Stress analysis of the regime surrounding Pine Valley Mts.PVM

PVM igneous Intrusion (Monzonite) is shown in Red, as contrasted to surrounding sedimentary Rocks


Google Earth photo shows the SE edge of PVM and its draining Wet Sandy Creek. Quail Lake is at the SW corner of the map, and Anderson Junction is near the Center (Orient onto the Occidental I-15)

There are flat and level beds almost alongside the Monzonite melt, indicating that they have not been distorted by sliding of large masses of monzonite Pine Valley Mountain Foothills, PVM, sedimentary Carmel outcrop, or by the intrusion of the hot mass

Pine Valley Mountain Foothills, PVM, sedimentary Claron outcrop

The PVM rose as a melt in Miocene times (21 mybp, million years before present, laccolith-shaped, and oriented NE-SW), and developed a mushroom-shaped overhang at about the kilometer depth in its late stages. This overhang is due to the mechanics of stress changes, as the melt rose. At approximately 1 km, the resisting stress of overburden is less than that of splitting the rock vertically, and the melt works laterally, lifting the over-lying rock (this same result has been noticed for rising Salt Domes in the Gulf Coast, USA). This is noticed in the photo shown for the Oak Grove Campground area, where the flat and level Tertiary rock is un-deformed by the rising melt, while downhill there are remnants of melt which have fallen past the weak sedimentary beds. This means that the dropped Monzonite mass has not slid over the sediments; rather it was above and to the SE of them in days when there was a lot more Mesozoic and Tertiary rock (before erosion). As the mountain and its surrounding sedimentary rock eroded in the 21 million years just past, the detached Monzonite sunk further in elevation, now displaying portions SE of the older and once deeper sedimentary rock.

Tertiary Claron (pink to white youngest limey siltstone) within 200 meters of Tm Intrusion is almost flat and level- indicating that it has not incurred monzonite sliding past it .
Study the accompanying Google Earth geographic map, to notice several things:
1. Wet Sandy Creek traces SE, to point toward Toquerville and its large Ash Creek spring;

Though SE would be the normal eroding direction for streams falling off the NE-SW oriented Pine Valley Mts, the wash east of Oak Grove trends toward the south


2. While most creeks and washes descending from PVM are somewhat irregular, Wet Sandy is fairly consistently tracing NW-SE, making a linear trend with the headwaters of Santa Clara River on the NW side of PVM;
3. Large isolated portions of Monzonite outcrop near Anderson Junction, which are seemingly anomalous; and
4. The NW-SE fracture trend is not obvious, once the Hurricane fault is intersected.
Although the trend is strongly indicated, we have no proof that this is the same NW-SE trend noticed in the Fissures near the Virgin River, or of that noticed at the Laverkin Quarry.

Re-tracing the path to the Monzonite (PVM) outcrop east of the Oak Grove campground:
Hikers in November 07 returned to the arroyo just east of the campground, to view the contact of Tertiary Claron, Tc, with the 21 mybp laccolith- seen as pink sedimentary beds within a few hundred meters of the Monzonite, Tm, sheer cliffs.
Although rubble obscures the actual contact, the continuity of Tc can be seen over one hundred meters NE-SW, to determine that Tc has not been shoved or rotated significantly (the dip of Tc into the igneous uplift is not over 5 degrees, and the formation on both sides of the arroyo is concordant). This is shown in the accompanying photos.
Also shown is a photo with an outcrop of Tm which indicates that the arroyo yielding the bare Tc has been caused by a large fracture (insufficient displacement to classify as faulting) tracing N-S into and through Tm. This was surprising, since most of the fractures in the surrounding sedimentary rock show NW-SE orientation. The N-S fractures, and faulting, are considered to be less than 2 million years of age (similar to the latest Hurricane fault expression), as contrasted to the NW-SE older set in this area. Hence the stresses causing Hf are present in this area, some 10 km distant from the fault.

