Volcanic Materials Identification

Volcanic Lava Flows and Pyroclastic Materials

Volcanic materials are divided into two main groups: Pyroclastic materials and lava flow materials. Below is a list of the various volcanic material definitions describing the general characteristics of those materials, and in some cases explanations on their formation. Read through these definitions and become familiar with them before proceeding to the online activities.

Lava Flow Materials

Aa: Aa (pronounced "ah-ah" - a Hawaiian term), is lava that has a rough, jagged, spiny, and generally clinkery surface. In thick aa flows, the rubbly surface of loose clinkers and blocks hides a massive, relatively dense interior.

Block: Fragments of lava or rock larger than 64 millimeters in size which form due to the fracturing of viscous lava flow surfaces during flow.

Lava: The term used for magma once it has erupted onto the Earth's surface. Molten rock that erupts from a vent or fissure.

Lava flow: Stream of molten rock that erupts relatively non-explosively from a volcano and moves slowly downslope. An outpouring of lava onto the land surface from a vent or fissure. Also, a solidified tongue-like or sheet-like body formed by outpouring lava.

Pahoehoe: Pahoehoe (pronounced "pah-hoy-hoy" - a Hawaiian term), is a very fluid lava flow, that in solidified form, is characterized by a smooth, billowy, or ropy surfaces.

Pillow Lava: Fluid lava erupted or flowing under water may form a special structure called pillow lava. Such structures form when molten lava breaks through the thin walls of underwater tubes, squeezes out like toothpaste, and quickly solidifies as irregular, tongue-like protrusions. This process is repeated countless times, and the resulting protrusions stack one upon another as the lava flow advances underwater. The term pillow comes from the observation that these stacked protrusions are sack- or pillow-shaped in cross section. Typically ranging from less than a foot to several feet in diameter, each pillow has a glassy outer skin formed by the rapid cooling of the lava by water. Much pillow lava is erupted under relatively high pressure created by the weight of the overlying water; there is little or no explosive interaction between hot lava and cold water. The bulk of the submarine part of a Hawaiian volcano is composed of pillow lavas.

Pyroclastic Materials

Agglutinate: Cinders, scoria or pumice fragments that have partially welded together to form a cohesive mass. Agglutinate forms when the individual pyroclasts retain a high enough temperature after impact to partially melt together (welding). If the fragment are completely molten after impact, they may begin to flow downhill in what is known as a rootless flow.

Ash (volcanic): Fragments less than 2 millimeters in diameter of lava or rock blasted into the air by volcanic explosions.

Blocks: Fragments of lava or rock larger than 64 millimeters in size that are blasted into the air by volcanic explosions. Blocks are ejected during the eruption in a solid state, while bombs are ejected during the eruption in a semi-solid, or partial molten, condition. Generally, blocks often have an angular appearance, due to the fracturing of solid material during the eruption.

Bombs: Fragments of fluid or partially fluid lava or rock larger than 64 millimeters in size that are blasted into the air by volcanic explosions. Bombs are ejected during the eruption in a semi-solid, or partial molten, condition, while blocks are ejected during the eruption in a solid state. Volcanic bombs undergo widely varying degrees of aerodynamic and/or impact shaping, depending on their fluidity, during the flight through the atmosphere and subsequent impact with the ground. Based on their shapes after they hit the ground, bombs are variously described, in the following graphic terms:

Bread-crust Bombs - These are bombs that have had their surface fractured, without breaking the bomb apart. These fractures form in one of two ways. In both cases, the exterior shell of the bomb cools and solidifies, while the interior remains molten or partially molten. Impact with the surface can cause the bomb to flex, thus fracturing the solid surface. Once on the ground, gases in the molten interior begin to expand, causing the solid surface to fracture and expand.

Cow-Pie Bombs - Also called cow-dung bombs. These are very fluid bombs that are greatly deformed during impact. The fluid nature of the bomb causes the lava to flow outward from the center of the impact, forming a roughly circular, pancake-shaped bomb.

Fusiform bombs - Also called spindle or almond bombs. These bombs have an elongated form which tapers down at each end, with a relatively smooth surface. Many have an almond shape.

Irregular Bombs - These are bombs that have no distinguishing shape, but clearly lack the angular nature common with blocks.

Ribbon bombs - Bombs that have a long flat, ribbon-like form.

Spherical Bombs - As the name implies, these are bombs with a spherical or ball-like shape.

Cinders: Cinders are vesicular lava fragments 1 centimeter or larger in diameter. See Scoria.

Lapilli: Fragments of lava or rock between 2 and 64 millimeters in size that are blasted into the air by volcanic explosions.

Pumice: A light-colored, frothy, vesicular volcanic rock, usually of intermediate and felsic composition, formed by the expansion of gas in erupting lava. Commonly perceived as lumps or fragments of pea size and larger but can also occur abundantly as ash-size particles. Because of its numerous gas bubbles, pumice commonly floats on water.

Pyroclastic: Pertaining to fragmented (clastic) rock material formed by a volcanic explosion or ejection from a volcanic vent. See Tephra.

Reticulite: During the exceptionally high fountaining episodes of some eruptions, an extremely vesicular, feathery light pumice, called reticulite or thread-lace scoria, can form and be carried many miles downwind from the high lava fountains. Even though reticulite is the least dense kind of tephra, it does not float on water, because its vesicles are open and interconnected. Consequently, when it falls on water, it becomes easily waterlogged and sinks.

Scoria: A dark to reddish-colored, scoriaceous, vesicular volcanic rock, usually of mafic composition. Scoria forms when blobs of gas-charged lava are thrown into the air during an eruption and cool in flight, falling as dark volcanic rock containing cavities created by trapped gas bubbles.

Tephra: Solid material of all sizes explosively ejected from a volcano into the atmosphere. Tephra is the general term now used by volcanologists for airborne volcanic ejecta of any size. Historically, however, various terms have been used to describe ejecta of different sizes. Fragmental volcanic products less than 2 mm in diameter are called ash, between 2 and 64 mm in diameter are called lapilli, fragments larger than 64 mm are called blocks if they were ejected during the eruption in a solid state, while bombs are ejected during the eruption in a semi-solid, or partial molten, condition. See Pyroclastic.

All definitions have been taken and/or modified from the CVO Glossary of Volcano and Related Terminology web page. References for original sources are available on the web page.

Sample Identification

On each of the following pages you will find an image of a sample of lava flow or pyroclastic material, and a Volcanic Materials Identification Worksheet. You may wish to review the material in the Igneous Rock Identification activity before proceeding.

Many of these samples have more than one texture. If there is more than one texture for a sample, the first texture will be that texture which appears higher on the list of texture. The second texture will be lower in the list. For example, if you determine that the sample has both glassy and vesicular textures, select glassy (higher in the list) for Texture #1, and vesicular (lower in the list) for Texture #2. If you reverse the order of the textures in the list, the textures will be marked as incorrectly identified.

Select a Sample to Identify:

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