Friday, February 1, 2008

Hikes onto Volcanic Craters in Hurricane, UT

Google Map shows the area of interest in the Town of Hurricane, where 3 dormant volcanoes exist.
Red Ash from West Crater was used for Road Metal (Against icing and for shoulders)

Google Space Photo shows the cones within the city limits of Hurricane (notice the Red color from ash broadcasted locally- not a violent blast), UT

Approaching West Hurricane crater from the East, near 600N gravel Road

After a Snowstorm, the View to the South of Radio Tower Crater is Subdued.View from West Hurricane Crater (S27 T41S R13W)
Included are several photos, which were taken from the rim of the W Hurricane Crater. This cone and crater are located about midway between the crater at the Stout Park, SPC, (named Radio Tower Crater by UGS) and that at Sullivan’s Knoll, (SK, above or near Ivins Knoll) - all within 2 km of each other. Although they are not aligned, WHC lies between the two others within the city limits. Both of these other craters are shown in the photos, and you should try to answer the following question as you view them:
Q. Considering the three craters as a set of extrusions from a similar source, what is the relative age of WHC (oldest, youngest or in-between)?
You will see the following clues, as you hike to the summit of the rim:
1. This crater, although better preserved than the one to the east, is the lowest in elevation;
2. The other cones have basalt exposed at their base, while this cone is predominantly red ash and scoria, or lapilli;

West of WHC lies a tongue of Basalt, originating from SK
3. This cone has an erosional path just off its east side, which is lower than the basalt levees exiting from the SPC;
4. This crater rim retains a rough shape of a crater, with erosional breaks, but then it is lower in elevation and has been subject to a smaller gradient for precipitation-causing erosion; and,
5. WHC contains the best deposit of red ash for Road Metal (makes excellent shoulders and coating for icy roads).
Facts and Terms:
The initial basaltic eruptions occurred about 100-200 kilo-years before the present time (kybp) for all three of these craters, although the cones and explosive craters are much younger and are more siliceous-basalt. Surface evidence can only be applied to the final emission for each case, since the rock at the base has been eroded and mostly covered. Basalt came first, evidently from large fractures which had insufficient time to incorporate wall rock containing larger amounts of silica (sand and shale) because of the faster velocity of the magma.
Crater is the interior depression of a cone, much smaller than a caldera (arbitrarily, a crater is less than one kilometer to mile diameter, as contrasted to a caldera, which is larger than a mile).
Extrusions are eruptions which have occurred on the earth’s surface as flows, blasts, solid movements, or gaseous displays.
Intrusions are magma residuals which never reached the earth’s surface, but which may have a similar composition to extrusions which did- e.g. granite (which is a crystallized rock, compared to rhyolite whose crystals are too fine to be seen with the eye), diorite, or gabbro, found at the earth’s surface, after erosion of the cover rock (going from left to right yields the less common outcrops). Granite is equivalent to extrusive rhyolite, diorite to andesite, and intrusive gabbro to basalt in composition.
Lapilli are sand size particles, from 2-64 millimeters (about 1/13 inch to 2.5 inches pebbles), up to cobble size.
Scoria is deposition from the original blast, as from a solid and gaseous suspension in the sky (cinders, clinker, and ash may be other names).
Types of Vulcanism:
a. Cone
: a small protrusion from the main volcano or magma source, which is cone-shaped before erosion, e.g. Sullivan’s Knoll or Veyo;
b. Shield, or volcano with a slight slope (resembling a warrior’s shield), such as Mauna Loy in Hawaii- this is predominately composed of basalt; and,
c. Strata or composite: A large cone with large slope and indications of multi- eruptions- alternately flows, blasts, and puddling of lavas (doming), e.g. Fuji.
d. Vulcanism around the Pacific Rim is mainly Andesitic (from the Andes Mountains name), and is explosive- high silica content- while volcanoes from the interior of continents and isolated islands are mainly basaltic and slow-flowing (high iron content from the Mantle). The reason for the explosive vulcanism around the Rim is due to the higher silica content as the various plates dive into the Crust and absorb continental sedimentary rocks (with their large amounts of sandstones and shales). This feature along with the larger fraction of water in the crust yields explosive magma before the subduction reaches the Mantle and its mainly basaltic composition.
e. When basalt is encountered, it indicates mantle or deep Crustal flow, whereas explosive vulcanism denotes recycling of Crustal sediments or Metasediments.

2nd Hike around the rim of the WH Crater

A complete circuit was made around the rim of the West Crater, WHC, to determine the age relative to the other cones occurring within the Hurricane city limits. The rim is eroded in only one location- the southern edge, where erosion has opened a drainage way. Next to this drainage there is a 4th dome, which is obviously younger than WHC (no red ash cobbles overly it), and it is composed of rock which has flowed rather than blasted- (it has a light color, and is layered). This rock is younger than WHC, since it has been eroded less than the red ash cone, and there is a dike of similar material which is on the edge- protruding from the red ash. The dome has no crater (or if so highly eroded), and it is higher than SWHC in elevation; it appears to have flowed laterally and vertically, without volatility.
This dome is closer in distance to SK, relatively aligned with it and SPC, in an arc.
It appears that the dome has risen after the basalt flows of SK and SPC (having a more silicic basalt than either of them). Its lack of excessive erosion indicates that the following eruptions occurred (older time first- see the Google map of the Hurricane- Virgin area, above):
1. Basalt flowed near WHC location, and then proceeded to flow further west, finally damming the Virgin River near hiway 9 bridge (at least 3 flows).

2. Blasts occurred first at SPC?, then at WHC (these first two eruptions may be reversed in time), and lastly at SK;
3. A dome of more siliceous basalt rose, before SK cone was formed, with dikes, west of WHC; and

4. Erosion affected SK the most, followed by SPC, and finally WHC, because of the lesser erosional gradient (elevation) there.

5. There is no red ash covering the Stout Park outcrops, hence West Crater may be the oldest of the 3 volcanoes. (note: UGS has measured the ages of all outcropping volcanic rock in WA County, and finds that Ivins Knoll beneath or nearby Sullivan Knoll is 1 mybp age (volcano Mt. adjacent is 258 kybp), Cinder Pits or West HV is 240 ky, and Radio Tower or Stout Park volcano is 140 ky. This indicates that the vulcanism proceeded northeasterly from Ivin's Knoll and then reversed to form Sullilvan's Knoll last- all within 200kybp.

PVM to the NW shows how this 21 mybp Intrusion has "Jacked up" the whole Region and now has been UNEARTHED by erosion (Previously, before vulcanism, there was at least 10,000 feet of sedimentary Rock above Sea Level).

West of WHC lies a tongue of Basalt, originating from SK (notice the trace of a Fault trending N-S, which allowed basalt to enter a fissure, and which is traced through Jn

A Fourth Dome occurs on the rim of WHC- probably younger than it

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