2. Pumice is used as an abrasive material in lava soap
II. Volcanism (Volcanic activity)
- A. Some common terms
- 1. Volcanism
- refers to the processes by which magma and/or gases or volatiles are transferred to the Earth's surface
- 2. Volcano
- is an elevated area or mountain formed from the accumulation of lava and/or pyroclastic material
- 3. Active, dormant, and extinct volcanoes or volcanic areas
- active
- eruption can occur in the near geologic future
- an example is Mt. St. Helens and other Cascade Mountains
- dormant
- presently inactive but believed capable of future eruptions
- extinct
- expected not to erupt again
- B. Origin and global distribution of volcanism
- 1. Origin of volcanic materials
- basically the same as for igneous activity (areas of subduction, ridges and magma plumes)
- see page 101 again
- 2. Global distribution
- "ring of fire" or the circum-Pacific region is the greatest concentration of volcanic activity on Earth
- some individual volcanoes listed on page 100 will be cited later as specific kinds of eruptions---
there is an interesting historical eruption mentioned by Plato who suggested that Santorin, a group of islands in the Mediterranean Ocean was once a continent which exploded and thought to be
the "lost continent of Atlantis"
- click here to see the world distribution of volcanoes
- C. Volcanic rocks, volatile (gases and vapors) emissions and manner of eruption
- 1. Volcanic rocks
- are extrusive rocks and include basalt, andesite, rhyolite, pumice, scoria, tuff, and obsidian
- refer again to figure 1
- 2. Volatile emissions
- steam or water vapor (H2O) is most abundant, carbon dioxide (CO2) is next and sulfur dioxide (SO2)
nitrogen (N2), and sulfur trioxide (SO3) are others typically associated with volcanic eruptions
- carbon dioxide additions to the atmosphere can add to the "Greenhouse Effect" while SO2, N2, and SO3
can contribute to the formation of acid rain
- although the burning or decomposition of fossil fuels produces carbon dioxide and the other gases emitted
into our atmosphere we are able to control only the synthetic emissions
- 3. Manner of eruption
- QUESTION: why are some volcanic eruptions explosive or violent while others are passive?
- ANSWER: the manner of eruption depends largely on the viscosity of the magma associated with the eruption
--gases cannot escape as readily from a more viscous (syrup or oil like) magma causing a gas pressure buildup which
eventually explodes
- magma viscosity may depend on 2 factors
- magma composition--the higher the silica (SiO2) content, the more viscous the magma
- magma temperature--the cooler the temperature, the more viscous the magma
- QUESTION:based on composition alone a more explosive eruption would be associated with which magma composition; rhyolitic,
andesitic, or basaltic? (HINT: which extrusive rock has the most quartz in it?). For the same reason which associated magma composition
would result in a passive eruption?
- QUESTION: two magmas start out in the mantle with the same composition, one magma moves through the ocean basin crust (basalt) and
erupts on the ocean basin surface, while the other moves through the continental crust and erupts on the continental surface. If there is assimilation
of rock in each magma, based on composition at the time of eruption which magma would be more explosive?
