Pyroclastic rock

Classification

Pyroclasts include juvenile pyroclasts derived from chilled magma, mixed with accidental pyroclasts, which are fragments of country rock. Pyroclasts of different sizes are classified (from smallest to largest) as volcanic ash, lapilli, or volcanic blocks (or, if they exhibit evidence of having been hot and molten during emplacement, volcanic bombs). All are considered to be pyroclastic because they were formed (fragmented) by volcanic explosivity, for example during explosive decompression, shear, thermal decrepitation, or by attrition and abrasion in a volcanic conduit, volcanic jet, or pyroclastic density current.

Transportation

Pyroclasts are transported in two main ways: in atmospheric eruption plumes, from which pyroclasts settle to form topography-draping pyroclastic fall layers, and by pyroclastic density currents (PDCs) (including pyroclastic flows and pyroclastic surges), from which pyroclasts are deposited as pyroclastic density current deposits, which tend to thicken and coarsen in valleys, and thin and fine over topographic highs.

Formation

During Plinian eruptions, pumice and ash are formed when foaming silicic magma is fragmented in the volcanic conduit, because of rapid shear driven by decompression and the growth of microscopic bubbles. The pyroclasts are then entrained with hot gases to form a supersonic jet that exits the volcano, admixes and heats cold atmospheric air to form a vigorously buoyant eruption column that rises several kilometers into the stratosphere and cause aviation hazards. Particles fall from atmospheric eruption plumes and accumulate as layers on the ground, which are described as fallout deposits.

Pyroclastic density currents arise when the mixture of hot pyroclasts and gases is denser than the atmosphere and so, instead of rising buoyantly, it spreads out across the landscape. They are one of the greatest hazards at a volcano, and may be either 'fully dilute' (dilute, turbulent ash clouds, right down to their lower levels) or 'granular fluid based' (the lower levels of which comprise a concentrated dispersion of interacting pyroclasts and partly trapped gas). The former type are sometimes called pyroclastic surges (even though they may be sustained rather than "surging") and lower parts of the latter are sometimes termed pyroclastic flows (these, also, can be sustained and quasi steady or surging). As they travel, pyroclastic density currents deposit particles on the ground, and they entrain cold atmospheric air, which is then heated and thermally expands. Where the density current becomes sufficiently dilute to loft, it rises into the atmosphere as a 'phoenix plume' (or 'co-PDC plume'). These phoenix plumes typically deposit thin ashfall layers that may contain little pellets of aggregated fine ash.

Hawaiian eruptions such as those at Kīlauea produce an upward-directed jet of hot droplets and clots of magma suspended in gas; this is called a lava fountain or 'fire-fountain'. If sufficiently hot and liquid when they land, the hot droplets and clots of magma may agglutinate to form 'spatter' ('agglutinate'), or fully coalesce to form a clastogenic lava flow.

Real-world examples

Mount Vesuvius

In 79 AD, Mount Vesuvius erupted, causing a large cloud of ash, pumice, and gas to rise in the sky before collapsing back onto the mountain in the form of pyroclastic flows. The pyroclastic flows rushed down the mountain side, burying nearby cities, such as Pompeii and Herculaneum, and killing thousands of people. The eruption covered the cities in pyroclastic rocks; preserving the cities underneath for centuries.

Mount St. Helens

In May of 1980, Mount St. Helens erupted, causing a massive landslide and a large sideways explosion, that sent ash, gas, and rock into the surrounding forests, causing substantial damage. The eruption sent a large column of ash into the sky, which then turned into pyroclastic flows that settled on the base of the mountain in the form of pyroclastic rock material.

Yellowstone Caldera

Approximately 640,000 years ago, a huge amount of magma erupted into the air, spreading ash, pumice, and other volcanic rock across thousands of miles, leaving behind a thick layer of pyroclastic material that "welded" into hard "tuff" rock. This eruption, along with two other earlier blasts, formed what is known as the Yellowstone Caldera in the Yellowstone Plateau Volcanic Field, which is one of the most studied volcanic systems in the world.

Mount Pinatubo

On June 15, 1990, in the Philippines, Mount Pinatubo erupted, causing a large column of ash to travel into the air and pyroclastic flows to slide down the mountain and fill surrounding valleys with thick pyroclastic rock debris. The eruption blanketed huge areas, as far as the Indian Ocean, which is approximately 2,500 km away, in volcanic rock fragments and fine ash.

Other reading

• Blatt, Harvey and Robert J. Tracy (1996) Petrology: Igneous, Sedimentary, and Metamorphic, W.H.W. Freeman & Company; 2nd ed., pp. 26–29; ISBN 0-7167-2438-3
• Branney, M.J., Brown, R.J. and Calder, E. (2020) Pyroclastic Rocks. In: Elias, S. and Alderton D. (eds) Encyclopedia of Geology. 2nd Edition. Elsevier. ISBN 9780081029084