2/11/16
PYROCLASTIC ACTIVITY AND VENT STRUCTURES
ON HAWAIIAN VOLCANOES
Modified from Scott K. Rowland, University of Hawai‘i at Manoa
ERUPTION STYLES AND
VENT FORMS
Today:
Pyroclastic deposits
and vent types
No class Monday
Review next Fri
Expanding gas drives a lava fountain. The
highest fountains in Hawai‘i are >500 m
hmmm
Technically, the base of the fountain is where the gas
becomes 75% by volume, and this is usually 10s to 100s
of meters below the vent rim.
1
2/11/16
Fountain evolution
With decreasing flux, or decreasing volatile content,
fountain slowly decreases in size
2
2/11/16
-Pu‘u ‘O‘o (90s): result of fountain = scoria cone
Scoria: crude bedding and large bombs
~2 m
Tephra
Fragments of airborne volcanic material!
 Classified by size!
 2 types by composition!
• Scoria (basalt-andesite)!
• Pumice (rhyolite)!
 Rock types depend on loose vs. consolidated/welded!
Wikipedia: volcanic ash
3
2/11/16
Ash
Smallest fragments of volcanic material!
 Ash results from high level of fragmentation!
 Widest spread away from eruption!
Wikipedia: volcanic ash
South America
http://www.seablogger.com/images/200805/chaiten2.jpg
http://www.terradaily.com/images/ecuador-tungurauavolcano-buried-village-roof-ash-2006-afp-bg.jpg
Lapilli
Intermediate fragments of volcanic material!
 Small pieces of rock (fragments)!
 Accretionary lapilli (concentric ash layers, wet condition)!
http://www.decadevolcano.net/
photos/keywords/photos/
P5091515.jpg
http://volcanoes.usgs.gov/Imgs/Jpg/
Photoglossary/lapilli_large.jpg
Wikipedia: lapilli
Bombs and blocks
Large fragments of volcanic material!
 Solidified melt (prior to eruption): blocks!
 Plastically deforming: bombs -- drop/aerodynamic !
shapes!
Breadcrust!
bomb
http://www.nps.gov/archive/crmo/
glossary/breadcrust-bomb.jpg
http://www.daviddarling.info/encyclopedia/
V/volcanic_bomb.html (USGS image)
Spindle!
bomb
4
2/11/16
Pumice
Lost of volatiles => bubbles!
Rhyolitic composition!
 Individual bubbles => pumice (floats)!
 Interconnected bubbles => reticulite (sinks)!Wikipedia: pumice
http://volcanoes.usgs.gov/Imgs/Jpg/Photoglossary/reticulite1_large.JPG
Typical high-fountaining pyroclasts:
reticulite, scoria, Pele’s tears, and Pele’s hair
~2 cm
(from Volcanoes in the Sea)
Melt blobs
Melt blobs erupted as:!
 Spatter (more basaltic)!
 Agglutinates (more granitic) !
blobs welded together!
http://records.viu.ca/~earles/geol390/photos/spatter01.jpg
5
2/11/16
On flank of Pu‘u ‘O‘o, spatter is erupted
Spatter is still fluid
when it lands
~20 cm
Mauna Iki satellitic shield, Kilauea SW rift zone
-
- scoria cone
Kupaianaha
lava pond and shield, with Pu‘u ‘O‘o
in the background
6
2/11/16
- Kilauea (1986-1992)
Kupaianaha
lava pond,
~20 m
Phreatomagmatic
Melt interacts with groundwater:
H2O provides volatile-driven
force for eruption:
e.g. Kilauea, 1924
7
2/11/16
Keanakako‘i
tephra exposed in upper SW rift zone fractures
photo by P. Mouginis-Mark
Hydromagmatic (surtseyan) eruption
Explosive eruption at water level
Surtsey island built as continuing explosive eruptions Vent area: mix of glass shards, gas, water and melt coming in
⇒ H2O expands and melt cooled to fragments
⇒
Youtube
Wikipedia
Examples of “wet” eruptions
Koko Rift, Ko‘olau,
O‘ahu:
-most of these eruptions
occurred off the shoreline
at the time (pyroclastics,
coral debris)
-tuff cones, many nested
and/or coalesced
Molokini, E. Maui Volcano
8
2/11/16
Wet eruptions often make “surge” deposits, from
lateral, turbulent deposition
Note surge deposits can make cross-bedding
Cross-bedding
  Small beds at an angle to main sedimentary layers
 Wind, waves, currents
Pau
9