Volcanic and Plutonic Landforms

Assignments

1. Earth Revealed: movies. Courtesy of Anneberg Media, URL <http://www.learner.org/resources/series78.html>.  Requires Windows media Player.  Sign in and view#13 Volcanism and #14 Intrusive Igneous Rocks
2. Jones and Jones: Chapter 3 Exercise 7 www.mhhe.com/jones5e

Explore

• Smithsonian Institution, Global Volcanism Program (GVP),Types and processes gallery
• Tilling, Robert, 1997, Volcanoes: <http://pubs.usgs.gov/gip/volc/intro.html> USGS online text
• Vic Camp: How volcanoes work> volcanic landforms
• NASA GFS: Volcanic Landforms
• Erupt 2.5by Dr. Ken Wohletz, Los Alamos National Laboratory: free PC software that allows you to build your own volcano.
• Other helpful sites
  • USGS Photo Glossary of Volcanic Terms
  • NationalAtlas: Active Volcanoes of the US (shockwave required)
• Additional sites are listed at the end of this outline.

Terms: exhalative (phreatic) eruption, geyser, sinter mound, phreatomagmatic eruption, felsic, intermediate, mafic, pahoehoe, tephra, obsidian, columnar jointing, exogenous and endogenous domes, tholoid dome, shield volcano, composite cone, pumice, scoria, cinder cone, mar, caldera (Hawaiian type, Yellowstone type), discordant, concordant, dike, sill, laccolith, diatreme, volcanic neck, batholith, maar, lava (basalt plateau), flood basalt, topographic inversion

Non-magnetic landforms

Exhalative landforms: These features are formed by eruptions of hot water, steam, or gas. They most commonly occur where groundwater is heated by an underlying magmatic source or where water is flowing into an active vent. Eruptions can range from boiling pools and fountains of hot water to violently destructive phreatic blasts capable of forming giant craters (maars) and explosion pits.  The geysers, mudpots, mud volcanoes, and sinter mounds of Yellowstone  National Park are exhalative landforms.  Violent phreatomagmatic eruption occurs when water and magma interact.

• geysers, fumaroles, solfateras, sinter mounds (CVO)
• mud volcanoes

Extrusive Landforms

Factors governing the type of volcanic landform:

1. Feeding conduits (vents or fissures): The number and character of the conduits controls the number, shape, and distribution of landforms in a volcanic field. Volcanoes are created from a central vent or radiating fissures, whereas lava plains are develop from one or more large fissures that distribute lava over a broad region.
2. Eruptive style (effusive or explosive): Magma composition (silica and gas content) is the underlying factor controlling eruptive style (fig. 2).  Eruptive style depends on the interplay between viscosity, which inhibits flow, and gas content which drives it.  If the viscosity is low and the gas content high the magma will flow readily.  However, if both are high the eruptions will be explosive. Viscous magmas are typically high in silica.  Therefore, rhyolitic or dacitic eruptions tend to be the most explosive, while basaltic eruptions are least explosive(fig. 2).  For details about types of eruptions go to Tilling (1997) and Camp.
3. Volume of material extruded

Through magma degassing the eruptive style can change in the following ways:

• Mafic eruption: A gas charged mafic flow will fountain and later transform into a pahoehoe   or clinkery aa flow (fig. 1).
• Felsic (rhyolitic, dacitic) eruption: A pyroclastic eruption dominated by tephra may later produced short, thick blocky flows of obsidian. (figs. 6 and 7)

Highly explosive events can result from the addition of meteoric water to the magma chamber.

The above factors are ultimately controlled by tectonic regime, which is why a suite of volcanic landforms along a continental rift is different than that formed along a subduction zone.  Compare for instance the Long Valley caldera, felsic domes, and cinder cones (Poverty Hills) of the Owens Valley in eastern California with the stratovolcanoes of the Cascade Range in the Pacific northwest.

Figure 1. Rubbly basaltic aa flow in the San Francisco Volcanic Field, Northern AZ. (quicktime VR version) (Photo by LSH)

Figure 2. Magma compositions and characteristics and associated volcanic landforms.  Modified from Smith and Pun (2006).

