Texas Bureau of Economic Geology Down to Earth at Tuff Canyon, Big Bend National Park, Texas

Italicized words are cross-referenced to terms defined elsewhere in this glossary.

air-fall deposit material that falls out of an eruption cloud particle by particle, like rain drops or snow flakes, and accumulates on the Earth's surface. Has not been identified in Tuff canyon, probably because small, loose particles were easily picked up by debris flows and by pyroclastic flows and surges.

alluvium silt, sand and gravel deposited by running water.

ash pyroclastic particles that are not stuck together and that are smaller than roughly 0.1 inch in average diameter.

ballistic block fragment that is blown out of a volcanic vent and travels in a parabolic trajectory like an artillery shell or rocket.

basalt dark, fine-grained, igneous rock.

blocks and bombs rock fragments larger than an orange in a pyroclastic deposit. A bomb has a round, smooth outline and was probably molten when it was thrown out. A block has an angular outline (straight edges, sharp corners) and was solid when it was thrown out. Many blocks can be seen in Tuff canyon, but no bombs.

breccia rock containing angular fragments of older rock in a finer matrix.

case hardening process by which the outer part of a rock is made more resistant to erosion by precipitation of minerals from water that percolated through the rock and then evaporated, leaving the minerals behind.

channel small valley, formed by erosion, It may be preserved by being filled by alluvium or debris-flow deposits. A channel filling is usually recognizable because it has different particle sizes than do the deposits above and below.

debris flow moving mixture of rock particles and water, commonly having the consistency of freshly mixed concrete ready to be poured. Also called a "mud flow," but that name is not correct because some of the rock particles can be the sizes of cars or houses.

deposition laying down of rock particles.

dike intrusive igneous rock body shaped like a wall or a knife blade and formed by magma filling a crack.

erosion process by which material on Earth's surface is loosened and carried away. Running water is the most influential erosional agent on Earth, even in deserts.

fault break in Earth's crust, along which displacement has occurred; rock on one side of a fault slides past rock on the other side.

flow unit in pyroclastic-flow deposits, a layer or group of layers that can be distinguished from those above or below by sorting, degree of welding, or proportions of rock and pumice fragments. It may have been deposited only seconds before the one above it.

gas escape pipe narrow, vertical tube formed in a still-hot pyroclastic-flow deposit by gas moving rapidly upward and carrying away smaller particles in such a way that the pipe has a higher proportion of large particles than does the surrounding deposit.

glass in volcanic rocks, material that forms naturally when the liquid in lava cools before the atoms within it can organize into crystals.

gravel sediment having particle sizes larger than roughly 0.1 inch.

igneous rock rock that was once molten (magma or lava). The two general kinds are intrusive and volcanic.

imbrication leaning or shingling of flat rock or pumice fragments; the higher ends point the direction the depositing current was moving. Imagine a row of closely spaced dominoes all standing on end; if the domino at one end of the row is toppled so that it falls against another, the imbrication shows the direction the toppling wave moved.

impact sag dent left in the surface of the ground by a ballistic fragment.

intrusive rock igneous rock injected below Earth's surface as magma, which cooled and became solid long before it was exposed by erosion of overlying rock.

laminar flow fluid motion in which suspended particles follow smooth, parallel paths, like racehorses going around a track. Contrasts with turbulent flow.

lapilli pyroclastic particles having diameters between 0.01 and 2.6 inches.

lava igneous rock material so hot that it is partly or wholly liquid when it flows onto Earth's surface. Term used for both the hot liquid and the rock that forms when the liquid freezes.

lava dome steep-sided pile of lava having such high viscosity that it did not flow far from its vent. Lava domes can act as plugs in vents, holding in gas until an explosive eruption results. They can also build up so high that their sides collapse, triggering pyroclastic flows and surges.

magma rock material, partly or wholly liquid, within the Earth. When it erupts on the surface, it becomes lava.

