Volcanism Dr. Dan Britt Department of Physics
Pompeii and Herculaneum Buried in a AD 79 eruption of Mt. Vesuvius. Pompeii was under ~20 feet of ash and pumice Herculaneum was under ~80 feet of welded tuff. This 18th century print of the excavation of Herculaneum gives you a sense of how deeply these cities were buried.
What Happened? This was an explosive, Plinian eruption. So called because Pliny the Elder set out to rescue the victims from Pompeii during the AD 79 eruption of Mount Vesuvius (he commanded the local Roman fleet). He famously said Fortune favors the brave when it was suggested to turn back. Pliny the Younger was a prolific writer and provided an account of his death.
What Happened? A classic Plinian eruption is dominated by sticky lavas that make it very difficult to out-gas volatiles. Gases accumulate (reaching ~75 vol % of the magma) and erupt explosively. Produce rapidly expanding eruptive columns that reach up to 45 km.
The column cools at altitude Without the expanding heated gases it becomes heavier than air and collapses. The kinetic energy gained from all that altitude goes into producing a rapidly moving pyroclastic (glowing debris) flows that move downslope like a fluid. Note that while the column has cooled it is still very hot.
Pompeii and Herculaneum The pyroclastic flows hit the towns at 100 mph with temperatures up to 932 F The hot gases were quickly fatal. About 16,000 dead 1100 casts of bodies in Pompeii 332 in Herculaneum
AD 79 Eruption
Pyroclastic Flows These can be very bad news..a couple of tips. Don t run downhill..that is were the flow is going. Run normal to the terrain. They are very fast. Don t try to outrun them like the guy on the left...
One more tip..don t look back!
St. Pierre, Martinique, May 8, 1902, a pyroclastic flow from Mont Pelée destroyed the town of, killing almost all of its 29,000 inhabitants
Mt. Vesuvius Risks Eruptions are both frequent and violent. Since the eruption of 79 AD, Vesuvius has erupted around three dozen times. The activity goes in phases and it can be quiet for ~ 300 years. The quiet periods often result in population build-up on recent lava flows (this is Italy after all). New eruptions destroy the villages and kill the inhabitants. Last eruption was 1944
Note the relative distances of Pompeii and Naples from Vesuvius. There are about 4 million people in the area of this image
Styles of Volcanism Effusive (Hawaiian, Flood) Explosive (Plinian) Intermediate (elements of both) Intrusive
Eruption Products Pyroclastic Flows Ashfalls Lava Flows Mudflows/Debris Flows
Effusive volcanism is associated with zones where the mantle plumes are close to the surface. Mid-ocean ridge spreading centers Hot spots Continental spreading centers Explosive volcanism is associated with zones where magma is generated from subducting plates and travels some distance thru the crust.
Effusive Eruptions Lava is runny and flows downhill quite quickly and easily. Effusive eruptions build up gently-sloping shield volcanoes such as Mona Loa and Mona Kea in Hawaii, as well as flood volcanism. This is Skjaldbreiður in Iceland..extra points for anyone who can correctly pronounce it.
Dissolved gases escape easily as the magma erupts. Sources: Mid-ocean ridges, hot spots, continental fissures. This is Mona Kea in Hawaii (note observatories on the top)
The Royal Gardens Subdivision Hawaii
Surging Lava.. While Hawaii-style eruptions tend to be slow, lava can surge.
What is wrong with this picture?
Pahoehoe: Hawaiian for ropy. Highly fluid Aa: Hawaiian for ouch. This is pahoehoe that has cooled and lost gases. It turns into rough, jagged blocks. Essentially very large collections of broken glass. Basaltic Lava AA Pahoehoe
Fissure (flood) Eruptions Actually the most common and important type of eruption. Where magmas flow from linear cracks rather than spots. Creates the ocean floor at the midocean ridges and flood basalts on the continents.
Bridge across the Álfagjá rift valley in southwest Iceland, on the boundary between the Eurasian and North American plates. How often do they need to lengthen the bridge?
Volcanic Products
Types of Ejecta Ash: The finest fragements. Less than 2 mm in diameter Volcanic Breccias: Larger fragments Bombs: Smoothed rounded rocks formed from magma blown out of the volcano and cooled in flight.
Volcanic Bomb
Ash Can be a Problem
Debris Flows
Mt. Shasta Debris Flows: Roughly 46 cu km Debris avalanche swept some 50 km to the north
The Osceola Mudflow: volume 3 cubic kilometers The smallest, but most frequent, debris flows at Mount Rainier begin as glacial outburst floods, also called by the Icelandic term "jokulhlaup" (pronounced "yokul-h-loip")
When you see things like this near your house, you should consider putting it on the market and moving to higher ground.
Mt. St. Helens Erupted explosively Sunday May 18, 1980, at 8:32 am PDT Fifty-seven people were killed The USGS had decided that eruptive danger was low. The local forests were to reopen to logging on Monday
260 square miles of blow-down
Hot Spots Probably due to hot mantle plumes that rise from the core-mantle boundary.
Hot Spot locations. What is wrong with this picture?
Intrusive Volcanism Intrusive rocks are formed from magma that solidifies within the crust of a planet. The magma cools slowly, and as a result these rocks are coarse grained. Turns out that this is by far the most prolific type of volcanism The ratio of intrusive to extrusive volcanism is about 40 to 1.
Intrusive Volcanism This type of volcanism is responsible for things like the Sierra Nevada range and the generation of most of the oceanic crust. For oceanic crust, most of it is formed from dikes (similar to the picture on the top right). Other forms include large masses of buried magma (batholiths) like the Sierra Nevada