Ash – Tiny pieces of rock and glass blasted into the air by a volcano. (Very different from wood ash!)

Basalt (Basaltic Lava) – The least viscous (most runny) kind of lava.  Basalt is black, reddish brown or gray when it cools.  Sometimes you can see a few crystals (phenocrysts) spread out in a piece of basalt, but most of the minerals are often too small to see without help from a microscope.

(Volcanic) Block - Solid, juice-box size pieces of lava ejected from a vent.  Can be called either a volcanic block or just a block.

(Volcanic) Bomb - Bowling-ball size pieces of frothy lava ejected from a vent in a semi-solid, or plastic, condition (like a hot marshmallow).  Can be called either a volcanic bomb or just a bomb.

Caldera – A large depression (hole) in the ground formed by very large eruptions.  Some calderas are 10 to 25 kilometers across (about the area of Anchorage, AK) and deeper than 100 football fields standing end to end.

Cinder - A cooled blob of lava that can be the size of a pea, a gumball, or a small potato.  Cinders are usually red or reddish-brown with lots of vesicles (holes from bubbles) in them.  Cinder is another name for scoria.

Cinder Cone – The simplest type of volcano.  Cinder cones are built from cinders ejected from a single vent (hole) that fall around the vent to form a circular cone no taller than a 100-floor building.  Few cinder cones are well known, Paricutin, Mexico is one of the most famous.  The caldera of Okmok Volcano is full of cinder cones.

Composite Volcano – A steep sided, symmetrical (same shape in all directions) cone built from layers of lava flows, ash, and other volcanic materials. Augustine, AK, Mt. St. Helens and Rainier, WA are examples of composite volcanoes.

Convergent margin (subduction zone) - Tectonic plates move toward each other and collide.  One is subducted (dragged down) beneath the other.

Crust – Thin outer layer of the Earth.  If the Earth were the size of an apple, the crust would be about as thick as the apple’s skin.

Debris Avalanches (Volcanic Landslides) - Rock, snow, and ice sliding down hill in a big rush.  They can be small movements of loose material on the surface of a volcano or nearly half of the volcano!

Density – The density of any matter (everything around you – your computer, pencil, even the air you’re breathing!) is the mass (kind of like weight) divided by the volume (the space it takes up).  Imagine a brick and a loaf of bread of the same size… which is heavier?  So, if the volume is the same, which has the greater density?

Divergent (spreading) margin – Tectonic plates pull apart from each other.  This causes sea-floor spreading as new material is added to oceanic plates.

Dome – A steep sided mass of viscous (like Play-Doh®) lava extruded from a volcanic vent.  The shape of a volcanic dome is a bit like a squashed marshmallow – circular on the bottom and spiny, rounded, or flat on top.

Eruption - A volcanic event that may last from minutes to years, with solid material ejected from the volcano.

Extrude – To force or press out of an opening.

Fault – An area where two tectonic plates move past one another.  The direction the plates are moving does not change.  Sometimes the motion is slow and continuous (creep), or very sudden (earthquake).

Fumarole – A vent that releases volcanic gases, including water vapor (steam).

Harmonic Tremor - A type of continuous, rhythmic ground shaking (vibrations) related to underground movement of magma.  This is different from the sudden ground movement of a tectonic (more common) earthquake caused by slipping along a fault.

Island Arc – A chain of volcanic islands in an arcuate shape formed at a convergent margin.  The Aleutian Islands are an example of an island arc.

Lahar (Volcanic Mudflow or Debris Flow) – Mixtures of water, mud, and rock pieces that flow down a volcano either during an eruption or when the volcano is resting.  Lahars sometimes look like flowing wet concrete.

Lapilli – Pea to racket-ball size pieces of volcanic materials.

Lava – Red-hot rock that comes out of a vent in many different forms.  Lava may pour out as liquid, spray up into the air, explode into ash, pumice, or scoria, or extrude as a solid dome.

Magma – Molten (liquid or partly liquid) rock below the surface of the Earth.  Magma rises from many kilometers beneath the ground to the surface, where it erupts from volcanic vents as lava.

Phenocryst – A crystal in a rock that is big enough to see with your bare eye.

Pumice –Frothy (bubbly) pieces of lava.  Pumice has a density less than that of water – so it floats!

Oceanic Plates – The rigid plates of the ocean floor, made mostly from solidified basaltic lava erupted at divergent margins.

Scoria - A cooled blob of lava that can be the size of a pea, a gumball, or a small potato.  Scoria is usually red or reddish-brown with lots of vesicles (holes from bubbles) in them. Another name for cinder.

Shield Volcano –A gently sloping volcano that typically erupts quietly or non-explosively.  These volcanoes are built from fluid lava flows that sometimes flow great distances from the vent.  Mauna Loa, HI and Okmok, AK are both shield volcanoes.

Stratovolcano –  This is another name for a composite volcano.  A steep sided, symmetrical (same shape in all directions) cone built from layers of lava flows, ash, and other volcanic materials.  Augustine, AK, Mt. St. Helens and Rainier, WA are all stratovolcanoes.

Tectonic Plate - A rigid (non-squishable) piece of the Earth’s outer crust.  There are about 12 main plates moving around the Earth at about the speed your fingernails grow.

Tephra – A general term for all material ejected from volcanoes, including: ash, blocks, volcanic bomb, and lapilli.

Transform Fault – A zone where one tectonic plate slides past another. 

Vent – Hole in the ground where lava or volcanic gases come out of a volcano.  Vents can be a few meters to a few kilometers wide.

Vesicles – The holes in any volcanic product left from expanding (growing) volcanic gases – fossil bubbles!

Viscous (Viscosity) – A measurement of how easily a fluid flows.  Water is less viscous than molasses.

Volcanic Gases – Steam (H2O) is the main component (piece) of the gas coming out of a volcano.  Other components may include: carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen sulfide (H2S), carbon monoxide (CO), (COS), (CH4), hydrochloric acid (HCl), hydrofluoric acid (HF), hydrogen gas (H2), oxygen (O2), native sulfur (S2), nitrogen (N2), carbon sulfide (CS2), sulfur trioxide (SO3), ammonium (NH4), boron (B), bromine (Br)