Transcript Volcaniclastic Sedimentation and Facies

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
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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 subfacies (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, 1980. 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 18 1980
debris avalanche, Toutle Valley, Mount St Helens. Image:
Roger Suthren, 1988.
Skeletal
Remains
Watering Hole
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
Ashfall Fossil Beds State Historical Park
in the sandstone below.
86930 517th 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
86930 517th 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 (<1/16
mm; >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
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• 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 km3 to over 3000 km3. Some
pyroclastic flows of large volume are
erupted at such high temperatures that
they become welded.
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Welded Bishop Tuff (ignimbrite)
in Owens Gorge north of the
town of Bishop, California.
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