- refer to page 101(113) again (above) and to page 102(114) below
- D. Major categories of volcanic deposits (Classification of volcanoes)
- volcanic deposits are comprised of lava flows and/or pyroclastics
- 1. Shield type
- is a thick deposit of successive lava flows usually basaltic in composition with little or no pyroclastics and associated with ocean basin eruptions
- the Hawaiian island shield is comprised of basalt which averages about 28,000 feet thick with the individual islands sticking above sea level like pimples on the shield deposit
- click here to see the shield volcano
- 2. Cinder cone type
- 3. Composite or strato-volcano
- is a very tall somewhat layered deposit of alternating lava flow and pyroclastic deposits comprised of andesite or rhyolite composition rocks
- occur on continents and are usually explosive in manner
- examples include: Mountains in the Cascade mountain range in the western U.S. and includes Mt. St. Helens; Mt. Vesuvius, Italy; Mt.Fujiyama, Japan; Mt.Kilamanjaro, Africa
- click here to see the composite volcano
- E. Eruptions with special volatile emissions
- 1. Strombolian type
- is a cloud of volatiles emitted during the eruption and may hover for long periods of time over a volcanic cone--the molten material in the volcanic vent reflects light off the cloud--
the prime example of this is the incandescent cloud which hovers over Mt. Stromboli and acted as a navigation guide for early mariners--Mt. Stromboli was known as the "lighthouse of the Mediterranean"
- 2. Nuee Ardentes (fiery or glowing cloud) type
- is an emission of a very hot cloud of volatiles and ash materials during eruptions---
the prime example of this is an eruption of Mt. Pelee near St. Pierre on the island of Martinique in the Caribbean. A 700 degree (C) cloud killed about 30,000 people in a few minutes
---see the photo on page 93(100) of your text book
- 3. Lake Nios type
- carbon dioxide (CO2) trapped in subsurface rocks from previous volcanic activity can be later emitted at the Earth's surface with possibly disastrous results---the best example of this is Lake Nios, Cameroon (Africa)
where a total of 1746 villagers were killed in 1986 from an emission of CO2 from a caldera (collapsed volcanic cone--see later notes) on which they were living
- F. Special features, benefits and mass wasting associated with volcanism
- 1. Special features
- Continental lava flows
- Ocean basin lava flows
- mostly associated with the shield type of deposit
- pahoehoe is a ropy surface lava
- aa is a blocky surface lava
- lava tubes or tunnels develop when lava inside the lava flow continues to move while the outer lava stiffens and stays in place
- Volcanic neck and radiating dikes
- revisit page 68(107) and see photo on page 97(105) in the text book--one of the best examples of a volcanic neck and radiating dikes is Shiprock in New Mexico
- click here to see Shiprock
- Caldera
- is a volcanic cratered structure resulting from the collapse of a volcanic cone
- click here to see the stages in caldera formation
- best terrestrial example is Crater Lake, Oregon and the best extraterrestrial example is Olympus Mons on Mars which is 5 times the diameter of Crater Lake and covers over 45 times the area
- click link on comprehensive reference on volcanic terms terms and examples at the beginning of volcanism above or at the end of this section to see pictures of different calderas
- 2. Benefits related to volcanism and igneous activity
- fertile soils result from weathering of many volcanic rocks
- magma in contact with or in the vicinity of groundwater produces steam which in turn can be converted to a clean source of energy (geothermal energy)
- water related to igneous activity and trapped below surface of extraterrestrial bodies could be mined--this could influence colonization of other celestial bodies in the future
- 3. Mass wasting related with volcanism
- severe damage can result from mudflows on the sides of volcanoes--these mudflows are called lahars--lahars result when highly unstable layers of volcanic ash and debris become saturated with water from heavy precipitation and flow down slope
- click here to see a house buried in a lahar
- G. Prediction of volcanic eruptions
- Omens or precursor events related to eruptions
- the periodicity and magnitude of Earth tremors caused by movement of rock materials below the surface can be used as precursors to a volcanic event
--the closer the tremors in time and the greater the magnitude, the sooner the volcanic eruption
- bulging of the Earth's surface can be used as an eruption predictor: as magma rises prior to eruption the Earth's surface bulges
- tiltmeters are used to measure the magnitude of the surface bulge--the magnitude may be an indicator of the eruption
- laser beams reflected off mirrors can also determine the magnitude of bulge
- click here for more on volcano monitoring techniques
- H. Volcanism in our solar system
- 1. On our moon
- although there is no official citing of volcanism on the moon the rocks collected show evidence of large scale igneous and volcanic activities. Large abundances of gabbroic and basaltic type of rocks
are present on the moon
- the lunar maria contain massive lava flows
- 2. Mars
- 3. Io, a moon of Jupiter
- the only place other than on Earth where volcanism was seen taking place---a scientist observed a volcanic plume on the horizon of Io as the pioneer spacecraft passed by this moon
click here for a comprehensive reference of volcanic terms and examples