matrix finer grained part of a breccia, between larger rock fragments. Particles in the matrix may or may not be the same kinds of rock as those making up larger fragments.

pillow lava rounded masses, a foot to several feet long, formed by the quick cooling of an outer skin of lava in water or wet mud. The hot interior of a pillow cools more slowly, allowing the still pliable skin to sag and mold itself around any underlying pillows.

pothole deep, smooth hole worn in a rock surface by the grinding of suspended sediment that moved in a tight orbit in a water eddy near the edge of a stream.

pumice volcanic glass that is so full of gas bubbles that it is frothy and of low density. Some floats on water.

pyroclastic "Fire-broken" (literally) volcanic material that was blown out of a vent as solid fragments and/or liquid droplets during an explosive eruption. The other kind of volcanic material is lava, which flowed out onto the surface of the ground as a liquid.

pyroclastic flow dense cloud of hot particles and gas, moving by laminar flow close to the ground surface. Pyroclastic flows travel at 22 to perhaps more than 600 miles per hour.

pyroclastic surge hot cloud, less dense than a pyroclastic flow, moving with turbulent flow close to the ground surface, commonly leaving a stratified deposit. The layering is produced by the unstable and pulsating motion of the surge cloud; within seconds the sizes of particles being deposited at any one place can change from large to small and back again.

rhyolite fine-grained, generally light colored, igneous rock with the chemical composition of granite; when liquid, it has a much higher viscosity than does basalt.

sand sediment composed of particles between 0.002 and 0.1 inch in diameter.

sediment loose particles freed by weathering from older rocks and deposited on Earth's surface (including the bottoms of streams, rivers, lakes, and oceans).

sedimentary rock consolidated (hardened) sediment. If you can dig into a deposit with your fingers or with a garden trowel, the deposit is not consolidated.

silky pumice frothy volcanic glass in which the gas bubbles have been drawn out into long thin tubes.

silt an unconsolidated sedimentary deposit dominated by particles smaller than 0.002 inch but large enough to have a gritty feel.

sorting process by which particles of a specific shape or size are selectively separated from others of different shape and size. The result of sorting is a more uniform population of particles.

spatter pyroclastic accumulation of hot globs of lava that flattened on impact and partly stuck together. It builds mounds close to volcanic vents and forms no more than a few hundred feet from them.

tuff rock composed of consolidated pyroclastic particles (ash) that are less than 0.1 inch in diameter.

turbulent flow fluid motion in which suspended particles follow erratic paths, like roaches scattering when you turn on the kitchen light at night. Contrasts with laminar flow.

vent opening in Earth's surface from which volcanic (lava or pyroclastic) material erupts.

vesicle hole in an igneous rock, formed by the trapping of a gas bubble as lava or magma froze. It is commonly less than 1 inch in diameter.

viscosity resistance to flow or to stirring. Basalt liquid has low viscosity (as low as that of motor oil) and therefore can form thin, rapidly moving lava flows. Rhyolite lava has high viscosity (similar to that of cold honey) and tends to form thick, slowly moving lava flows or to pile up as a lava dome over the volcanic vent. High-viscosity liquids, which can trap more gas bubbles, are more likely to form pumice and to erupt explosively. On the other hand, some rhyolite lavas have lost their dissolved gas while sitting at or near Earth's surface.

volcanic rock igneous rock formed when magma breaks through Earth's surface and pours out, forming lava, or is blown out as particles, forming pyroclastic material.

weathering process by which rock is mechanically or chemically broken down, at or near Earth's surface.

welding high-temperature process by which glass particles in a pyroclastic deposit are stuck together and most vesicles are pinched shut. Welding, which occurs within the first few days or weeks after pyroclastic material is deposited, is most common in pyroclastic-flow deposits because these were hotter and commonly thicker than surge and air-fall deposits. Thickness is important because it helps hold heat in, and the weight of the overlying deposit promotes welding of the material below.