Driving through Highway 18 in southwestern Utah, the visitor sees forested terrain with few rock outcrops and may be tempted to think that Hutton’s cycle of uplift and erosion is just as applicable here as anywhere else.
However, detailed mapping of the area has revealed a remarkable story involving the rapid intrusion of laccoliths, with associated arching of overlying rocks, massive gravity sliding, and synchronous volcanic eruptions” (Willis, 2002). The dramatic nature of these past events, and the speed of formation of geological structures, should make us stop and think about our basic approach to geological interpretation.
The events took place in the Tertiary. Thick layers of Jurassic and Cretaceous rocks had been laid down, and covered by Eocene strata. Then came the igneous intrusion: the magma was a viscous combination of quartz monzonite and granodiorite. It moved up from deep within the Earth’s crust, but instead of breaking out at the surface, it found layers of weakness in the surface strata and moved laterally. It ponded and formed a dome-shaped structure called a laccolith. This uplifted the surface rocks and this is where the story really gets interesting.
A chain-reaction series of events followed the intrusions. “First, the doming magma fractured and oversteepened the overlying layered rocks. The elevated weakened layers then broke loose and slid down the flanks of the domes as massive gravity slides. The sudden removal of the slide masses produced an immediate large reduction in confining pressure. This then caused still-intruding magma to burst through the seal and erupt, first ejecting vapor-rich (high-volatile) tuffs, then vapor-poor lava flows, which draped across the new gravity slide masses.” (Willis, 2002) The whole cycle repeated itself several times, with new laccoliths being formed and then bursting their sediment seals.
Timescales for these events were comparatively short. “Ages and relationships of the outcrops throughout the area strongly indicate that each laccolith intruded and domed within just a few years, or less. The gravity slides probably occurred even faster. Most likely, they were catastrophic events that, once started, happened within minutes, similar to the massive landslide on Mount St. Helens in May 1980 that triggered the disastrous eruption that took 60 lives. It would have been an incredible sight to watch huge slabs of rock several hundred feet thick rush down a mountain slope and run out across the landscape several miles!” (Willis, 2002).
Some of these slides were huge: the largest is more than 60 square miles in area, 1800 feet thick and it traveled over 12 miles. When they moved, the laccolith was unroofed and pressure release led to the eruption of volcanic ash and some lava flows. The remains of all this activity has been identified in careful mapping work.
Some general thoughts follow.
1. Geologic processes in the past do not necessarily fit our preconceived ideas about rates of process or even types of process. Be prepared for some surprises when you investigate the Earth’s history. Some processes were very short – measured in hours, days or years.
2. Igneous processes tend to be stretched out over thousands or even millions of years, but this expectation needs to be revised. Whether we are considering magma segregation, magma transport to the surface, emplacement or cooling: all these processes can take place much faster than previous generations of geologists surmised.
3. In most cases, geological research has explored timescales in only a limited way – because “everybody knows” that the timescales are in millennia or longer. More rigorous work, that looks for clues about timescales, invariably finds that past estimates have been too long. A case in point was posted by Glenn Morton at
http://www.theologyweb.com/forum/showthread.php?t=20334. The Great Stone Dome is known from seismics and boreholes, but is not accessible to field geologists. In such a case, a variety of scenarios could be proposed to explain the data – but testing these scenarios would be quite difficult. It is more useful to argue from geological structures where we do have the opportunities to test hypotheses more effectively.
4. A Flood Geology Understanding of the Pine Valley Mountains. Those who have followed previous threads to which I have contributed will know of my interest in developing and testing the Recolonisation model of earth history. This involves a global catastrophe (linked to the biblical history of Noah’s Flood) followed by an extended period of more local catastrophism as the Earth recovered from the destruction of the pre-Flood world. These particular events would then be part of the post-Flood catastrophism affecting the North American continent. The timescales indicated in my notes above are fully consistent with this scenario.
Reference
UTAH GEOLOGICAL SURVEY NOTES, September 2002, 34(3), 1-3.
http://geology.utah.gov/surveynotes/snt34-3.pdf