Notice the Vertical Fracture which extends upward as far as can be seen- this has allowed increased erosion (forming an arroyo or wash in a N-S orientation)

Those taking this hike should remain in the arroyo until getting close enough to touch Tc, because of the thickness of manzanita and scrub oak- which is essentially impenetrable. This is a boulder-hopping hike, which requires climbing and jumping over the outhouse-sized boulders.
Re-stating the conclusions noted from a previous hike:
1. Tc was relatively unaffected by the younger intrusion of Tm, remaining fairly flat and level. This eliminates the conclusion that Tm fragments slid over Tc, moving SE downhill with time, in this area. There are segments of Tm further to the SE near Anderson Junction of I-15 which evidently did slide over the soft sediments, using them as a lubricant and carrying some Tc with the younger Tm.
2. There is possibility that Tc slid somewhat downhill, moving along the curved face (starting almost vertically, but gradually moving horizontally with time), which results in Tc dipping-down to the cliff face as the whole sedimentary column dropped (rotating due to the smaller cliff slope angle). This yields a small dip into the monzonite- less than 5 degrees for this location. Other locations to the east have a more marked dip, strongly indicating that Tc and older beds slid- taking their overlying Tm with them.

Notice the Large Dip Angle of Mesozoic beds tilting into the PVM Monzonite
Mesozoic and later Tertiary Rocks border Tm, and the elevations of each must be measured to determine which have fallen from original high positions after being lifted and penetrated by Tm
3. Underneath the pink Tc lake limestones and siltstones, there are thick and hard conglomerates, which were searched for imbrication. These showed no flow direction indication, and appear to be storm-generated- leaving pebbles and cobbles dumped into sand or silt.

Southeast of the Tc outcrop, there are blocks of Monzonite which have obviously slid down to their present position.


Linear Slickensides were found SE of the Tc outcrop in Mesozoic rock, indicating that some sliding occurred downhill of Tc


Oncoliths were found in the Mesozoic downhill of the Claron Tc

Fracture Influence on the Virgin Diversion Pipeliine

Ben Everitt scrambles to un-scramble the slickensides along a major splay of the Hurricane fault


The Canal Trail starts on the South Rim of Virgin River, accessed via Sheep Bridge gravel road south of Virgin Town



November Hiking into Virgin Gorge via Canal Trail

Morning Shadows Create Mystery in Virgin Gorge

Google Earth Map shows Fracture pattern influence on Virgin River


Fissures and Fractures near Virgin River and Town
Access to this Trail may be made by taking the Highway toward Kanab, from Hurricane, and turnng north after reaching the top of the switchbnacks (about 1-2 miles on top of the plateau). This is an improved road going to the town Of Virgin. After 2 miles on this gravel road there will be a trail marker for the Canal Trail. Turn left and proceed on the most used dirt road, until you reach a deadend at the Virgin cliffs.
Alternately, take the Virgin hiway 9 towards Zion NP and turn south across from Mesa Road onto Sheep Bridge road (mostly dirt) for 3 miles (past the flagstone quarry). Look for the Canal Trail marker where you turn right. You can see the Virgin gtorge in the distance, and you will have to feel your way to reach it by seeing an imposing outhouse on the north skylinie.