Constructional landforms

Cones and Domes

Volcanic domes are mountains or hills created by the extrusion of lava from a point source. For an excellent online summary see The principal types of Volcanoes (Tilling, 1997).

Volcanoes (Exogenous Domes)

Volcanoes are built of layers of  lava and/or ash.  The relative proportion of each is determined by the eruptive styled, which in turn is controlled by magma composition and gas content. 

1. Shield Volcano (CVO)

• Large WxH ratio (slopes: 4-6°, may be 20° at summit)
• Largely basaltic in composition
• composed predominantly of lava flows (CVO) and a network of radial dikes
• Examples(GVP): Little Belnap, Oregon, Prestahnukur, Iceland and Hawaiian volcanoes

3. Cinder ("tephra") cones (CVO) / Pyroclastic cones (GVP)

• pumice or scoria cones that commonly occur as satellite cones
• highly symmetrical unless strong prevailing winds existed during eruption
• slopes governed by angle of repose (10-40°)
• the shape and slope of volcanic cones is govern by the balistic distribution of tephra, the rate of accumulation around the vent, and the angle of repose of ejecta.
• Scoria cones are very permeable and don't easily submit to erosion by surface runoff.

Figure 3. Shield volcano. Mauna Loa, Big Island, Hawaii. For more info see Mauna Loa, Earth's Largest Volcano (HVO) (Image Source: Wikimedia Commons, photographer Atelier Joly).	Figure  4. Stratovolcano. Mount Rainier, Was hinging, 4,392 Meters (14,410 Feet) is the third largest stratovolcano in the Cascade Range. (Image Source: Wikimedia Commons, photographer Walter Siegmund)	Figure 5. Cinder Cone. 71,000 year-old SP Crater with a basalt aa flow emanating from the north flank. Classic cinder cone in the San Francisco Volcanic Field, AZ. For more images to to geology.com. (Image source URL: http://wrgis.wr.usgs.gov/fact-sheet/fs017-01)

Endogenous Domes (Cumulo-domes, tholoids, and plug domes)

Endogenous domes (CVO): Cumulo-domes, tholoids (tholos--Gr. a dome or vaulted ceiling) and plug domes are formed by the extrusion of thick pasty lava that pushes outward from the vent, much like putty flowing from a cauking gun.  As such they lack the layering that characterizes exogenous domes. These domes are composed of obsidian.   In some instances the magma is so stiff that it forms a vertical spine (fig. 8).

Examples (SVP):

• Dacite lava tholiod dome in Novarupta, Alaska
• Tholoid dome in Mount St. Helens , WA
• Mono Craters (30 rhyolite cumulo-domes)
• Spines: Mammoth Mountain, CA (fig. 8), Mount Pelee, Martinique

Figure  6. Obsidian tholoid dome and pumice ring in Panum Crater, north of the Long Valley Caldera, California.  Ring morphology is controlled by a) the ballistic trajectories of pumice fragments, and b) the their angle of repose. Location Map for Long Valley Caldera Region (USGS LVO) (Photo by LSH)

Figure  7. Cumulo-domes or plug domes of Mono Craters, Long Valley Caldera region, CA. (Photo by LSH)

Figure  8.  Felsic lava spine inside Mammoth Mountain volcano on the western perimeter of the Long Valley Caldera. (Photo by LSH)

Flows, plains and Plateaus (formed by aerially extensive flows or pyroclastic sheets; slopes <1°)

• Lava Flows, Plains and Plateaus (CVO) are composed largely of flood basalterupted from long fissures. A single flow can extend form more than 100km. Eroding plateaus have a stepped-form controlled horizontal flows and vertical columnar joints. The term trap rock commonly applied to basaltic rocks is derived from treppen (germ. stairs) because of its associated geomorphology.

  Examples:

  • Columbia Plateau (1,300,000 km sq.) and Snake River Plain
  • Deccan Plateau (260,000 km x 2km, 2000 m thick)
  • Iceland
  • Paraná-Angola-Namibia Province
  • Treppen: Basalt Cliffs, Patagonia, Argentina
  • Siberia Trap (largest basalt province known)

  Topographic inversion and lava flows: Differential weathering and erosion of  variably resistant rocks can result in an inversion of topography.  nowhere is this process more evident than in volcanic fields.   Flood basalt will seek and fill in local valleys.  If surrounding rocks are weaker they will be preferentially eroded leaving the harder basalt behind.  What was once a valley becomes a black-capped mesa or butte.

