Monday, October 8, 2007
A grand view of The Hurricane Fault, Permian Kaibab Peak, Pk,and Sullivan's Knoll, SK, may be seen from Santa Fe Springs Fossil Dunes, Jn (Jurassic Navajo Sandstone), near Sand Hollow Reservoir:
Telephoto Image (above)of SK shown in wide field below:
View to NE of Sullivan's Knoll, from Santa Fe Springs (Notice the gradual basalt slope downward to the North):
Last Eruption (Sullivan’s Knoll) of Hurricane vulcanism:
Sullivan’s Knoll (SK) - an approximately 10 kybp (thousand years before present) crater- lies on the west side of Hurricane and was the last of at least three volcanoes to erupt within the present city limits. There are two other cones closer to the center of town which are older- on the order of 100 kybp- but they are much more eroded and show little of the original crater.
SK has most of its crater remaining, but the north side has been breached by erosion from rainfall. It is composed mainly of ash and lapilli- attesting to its more violent nature, as contrasted to the base, which is mainly basalt. The violence of the eruption is caused by large amounts of silica (volcanic glass), compared to relatively quiet flows at its base- which have large amounts of iron- and which flowed similarly to slag in a blast furnace.
That the basalt flows, which are older than the crater, came from south of the SK, can be seen in the accompanying pictures; the slope shows a downward trend towards the present Virgin River, from south of SK (where there are at least two older eroded basalt vents). These indicate that the basalt came first, and finally the explosive SK.
Considering that a melt of magma has risen from the bottom of the Earth’s Crust, or from the Upper Mantle just below the Crust, it is expected that the composition would contain more iron- which contributes to the heavy basaltic rock, and which is characteristic of Mantle material. Most of the Colorado Plateau eruptions are basaltic, which yield the characteristic dark color and higher density of these non-dangerous flows. These are similar to that on the big island of Hawaii, where Life has sufficient time to avoid the 2000 degree lavas. The explosive blasts, such as would come from SK, would be more sudden and dangerous. Further, they would be accompanied by noxious gases- which can kill. The older basalts have been measured for radioactive decay time since flow, and found to be somewhat less than 200 kybp in age. They would have flowed toward the low elevation of the area- that is the Virgin River of that ancient time. The photos show that the top of the basalts decreases in elevation to the north.
What about the explosive eruptions, which formed Sullivan’s Knoll? Shouldn’t they have arrived earlier, since they have more silica (which comes from shallower rock melting- such as sandstone)? As magma moves upward from the deep crust, it will incorporate the surrounding sedimentary rock into its melt, and form a volatile upper melt, and therefore blast out the melted glass, or silicates, first. The evidence indicates otherwise, that the basalt moved rapidly through large fractures and flowed out first- not having had time to incorporate the wall rock significantly. The magma, having lost some of its impetus, then slowly incorporates the surrounding sandstones and builds a more explosive mix- which comes last. A similar feature has been found on the Hawaiian Islands, where the last feature to occur on a volcanically-formed island is an explosive crater on the SE side of the otherwise dying magma-formed island. Later, or frequently earlier than the explosive cone, the basalt flows occur further to the SE on a new island.
What causes violent volcanic eruptions (compared to quiet slow flows such as with Basalt)?
The explosive eruptions are caused by the contribution of the shallow crust to the magma via sandstones and shales. But sandstones do not occur on Pacific Islands until the older basalts have been attacked by weathering processes to yield higher silica fraction sediments (quartz or silica- as in sand- remains practically insoluble, while feldspars and irons are dissolved in seawater). These sediments can then be incorporated into shallow magmas to form explosive eruptions such as Diamond Head or Koko Head, to the SE of the earlier basaltic eruptions (In addition, there is the complication of precipitation moving downward on a mountainous island- to cause steam explosions).
How to get There (SK is in S5 T42S R13W):
State road 9, which is the Hurricane Hiway from I-15 and St. George, allows a view of SK, from the road. To get to the location of the photos, take Sand Hollow Reservoir road south from highway 9 (the Santa Fe housing development road) turning at Santa Fe Springs Road to the east. There is an outcrop of Navajo Sandstone protruding on the otherwise rolling ground, which you can climb to view the Sand Hollow Park to the south or the Pine Valley Mountains to the north.
Navajo ss. or Jn (Jurassic Navajo sandstone) is a fossil sand dune deposit of about 150 mybp age. It was formed by the wind blowing from the north, as can be seen by studying the cross-bedding (the blowing sand falling away from the wind over the crest of the dune is preserved, and this is the downwind direction).
Influence of Latitude on prevailing wind direction, fgr a round globe:
How is it possible that the prevailing wind in those ancient years came from the north? Nowadays, the prevailing wind blows from the southwest. The answer lies in the formation of dominant wind patterns on the globe, according to the 30 degree latitude spread in which the rocks are formed. The first 30 degrees north of the Equator has winds blowing from the north at the surface of the earth, the next 30-60 degrees has wind blowing from west (varying over the year from NW to SW, which is the wind noticed now). These fossil dunes tell us with this single piece of information that Dixie and its sand dunes were in latitudes close to the Equator at that time, and that the land containing these rocks has since moved to the next higher latitude band- which geologists now realize occurs due to the crust moving northward, as it still is doing with the Pacific Crustal plate heading toward Alaska!
When you take an outing to the Sand Hollow Park and reservoir, make a left turn and climb the Jn fossil dunes to first view the beautiful lake, the fairly young volcanic cone, and finally the ancient dunes where dinosaurs once trod.