An interesting (unplanned) experiment occurred near the Hurricane Pipeline in the late 20th century, west of the town of Virgin. This involved the Diversion dam built there to divert water for a pipeline, the Pah Tempe hot springs, fissures in the earth, and The Virgin River flow.
Previously, the 19th century-initiated Hurricane Canal had carried water through an open flume on the south bank to the agricultural area of Hurricane Valley. Because of high maintenance necessary with this canal, a pipe was laid in the canyon of the Virgin valley toward the west, superseding the old canal. There was a large amount of spalling and rock fall annually, so that it was generally known that the cliffs would continue to deteriorate. A covered pipeline would minimize this problem, and the pipe would take water from approximately the same location as the canal- some 5 km west of the town of Virgin and deliver it to farmlands in the valley to the west. This metal pipe would not only eliminate the annual maintenance, but also deliver water more efficiently along the smooth interior of the pipe.
Here is the Third Party-communicated Sequence of Events
A holding reservoir upstream of the pipe was created via a dam across the Virgin, so that water would have time to drop some of its sediment load, and so that there would be a fairly stable flow into the pipeline. The process created an unstable subsurface with the operation of large machinery and with the diversion of water into the pipeline, the Virgin River downstream of the new dam diverted its water into the earth through a sinkhole in the river bed.
Within the first few months, an increased flow of water through Pah Tempe hot springs near the Hurricane fault, Hf, occurred, consisting at first of a slug of hot water. Then the springs delivered colder water (than the original 108 degrees Fahrenheit) making the springs unattractive for public baths. The loss of water near the dam occurred in one area assumed to be a limestone sinkhole, and this was filled with rock debris and isolated with a levee. This eliminated most of the problem of diversion of water into the earth, and allowed an analysis of the plumbing system of the near subsurface (underground water flow).

My analysis of all this (not having been present during the incident) is as follows:
1. Large Fractures were present (seen now in the cliffs and ground surface to the north of the river as fissures and fractures) in the earth below the river. These previously had been partly sealed off by siltation from the muddy river and cementation from precipitation of calcite cement;

Fractures are seen in vertical views as well as horizontally on the surface North of the Virgin (Pine Valley Mountains in the background).

2. Placement of the new dam not only decreased the sediment load in the river water (fluid viscosity decreased), but aggravated the ground surface by action of heavy machinery- both of which increased the movement of water into the earth;
3. The river water moved through solution channels and fractures in the earth downhill towards the Virgin River opening in the Hurricane Cliffs;


Review this Google Map to Notice the difference between Meanders and Fracture-induced river turns (Notice the Sharp Bends, compared to rounded Meanders, seen downstream of Hf- which you may study from a previous Blog entitled "Meanders")


Descent into the Gorge has been simplified by Trail Builders


Some Fractures cross the Virgin River, showing that they are not due to gravity spalling
4. There was a reservoir of hot water available in the caves and solution channels, which was flushed out in advance of the slug of cold river water. This yielded a temporary surge of hot water through Pah Tempe springs;
5. When the cold surge behind the hot water became faster than the upward movement of hot water, the spring temperature decreased; and
6. Finally, after isolating the sinkhole, the hot springs gradually re-heated the rock and the temperature rose to approximately the original equilibrated condition.

Conclusions pertinent to the Nature of the Subsurface near the Virgin River:
A. The Virgin River has a deep canyon (approximately 200+ meters) at this location and this has created spalling cliff walls, falling parallel to the river. But this is not the reason for the large fissures orienting at angles other than those parallel to the river channel in the plateau above; they are due to the similar circumstances for the fissures and sinkholes below the river- they pre-existed the river canyon. The path of the river seems to be determined by the two sets of fractures, seen by the perpendicular sharp bends in the river;
B. These fractures orient in two principal directions- NW-SE and N-S, as measured in the plateau above and in the river paths between bends;
C. That the river has found these (new? < 1 mybp) fractures is demonstrated by the occurrence of river gravels on the plateau above (on the south side, which orient to the SW) before the ancestral Virgin was captured by the present stream flow; and
D. Fractures seen now have been aggravated by spalling of the cliffs, regardless of orientation, but they were not necessarily initiated by the canyon formation.

Enclosed below is a report following a previous hike into the Timpoweap Canyon, describing the replacement of the old canal with a steel pipeline:

