Tuesday, October 23, 2007

Exercise in Dating (relatively) Hurricane's Cones and Craters


This View shows the eroded West Hurricane Crater. WHC, Rim, with Pine Valley Mountains in the distance

This Cone has been Quarried for red ash, which is excellent for Road Covering
View from W 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, and that at Sullivan’s Knoll, (SK)- all within 2 km of each other. Although they are not a line, WHC lies between the two others within the Huricane 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 take the hike to the summit of the rim:
1. This crater, although better preserved than the one to the east, is the lowest in elevation of all three;
2. The other cones have basalt exposed at their base, while this cone is predominantly red ash and scoria, or lapilli;
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 the 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- not 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.
Extrusions are eruptions which have occurred on the earth’s surface as flows, blasts, solid movements, or gaseous displays.
Intrusions are magma movements which never reached the earth’s surface, but which may have a similar composition to extrusions which did- e.g. granite, diorite, or gabbro (proceeding towards the less common outcrops, after erosion of the cover). Granite is equivalent to extrusive rhyolite, diorite to andesite, and gabbro to basalt in composition.
Lapilli are sand size particles, from 2-64 millimeters (about 1/13 inch to 2.5 inches), 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 high 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 flows, whereas explosive vulcanism denotes recycling of Crustal sediments or Metasediments.




Laverkin Hogback, formed by cooling of subsurface after vulcanism ceased is to NE




West of WHC lies a tongue of Basalt, originating from SK
2nd Hike around the rim of the SW 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 has a dike and possible sill protuding from the older WHC, and it is composed of rock from magma which has flowed rather than blasted- not the same as the iron-colored extruded blasts (it has a lighter grey color, and is layered, but containms some olivine). This rock is younger than WHC, since it has sliced through the red ash cone, and there are vertical and horiontal rocks exposed- protruding from the red ash. The dome has no crater but does have loose red ash, and it is higher than WHC in elevation; it appears to have flowed laterally and verically, without volatility.
This dome is closer in distance to SK, not quite aligned with it and SPC.

West of WHC lies a tongue of Basalt, originating from SK

It appears that the dome has risen after the basalt flows of SK and SPC (having a lighter color and composition than either of them), but of age later WHC. Its lack of excessive erosion indicates that the following eruptions occurred (oldest time first):
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. A dome of more siliceous (low density basalt formed, with dikes, west of WHC (this dike has somewhat bedded red ash sloping down toward it, so must have pierced the redbeds;
3. Blasts occurred first at SPC, then at WHC (this is a weak conclusion), and lastly at SK;
4. Erosion affected SK the most, followed by SPC, and finally WHC, because of the lesser erosional gradient (elevation) there.


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



West of WHC lies a tongue of Basalt, originating from SK

2nd Hike around the rim of the W Crater

A complete circuit was made around the rim of the SW 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 older than SWHC, and it is composed of rock from magma which has flowed rather than blasted- not basalt, but dacite or rhyolite (it has a light color, and is layered). This rock is older than SWHC, since it has been eroded to make way for 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 red ash, and it is higher than WHC in elevation; it appears to have flowed laterally, without volatility.
This dome is closer in distance to SK, not quite aligned with it and SPC.

West of WHC lies a tongue of Basalt, originating from SK


It appears that the dome has risen after the basalt flows of SK and SPC (having a more silicic composition than either of them), but prior to WHC. Its lack of excessive erosion indicates that the following of eruptions occurred (oldest time first):
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. A dome of Dacite formed, with dikes, west of WHC;
3. Blasts occurred first at SPC, then at SWHC, and lastly at SK;
4. Erosion affected SK the most, followed by SPC, and finally WHC, because of the lesser erosional gradient (elevation) there.


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