From the Depths of the Earth
Geologist David Foster studies birth and death of mountains
This article was originally published in the October 1999 issue of CLASnotes.
Foster is pictured using GPS (Global Positioning System) to obtain the location of a sample from the Bitterroot fault zone. GPS uses signals from 12 satellites to calculate specific locations.
These folded and tilted layers of sandstone and shale were once part of a large eastern Australian montain range (now eroded down to sea level).
The Bitterroot Mountains, on the Montana-Idaho border.
Though the Bitterroot Mountains on the Montana-Idaho border may not be as flashy as some—at 10,200 feet, the range's tallest peak is hardly a Denali, much less an Everest—geologist David Foster considers them one of the most important mountain ranges in North America.
"The Bitterroots are interesting because they expose rocks that formed in the earth's middle crust at depths somewhere around 20-30 kilometers (12-18 miles) below the surface," Foster says. "The rocks formed there mainly between 90 and 50 million years ago when Western Montana was part of a mountain range (known as the northern Sevier Orogen) more like the Andes, much higher and more extensive than it is today."
Fifty million years ago, the Sevier Mountains were torn apart by the extension or thinning of the continental crust. As the earth's crust thinned and the mountain belt collapsed, portions of the middle crust were transported upward to the surface on very large faults that are well exposed in a flank of the Bitterroot Range. "So by studying this mountain complex," explains Foster, "we can understand what happens at the middle part of the crust during the formation and destruction of many large mountain ranges. Additionally, we can use the Bitterroots as a proxy for what's now going on deep within the crust in the Andes or to understand what's occurring within the mountain ranges of the Himalayas and Tibetan Plateau."
This is especially valuable, says Foster, because recent evidence indicates that in Tibet, the middle crust is unusually thick and has build up enough heat that it may have started to partially melt. The presence of magma could weaken the crust enough that the mountain belt may eventually collapse, mimicking the evolution of the Bitterroots.
During the collapse phase of a mountain belt, the chances for large earth quakes, volcanic eruptions, landslides and other large scale geological hazards are greater because the rate of movement on faults is more rapid. "Certainly the recent events in Turkey are a reminder of just how unstable the earth's crust is," says Foster. "In the middle crust, rocks are the boundary between where layers of the crust fail and break by brittle fracture at shallower depths and where they start to flow like plastic at deeper levels," he explains. "It's a natural breaking point." Since many of the large earthquakes along major faults like the San Andreas are propagated from the same mid-crustal depths Foster studies, his work may potentially help pinpoint new areas prone to earthquakes or other geological disasters.
Foster's tectonic research is not restricted to Montana. "Over past eight to ten years I've been working on a very large scale project aimed at reconstructing the evolution of eastern Australia, from the time sediments were deposited on the ocean basin some 500 million years ago through a series of large collisions between that continent and oceanic island chains that occurred between 440 and 300 million years ago. During all of this time Australia was part of the super-continent of Gondwana, attached to India, Antarctica, Africa, and South America."
Those collisions formed major mountain ranges in eastern Australia, which, now worn down and nearly flat, contain some of the largest gold deposits in the world. "Our main objective is to examine the evolution of the mountain belts there because they are an ancient example of the current geologic activity in southern Alaska, but secondary follow-up research allows mineral companies to pinpoint areas where exploring for gold and other minerals would be more prospective."
But tectonics work is only half of what Foster does. He also utilizes temperature-sensitive isotopic dating methods or 'thermochronologic methods' in his research, allowing him to measure the temperature and time history of rocks. "Since most geological processes involve heat," he explains, "if we know when a rock was at certain temperature, it tells us about how it formed and about the history of the mass of rocks around it. This technique also gives us insight into the whole mountain building process, the extension and breaking apart of the continents, and the erosion process."
As part of the renovation of their new home in Williamson Hall, Foster and his geological sciences colleagues plan to establish a thermochronology lab here at UF. "The lab will feature a large mass spectrometer for measuring noble gases," Foster says, which will work in tandem with two other proposed labs that use additional thermochronological methods. "Each lab will allow us to look at different temperature intervals of a rock's thermal history. We'll be able to track the history a rock from temperatures of >500C to, effectively, surface conditions. Most of the lab applications will be to look at tectonic processes, but the lab will also be used to help the petroleum industry determine which sedimentary rocks are prospective for petroleum exploration."
The other thing these labs will be able to do, says Foster, is to date the precise age of volcanic eruptions, which will help establish the age of many geological events, as well as determine the periodic eruption intervals of certain volcanoes. Resulting research will also help geologists to understand the age of different parts of the sea floor and to date fossil localities, particularly Hominoid sites.
The department has made fundraising for the new labs a priority. "We're partly funded by UF, but we're seeking external matching funds from NSF or other outside sources for the remainder," says Foster. "In the next two years, we hope to have all this up and running."