Figure 9. Progressive stages of topographic inversion.  Basalt fields ranging in age from Miocene to Holocene are found along the Jemez lineament, a weak crustal zone that cuts diagonally through New Mexico.  On the left a young, possibly Holocene flow occupies a valley.  On the right an older flow caps a mesa underlain by weak shale. Like the flow on the left, the older basalt flowed in a valley. Differential erosion over time created the mesa.  The unique slumping along the mesa's margins is caused by failure of the underlying shale.  (Photo by LSH--took these out the window of a commercial flight to Phoenix. Not bad.)

 

 Pyroclastic sheets, plains, and plateaus formed by explosive Pelean eruptions of tephra)
• pyroclastic eruptions may occur from a series of vents around a caldera rim
Examples:
• Yellowstone Plateau (Lava Creek Tuff, Mesa Falls Tuff, Huckleberry Ridge Tuff), WY
• Bishop Tuff from the Long Valley Caldera, CA
• Valley of Ten Thousand Smokes (1912 ash flow sheet from Novarupta)
• Bandelier tuff from the Valles caldera, Jemez, NM (vwdoc)

Concordant plutons

Tabular and lenticular plutons

sill: tabular

• Palisades sill, NY-NJ

• laccolith: large lense-shaped pluton. Aerial dimensions may be similar to batholiths (e.g. Katahdin)

• Bear Butte, SD
• Henry Mountains, UT
• La Sal Range, UT
• Big Bend National Park: Intrusive features 

Distribution of igneous activity

1. Subduction zone
2. Oceanic-Oceanic rift zone
3. Continental-continental rift or transtensional zone
4. Hotspot (beneath oceanic or continental crust)

 Areas of Late Cenozoic volcanism in the United States

Figure 10. Created from http://www.nationalatlas.gov/dynamic/dyn_vol-us.htm

Text Bibliography

Bloom, Arthur. 1998, Geomorphology, A systematic analysis of Late Cenozoic landforms, (3rd edition): Prentice Hall, Upper Saddle River, N. J., 482 p.

Smith, Gary, and Pun, Aurora, 2006, How Does Earth Work? Pearson Prentice Hall, pp. 64-101.

Summerfield, M. A., 1991, Global Geomorphology. John Wiley and Sons, New York, NY, 536 p.

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Internet Sites

• Alaskan Volcano Observatory (AVO) <http://www.avo.alaska.edu/>
• Cascade Volcanic Observatory (CVO) USGS Cascade Volcano Observatory Global Volcanism Network at the Smithsonian Institution <http://vulcan.wr.usgs.gov/home.html>
• Creasy, John, and Fitzgerld, John, 1996, Bedrock Geology of the Eastern White Mountain Batholith, North Conway Area, New Hampshire, NEIGC
• Hawaiian Volcanic Observatory (HVO) <http://hvo.wr.usgs.gov/>
• Volcano World (VW) University of North Dakota VolcanoWorld <http://volcano.und.nodak.edu/>
• Long Valley Observatory (LVO) <http://quake.wr.usgs.gov/VOLCANOES/LongValley/>
• LVO Team, 1999, Geologic History of Long Valley Caldera and the Mono-Inyo Craters volcanic chain, California, U.S. Geological Survey Long Valley Observatory, 1999 <http://lvo.wr.usgs.gov/History.html>
• Priest, Susan, and others,  2001, The San Francisco Volcanic Field, Arizona: U.S. Geological Survey Fact Sheet 017-01, URL: <http://geopubs.wr.usgs.gov/fact-sheet/fs017-01/>
• Tilling, Robert I, Topinka, Lyn, and Swanson, Donald,  2002, Eruptions of Mount St. Helens: Past, Present, and Future: Online version 1.1, U.S.G.S., URL: <http://pubs.usgs.gov/gip/msh/revision.html>

Online Exercises

Vic Camps Volcano crossword

   Lindley Hanson/Department of Geological Sciences/Salem State College/Geomorphology/GeoIndex/QkRef