Page 1
April 2007 Field Trip Report, By Ben Everitt, DGS President
The April 28 hiking field trip in Timpoweap Canyon was a great success, even if it was a little quiet out there by myself. The route is well constructed and easily followed, except for one short section that seems to exist only in the mind of the cartographer. Recent geologic mapping by Bob Biek (Hurricane Quadrangle) and Janice Hayden (Virgin Quadrangle) provide good reference to stratigraphy and surface geology. Signage explains the history of the diversions and canals built to lead water out of the canyon to the Hurricane and LaVerkin benches.
The canals are an interesting example of pioneer engineering and sheer determination in the face of difficult topography and geology. The builders made use of breccia zones and open fractures, and the easy tunneling in the gypsiferous units. Irrigators paid dearly for these shortcuts over the next century in lost water and maintenance nightmares. The canals were abandoned in 1985 and replaced with a steel pipeline.
1) Hurricane canal routed through an open fracture in the Fossil Mountain Member
This tour continues the theme that “there is no such thing as bad geology, just different kinds of interesting geology”. The canyon exposes the Timpoweap limestone and Rock Canyon members of the Moenkopi Formation, and the gypsiferous Harrisburg and cliff-forming Fossil Mountain members of the Kaibab Limestone, and the upper gypsiferous Woods Ranch member of the Toroweep Formation. There are excellent exposures of local deformation and breccias associated with paleokarst (ancient collapse due to dissolution of gypsum), and much evidence of continued collapse in recent times. Both the Harrisburg and Woods Ranch members of the Kaibab
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formation are tilted and toppling toward the river from both sides, indicating that the thick gypsums in the underlying Toroweep continue to dissolve and undermine the canyon.
2) Just below the new diversion dam, open fractures extend from the top of the Harrisburg Member into the Fossil Mountain Member, but are not yet evident in the pipeline pad.
Above the canyon rim, remnant gravels resting on the lower red member of the Moenkopi mark the course of the Virgin River before there was a Timpoweap Canyon, and therefore before the Hurricane Cliffs were there. Bob Biek (2003, Geologic Map of the Hurricane Quadrangle) estimates the age of this gravel at middle Pleistocene. Since they contain basalt cobbles, they are probably younger than about 1.5 million years. Therefore the entire canyon and its interesting features are geologically quite young.
Test Drilling by the US Bureau of Reclamation in the 1950’s for the proposed Virgin Dam, named for the town of Virgin which it would have flooded, found rock so fractured, cavernous and permeable, that they walked away from it and never looked back. The sink-hole that swallowed the river for 3 months in the spring of 1985 and recharged Pah-Tempe hot spring is described in Everitt and Einert, 1994 (Utah Geological Association Publication 23, p. 189 – 194). These interesting geologic units underlie much of Washington County, and are sure to present challenges as future development moves out into the hinterland.

Volcanoes Near Hurricane Fault exit westward with time

Sullivan's Knoll, SK, west of Hf has basalt base age on order of 200 kybp, cone at 10 kybp

Grass Valley Flow (on Hf, Hurricane fault, south of town 15 km) is slightly less than 1 mybp in age

Cones and Domes are Generally older than 3/10 mybp (.3) East of Hurricane Fault; shown is Crater Hill, near Virgin Town

Slickensides in the Hurricane fault zone (scarp)

These Slickensides are more extensive on hiway 9 cliffs, east of the shoulder, on the north part of hte Graben
This is the most massive Slickensides found- you should study it to use as an
Example for Future Finds; notice the Linear Striations and Grooves of hardened surface (probably silicates) precipitated from warm water flowing along the previous Subsurface Fault Face (original composition was Limestone, CaCO3)

The South view of Graben shows where it started (Water tank sits on Graben Downthrown Block)

Westward, the Upthrown Block shows a 15 Degree departure from Vertical (West Edge of Graben wall moved northward, as well as Vertically)

Eastward, the Upthrown Block Has Almost-Vertical Striations, indicating that the Plunge to north has little influence on the orientation
Graben Slickensides show Direction of Crustal Movement

