The type of volcanic eruption and the variety of volcano that forms depends in large part on the composition of the magma at the time of the eruption. Please read the background information about the concepts for this activity on p.155-157 of this activity. Page 154 gives a brief overview of some volcano types.
Peaceful eruptions of low viscosity basaltic lava are commonly associated with shield volcanoes, cinder cones (called scoria cones in your book), and fissure eruptions
Shield volcanoes are large broad, and gently sloping (2° to 10° slopes) volcanic cones. The large volcanoes that make up the big island of Hawaii (Mauna Loa and Mauna Kea) are classic examples of shield volcanoes. The Hawaiian volcanoes erupt basaltic lava from rifts that radiate from the summit of the volcano and basaltic lava flows easily and spreads out broadly. Countless lava flows spread out and build up to tremendous heights by one on top of the other over time. For example, Mauna Loa’s circumference is about 400 miles and it has up to 6 miles of relief (if you start at its base, which is at the bottom of the ocean).
Fissure eruptions occur when basaltic lava erupts through linear fractures or fissures instead of from a round volcanic cone. Large-scale fissure eruptions are responsible for building the Columbia River Plateau up in Oregon and Washington (you can see the countless basalt lava flows that formed this geomorphic province in the Columbia River Gorge, for example). Small scale fissure eruptions are seen in rift zones that split the Hawaiian Shield volcanoes – like the East Rift Zone associated with Kilauea.
A cinder cone is a volcanic cone built of basaltic ash and cinders. When fluid basaltic lava fountains high in the air, the droplets of lava form the ash and cinders that build the cone as they fall to the ground. This fountaining lava is sometimes caused when basaltic lava encounters groundwater on its way to the surface. The hot lava superheats the groundwater causing it to flash to steam, which carries the lava up into the fountains that produce cinders and ash as the lava is thrown into the air. The finer-grained ash is blown downwind, and the larger heavier cinders fall around the vent to build up the cinder cone.
Cinder cones are the classically shaped volcanoes that you imagine when you think of a volcano. They are relatively small (height <1500 feet, typically 300 to 650 feet) with sides that slope at an angle of about 30°-40°. Cinder cones usually erupt over a short period of time (months to years) and during that time, once the steam that is causing the fountaining lava is used up, then massive basaltic lava flows breach the sides of the volcano. Once eruption stops, the cinders that make up the cone are easily eroded. Cinder cones usually only endanger property (lava flows and cinders are avoidable).
Examples: Paricutin, Mexico and Sunset Crater, Arizona - I went so see Sunset Crater a couple of summers ago (its a National Monument not far from Flagstaff). Here’s my photo:
Explosive eruptions and pyroclastic flows associated with high viscosity rhyolitic lavas are commonly associated with stratovolcanoes and lava domes
Stratovolcanoes are tall volcanoes built from alternating layers of lava flows and pyroclastic material (from pyroclastic flows and ash fall). They are generally 1.5 to 2 miles high. Their elevation is so high that it is common to see a snowcap on its summit. Some even sport year-round glaciers.
Most stratovolcanoes form in subduction zones. The initial magma formed by partially melting the mantle in a subduction zone is basalt. But this magma commonly ends up becoming enriched in silica as it works its way up to the surface through the overlying thickened crust. The different layers in a stratovolcano indicate that different kinds of magma (varying from basalt to rhyolite) may erupt over its long life. The average composition of the magma in a stratovolcano is andesite, but composition varies depending on what happens to the originally basaltic magma on its way to the surface. Thus, stratovolcanoes exhibit a wide variety of eruption types over time (peaceful lava flows, explosive pyroclastic flows). Examples:
Mount Vesuvius in Italy
Mount St Helens in the U.S.
Mount Fuji in Japan
Mount Pelee in Martinique
Click here for a video about the 1980 eruption of Mount Saint Helens (a stratovolcano): http://www.youtube.com/watch?NR=1&v=xP2dreOI8gI
Lava Domes are relatively small, rounded, steep-sided volcanoes that form from silica-rich rhyolitic magma. Lava domes form because this kind of lava (rhyolite) barely flows (extremely high viscosity), it just kind of bulges upward (instead of flowing outward). As the lava hardens it forms a plug, which traps gas below it. Lava domes are extremely dangerous because they tend to collapse and when they do, the pent up gas is released and forms nasty pyroclastic flows.
Most lava domes form in subduction zones. They may grow as separate isolated volcanoes (such as Lassen Peak, or Black Butte) or they may form within the crater of a stratovolcano (as has occurred in the crater of Mount Saint Helens) or even off the side of a stratovolcano (as in Shastina on Mount Shasta).
Some of my photos of Lava Domes:
This activity involves internet-based research - bring your laptop if you have one. A few 8T laptops will be available if you don't have a lap top or forget to bring yours.
What is tephra and what are the hazards?
Actions to take for ashfall:
Tephra in the Long Valley Area:
Air Routes Above Long Valley: http://volcanoes.usgs.gov/volcanoes/long_valley/long_valley_hazard_41.html
LAVA FLOWS AND LAVA DOMES
Effects of lava flows:
Danger of dome collapse: http://vulcan.wr.usgs.gov/Volcanoes/Hood/Publications/EV24-6/dome_collapse_hood.html
Other lava domes: http://volcanoes.usgs.gov/images/pglossary/LavaDome.php
Lava Flows and Lava Domes at Long Valley:
What is a pyroclastic flow?
Pyroclastic Flow Video: http://www.youtube.com/watch?v=vPRoTQYXwuY
Long Valley Hazard Zones: http://volcanoes.usgs.gov/volcanoes/long_valley/long_valley_hazard_2.html
Gases at Mammoth Mountain:
General Info about Volcanic Earthquakes:
Info about earthquakes in the Mammoth area:
http://pubs.usgs.gov/bul/b2185/appenda.html (scroll down to the text and figures starting below figure A2)
http://earthquake.usgs.gov/monitoring/deformation/data/plots/?region=LV (shows the faults in the Long Valley area)
Read the material in your manual for each activity with care so that you understand what you missed, talk to a classmate to find out more, visit the internet sites for Long Valley Analysis. Download the assignment and attempt to answer the questions - turn it in prior to the next class period. Pick up the information about next week's quiz. Click here for the syllabus information about missing class.