Volcaniclastic Sedimentation and Facies

The Interaction between Volcanism and Sedimentation Active volcanism produces abundant sediment that is rapidly delivered to sites of deposition Lateral changes are the result of flow transformations During eruptions, large volumes of pyroclastic and hydroclastic sediment are released far more rapidly than any process of production of epiclastic particles

Volcaniclastic Facies Magma Composition –Eruptive Rates –Types of Particles –Manner of Emplacement –Total Volume and Type of Volcano Differences between volcanoes require that different facies aspects be considered in order to reconstruct volcanic areas These facies aspects are –1 distance-related facies –2 the type of source volcano –3 whether vents were single, multiple, central or flank.

Facies Proximal facies rocks can be defined by type of transport such as lava flows (short travel distance), lahars, and fallout layers (most far-traveled) and, in the case of reworked pyroclastics or volcanic epiclastic materials, on their coarsest and thickest parts Pyroclastic facies may be divided into different subfacies, such as lahar or pyroclastic flow and pyroclastic surge subfacies (mechanisms of transport), lacustrine, submarine fan or alluvial sub- facies (environment of deposition), etc.

Mount St. Helens As shown by the 1980 Mount St. Helens eruptions, one facies lineage, linked by flow transformations, is as follows (Scott, 1988): Eruption of pyroclastic surge or flow > lahar > hyperconcentrated flood flow > normal fluvial transport (in the Columbia River). Another lineage is fallout ash from vertical eruption plumes > initial large-scale debris avalanches > stop-gap storage of sediment on submarine shelves or slopes > submarine landslides > subaqueous lahars > turbidity currents.

Volcanic Ash

Pyroclastic Flow Mount St. Helens During the May 18, 1980 eruption, at least 17 separate pyroclastic flows descended the flanks of Mount St. Helens. Pyroclastic flows typically move at speeds of over 100 kilometers/hour and reach temperatures of over 400 degrees Celsius

Debris Avalanche Downstream view of the North Fork Toutle River valley, north and west of St. Helens, shows part of the 2.3 cubic kilometers of debris avalanche that slid from the volcano on May 18, The avalanche traveled approximately 24 kilometers downstream at a velocity exceeding 240 km/hr. It left behind a hummocky deposit with an average thickness of 45 meters and a maximum thickness of 180 meters.

Miniature alluvial fan developed by local reworking of volcaniclastic sediment on top of the May debris avalanche, Toutle Valley, Mount St Helens. Image: Roger Suthren, 1988.

About 12 million years ago, a volcano in southwest Idaho spread a blanket of ash over a very large area. One or two feet of this powdered glass covered the flat savannah-like grasslands of northeastern Nebraska. Most of the animals which lived here survived the actual ashfall, but as they continued to graze on the ash covered grasses, their lungs began to fill up with the abrasive powder. Soon their lungs became severely damaged and they began to die. Undisturbed except by an occasional scavenging meat-eater, the skeletons of these animals are preserved in their death positions, complete with evidence of their last meals in their mouths and stomachs and their last steps preserved in the sandstone below. Watering Hole Skeletal Remains Ashfall Fossil Beds State Historical Park th Avenue Royal, NE 68773

All Ashfall skeletons are buried in a bed of pure volcanic ash. Volcanic ash consists of tiny shards of glass from broken glass bubbles. These glass bubbles form and then break apart during powerful volcanic eruptions. Ashfall Fossil Beds State Historical Park th Avenue Royal, NE 68773

DEPOSITS OF PYROCLASTIC SEDIMENT GRAVITY FLOWS There are two end-member kinds of pyroclastic sediment gravity flow deposits: –(1) pyroclastic flow deposits that are relatively thick, poorly sorted, commonly containing abundant fine-grained ash in the matrix ( 4 phi), and with crude or no internal bedding –(2) pyroclastic surge deposits that are relatively thin, better sorted than flow deposits, with or without abundant matrix fines, and well bedded to cross bedded volcanology.geol.ucsb.edu

Pyroclastic flow deposits composed of mixtures of non-vesicular to partially or wholly vesicular, fine- to coarse-grained juvenile lithic particles, are known as block-and-ash flow deposits. Pyroclastic sediment gravity flows can move rapidly for long distances, their deposits generally being much thicker in valleys than on ridges. Deposits from single flows range in volume from less than 0.1 km 3 to over 3000 km 3. Some pyroclastic flows of large volume are erupted at such high temperatures that they become welded. volcanology.geol.ucsb.edu

Welded Bishop Tuff (ignimbrite) in Owens Gorge north of the town of Bishop, California. volcanology.geol.ucsb.edu