Shown above and below are photos of “Skid Marks” or slickensides in the walls and Scarps above the town of Hurricane, in the Hurricane Fault, Hf to the east. These are the result of the rasping of one block of the Crust, as it moved relative to another block which is more stationary. These are primarily the work of normal faulting, which is unknown as to origin, but they give details of the movement in one local area. They may yield understanding of the larger underlying event which has Hf as a final result. The movement which produced the slickensides is very deliberate and slow, so that the final product is slick and contains regular lines and a glassy portrait of the direction and type of movement.
First, notice that the surface of the slickensides is so well-polished that it shines in the sunlight. The two blocks which slid relative to each other are of similar composition, and therefore of similar hardness. This causes the honed surface to appear extremely fine-grained.
Secondly, the surface is discolored in the millimeter or so thickness which is preserved. This is caused by the heat which is generated by the friction of rubbing, which changes the solubility of compounds in the water which has found the fractured and faulted space. The drop-out of iron compounds and other solutes in the water makes the surface harder than it was originally- initially calcite or limestone, which is soft (3 on Moh’s hardness scale).
Thirdly, small grains of hard material entrained in the moving block remain un-dissolved long enough to leave a line or trough, which indicates the direction of the movement. For a graben, this movement should be vertical, since this is a normal fault, happening under extension. However, the deviation from vertical is striking and regular for the west wall of the graben, which is the upthrown block (the downthrown block cannot be seen, because it has fallen into the earth).

Preliminary Conclusions:
1. The west wall of the graben has slickensides with striations dipping down into the earth- Down-to-the-South. Since the graben plunges down to the north, part of this 15 degree angle is due to rotation of the graben as it plunges down to the north (look at the larger photo of the whole graben, where the incipience or start of the graben is shown to be less than a kilometer to the south).

A Closeup of east wall of the Graben shows Striations resulting from "Drag" of Downthrown Block hard pebble inclusions- which leave parallel grooves.


2. The east wall of the graben (again the upthrown block) has a few degrees dip down to the north. This is not significant, and may be approximated as vertical, but the importance is that it shows that the plunge of the graben to the north is not a significant contribution to the deviation from vertical.

The Detailed Closeup of Striations (East Wall) show the angle of deviation as less than 5 Degrees, indicating that Plunge of Graben is not the Primary Factor causing the movement away from Verticality in the West Graben Wall
3. Since the graben west wall is the one showing deviation from vertical of the drop of the downthrown block, this indicates that we are looking a phenomenon which is occurring closer to Hf proper. Although the entire graben is part of Hf, and there is transfer of stress to form the graben, the entire result can be interpreted as part of the total movement of Hf. In this case, generalizing, the rim of the graben is moving north with the time of fall of the graben relative to the downthrown block. This would be a right-lateral fault, with movement laterally caused by something other than gravity (a weight of rock simply falling- due to gravitational attraction- as the earth is pulled apart). The lateral movement would be due to a compressive force, such as rotation of the Colorado Plateau, CP.
4. This right-lateral faulting is representative only of the graben where it is found but could be an indication of further investigation to determine whether it is representative of CP as a whole (at least on the western side, near the Pine Valley Mountains and local volcanism which bear on it- all Post-Laramide features).

The Massiveness of the Slicks shows that they were the Result of a continuous Sliding, and the Regularity indicates a slow movement which was heated by the Friction- allowing Water to Rearrange its Chemical Contents over significant time (NOT lurching)



A Rare image shows that there was occasionally a Disruption in the Graben Sliding- an interference

Hiking in Arroyos East of Laverkin, UT quarry

Large Slumping Angles (faulting Dip- down to West)occur East of Quarry



Laverkin Quarry and slumping near Hf (S11, 13 T41S R13W)

