, and they're a major concern even when a volcano isn't erupting (especially if the volcano is located in a tropical area that gets lots of rain.)
|Panorama of a debris avalanche deposit near Jack Boy Hill. Different colors mark chunks of preserved stratigraphy.|
Debris avalanche deposits contain mm to m sized clasts (or larger), as well as chunks of material from older deposits that retain their original stratigraphy. These clasts and chunks are poorly sorted, and rounded to angular in shape. Clast compositions are almost always mixed, and the matrix is often clayey and highly altered. Jigsaw jointing is apparent in individual clasts, with clast fragments separated by matrix material but orientation with respect to neighboring clasts preserved. Clasts or deposit blocks may be "smeared" out into trains or show internal faulting.
|A "jigsaw" fractured block in a debris avalanche deposit|
|House buried in lahar deposits in the Belham River Valley|
|Cross-section of a fairly recent lahar deposit. There's not much in the way of sorting or grading here.|
A fourth type of pyroclastic current, called a surge, also shows up on Montserrat, but they're not often preserved because they consist of a blast of ash and hot gases (sometimes derived from block and ash flows), and they leave very thin deposits. I don't have any good photos of them, but they tend to be fine-grained (mm to cm sized clasts at the most), and sometimes show cross-stratification. It's rare that they're preserved, especially in a tropical environment, because the fine material washes away very easily.