March 24, 2003
Sir:
The USGS has begun to use LAVA DOMES (see http://volcanoes.usgs.gov/Products/Pglossary/LavaDome.html ) when referring to VOLCANIC DOMES.
Lava domes - are whole volcanic edifices, otherwise known as shield volcanoes, built by fluid basaltic lavas .
Volcanic domes - are features on volcanoes built by viscous lavas such as rhyolite.It does no service to degrade vocabulary when there are so few descriptive words to go around.
The confusion might stem from the inappropriate word "viscous" that in the following would seem to refer to the whole of Mauna Loa rather than a feature on it AGI, Glossary of Geology, 1974, definitions:
LAVA DOME (a) A dome-shaped mountain of solidified lava in the form of many individual flows, formed by the extrusion of highly viscous [sic] lava, as Mauna Loa, Hawaii. Cf: volcanic dome. Syn: dome [volc]. (b) shield volcano. (c) A tumulus developed on a lava flow
But in the same glossary is
VOLCANIC DOME A conical hill of lava and/or pyroclastics that is built up around a volcanic vent. It may be intersected by dikes. Syn: cone [volc].
The confusion is not continued in the AGI, Dictionary of Geological Terms, 1984, in which the word "viscous" in the old definition is replaced by "fluid":
LAVA DOME 1. A dome-shaped mountain of solidified lava in the form of many individual flows, formed by the extrusion of highly fluid lava, e.g. Mauna Loa, Hawaii. Cf: volcanic dome. 2. shield volcano.
VOLCANIC DOME A steep-sided protrusion of viscous lava squeezed out from a volcano, forming a dome-shaped or bulbous mass above and around the vent. Older lavas may be lifted by the pressure of new lava rising from below. The structure generally develops inside a volcanic crater or on the flank of a large volcano, and is usually much fissured and brecciated. Cf: lava dome; plug dome. Syn: dome; cumulo dome.
Could you see fit to correct your website offering by substituting the correct term volcanic for lava for what is being described on your website?
I teach a course in Introductory Physical Geology, and sloppy terminology on websites is no help especially as I no longer can expect students these days to refer to many other sources for quick information.
Thanks,
Hugh Rance, Ph.D.
Professor of Geology
QCC of CUNY
March 26, 2003
Hugh,
Thank you for your comments. You are quite right that the common useage among USGS scientists is to refer to lava domes when referring to a mound-shaped extrusion of lava whether it is rhyolite, dacite or andesite and erupted onto the flank or central vent of an existing volcanic cone. In my 20 years working in the USGS Volcano Hazards Program, I've never heard or seen in scientific print my colleagues refer to the shield volcanoes of Hawaii as lava domes or volcanic domes. This would never make it past my editors and senior scientists of the USGS.
Tumulis that form on basalt lava flows are never referred to as domes in my experience.
The terminology you refer to is confusing as heck. When is a lava flow or many overlapping flows shaped like a dome or a mound or a hill to be called a volcanic dome or lava dome?
In Hawaii, the common terminology in use now for a conical hill or mound shaped structure built by pyroclasts and basalt flows is either a spatter cone, a cinder and spatter cone, or a shield (a shield built atop the shield of a larger volcano or vent) depending on the resulting shape and/or the primary constituents.
I suppose I could propose to include some discussion about the confusing application of the terms, but it wouldn't seem to clarify much to me. Any ideas?
Sincerely,
Steve
Steven Brantley
U.S. Geological Survey
Hawaiian Volcano Observatory
Crater Rim Road; PO Box 51
Hawaii National Park, HI 96718
tel 808-967-8827; fax 808-967-8890
email: srbrant@usgs.gov
USGS Volcano Hazards Program website
http://volcanoes.usgs.gov
March 29, 2003
Steven
That basaltic central vent volcanoes are referred to consistently by the USGS as "shield volcanoes" does free up the descriptive words "lava dome." One wonders when Mauna Loa became the archetypical type for a shield volcano. Did the descriptive words "lava dome" precede the discovery of the true shape of shield volcanoes? As far as I know, on earth all shield volcanoes are in deep oceanic areas. The true shape of Mauna Loa and like volcanoes could only have become known after deep sea soundings became routine.
The lava for the shield volcanoes and plateau basalts is produced by decompression melting of rising mantle plumes (hot spots). Typically the outpourings in intraplate oceanic areas are central vent and result in shield volcanoes and in continental areas or at divergent plate margins are fissure and result in plateau basalts.
The etymology of the descriptive words "lava dome" could be that this originally referred to small domes of basalt in continental areas. I speculate only.
But if so, presently the descriptive words "lava dome" are free to have a new application. Apparently the USGS regards "lava dome" and "volcanic dome" to be synonymous.
The quarrel to this could be:
Why use terms that could be misleading when "volcanic dome" has not wavered in its meaning or application?
Possibly to avoid contention, the Oxford Dictionary of Earth Sciences, 1999, includes no reference to "lava dome," but only to
VOLCANIC DOME, THOLOID - A mound of viscous lava, usually rhyolite in composition, which has grown and built up over a vent. The mound of solid lava is covered by coarse, angular blocks which form by chilling and brecciation of the growing dome's surface, The blocks accumulate around the growing dome to produce a scree slope of crumble breccia. Domes can grow by repeated injection of magma into the dome body (endogenous dome) or by repeated eruption of small volumes of magma from the surface of the dome (exogenous dome).
SHIELD VOLCANO (See HAWAIIAN ERUPTION)
HAWAIIAN ERUPTION - A type of volcanic eruption that produces very fluid, mobile, basaltic lava, characterized by spectacular fire fountains, with little explosive activity. The lava flows that result are often formed by the accumulation of fire-fountained lava and are termed 'spatter-fed flows'. Hawaiian-type eruptions produce a broad, low-angle cone (a shield volcano).
Hugh
March 31, 2003
Hi Hugh,
Thanks for the thoughtful reply. I've changed lava dome to volcanic dome on
our photo glossary list per reference to the Oxford Dictionary of Earth
Sciences. I appreciate your comments.
Sincerely,
Steve
Steven Brantley
U.S. Geological Survey
Hawaiian Volcano Observatory
Crater Rim Road; PO Box 51
Hawaii National Park, HI 96718
tel 808-967-8827; fax 808-967-8890
email: srbrant@usgs.gov
USGS Volcano Hazards Program website
http://volcanoes.usgs.gov
April 1, 2003
Steve,Your thoughtful position and willingness to edit website information in the
interest of clarity is a great example that I can share with my students.
You do know that the USGS website is viewed as the most authoritative
reference source for young up-and-coming geologists. Now by your kind
correspondence you have also put a human and welcoming face to the USGS.
Much thanks,
Hugh