A Google space photo is shown, to indicate the geographic and topographic changes near Hiway 9- ascending the cliffs, toward the town of Virgin, Utah. Orient your view by looking at the switchbacks to the east of the town and to the Black ridge to the north. Hf heads north between this mesa and the extension of it NE of Toquerville (the Pleistocene basalts have been sliced by Hf- which has several splays near Laverkin and Toquerville). Use the Freeway I-15 as a guide for focusing your gaze onto something familiar. Ash Creek is the N-S waterway to the west, and Laverkin Creek is east of that- flowing through the valley to the east of the Black Ridge. Hf has bifurcated, creating the weaknesses which allow the two creeks to parallel each other.
It is difficult to ascertain the quarry and faulting-slumping on this map, but it is just to the north of the obvious switchback on hiway 9. The quarry lies just east of the crossing of hiway 17 bridge over Laverkin Creek- being a light area, which is the result of digging and slumping. No hint is given of obvious fracturing, but Hf is easily followed by looking for the change of elevation along the scarp.
The N-S orientation of the fault can be readily seen, and it will be emphasized in photos, which show the fractures and their orthogonals (fractures which are at right angles to the dominant fractures seen in the outcropping rocks) along the path. The main interest is to answer the following questions:
1. Why does the fault create so many splays in the area of the quarry (helping to create the deposit, which has allowed more slumping as it is removed; the ancient river found these weaknesses and eroded more easily there, dumping its load of gravel in the ancient meanders or river bows)?
2. Is there movement laterally along the fault? Slickensides, near a graben on the rim above Laverkin, indicate that there is right lateral movement going toward Toquerville (a counterclockwise movement of the Colorado Plateau, CP).
3. Is there a continuity to the opening of a major fracture underlying Wet Sandy Creek NW of Toquerville- producing the large spring in Ash Creek there- proceeding along Hf to the south, and then to the quarry location and on up the Hf scarp towards the Virgin fissures to the SE? This would make a¯\ _ trace on a map, where the hyphens orient NW-SE and the diagonal underlies Hf.
4. In the field, we will look at the bedding angles displayed between the various faults or slumps, to see if there is rotation of the blocks as they slumped to the west and then rotated with a reverse movement later? This would happen if Hf has interfered, through time, with the slumping or faulting process.
5. Upon zooming in on the north side of the Virgin River (Search Google Earth), a faint NW-SE trace can be seen heading toward Laverkin quarry?, but the pixel density does not allow an inspection of this anomaly. This must be followed in the field.
6. There are various orientations of the major fractures shown on the map, both from the drastic turns of the river and with the mesas and scarps shown. Are these orientations consistent with a major fracture system?

Comments made after Re-Hiking the Laverkin quarry Arroyo:
a. A possible answer to 1, is that there is a transfer of movement (and of the stresses causing it) westward at Hf in this location. This has probably caused not only the multiple faulting splays, but has weakened the fractured zone to allow the ancient Laverkin Creek to enter the fractures.
b. There is movement laterally along Hf, but it has several possible reasons: CP may be rotating, the graben which abuts the slickensides has rotated downward (starting south just uphill from the slickensides), and there is slumping laterally- not only west from the slickensides, but also north. The entire lateral movement may just be a local feature, not part of the CP proper.
c. For 3, undoubtedly the large flowing spring at Toquerville indicates continuity for a fluid path, and the water flow is likely from the Pine Valley Mountains, but there are other large elevation gradients present, e.g. from the Zion NP region.
d. For 4, there is definitely a change of the bedding slope as seen in the Quarry Arroyo. The type of deposit is the only evidence showing reverse dip angle- large dip indicates there should be boulders in the deposit (there aren’t any in the lower strata), whereas slight dip angle in the overlying deposit indicate that there shouldn’t be boulders (they are present in the walls above the larger dip sediments).

e. The zoom-in of the area just north of the Virgin does show a color change on a line running NW-SE. This disappears before reaching the Hiway 9 switchback, but then this is the most distorted surface area (close to Hf, as it creates an up-to-the west bending of the crust);


and,
f. The answer to 6 is not yet found.


Google Earth Map shows the gross landforms without regard to Geological Terms

Laverkin Quarry and slumping near Hf (S11, 13 T41S R13W)

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

The "Big Picture", as seen in Zion Park (Model)

This Photo was taken askew deliberately, to allow you to look obliquely


Zion NP Model Frame orients N-S, with Virgin tracing the fractures of 2+ mybp


Notice that there are two sets of strains (Fractures), not quite orthogonal to each other in this National Park Model of the Colorado Plateau- Zion NP, but Coal Pits Creek to the west of this view shows that in the last million years a new orientation has superseded these.