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NEWS NOTES — GEOPHENOMENA
Appalachian Mountains becoming more rugged
Smoke on the water
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| Matt Kirwan collected samples of sandstone from the surface of Bear Rocks, on the eastern edge of the Dolly Sods plateau in West Virginia. Isotopic analyses of these rocks suggest that the Appalachian Mountains are eroding away so slowly that the difference in relief between summits and river valleys is growing, not shrinking. Photograph is by Greg Hancock. |
Even at their peak, North America’s ancient Appalachian Mountains may never have rivaled the Himalayas for sheer craggy majesty. Although geologists have long thought of the Appalachian topography as smoothing out over time, a new study suggests that the summits are eroding more slowly than river valleys are forming — and therefore, that the mountains are becoming more, rather than less, imposing.
Extending from Canada to Alabama, the Appalachians are famed more for their misty beauty than for their ruggedness; thrill-seeking climbers look elsewhere for a serious challenge. But the Appalachians’ advanced age — the range formed during a series of mountain-building events lasting from 480 million to 300 million years ago (see Geotimes, November 2005), compared to the Himalayas’ mere 65 million years of age — has suggested to scientists that the contrasts between high summits and low river valleys were probably much more imposing in the past, says Gregory Hancock, a geomorphologist at the College of William and Mary.
“We think of the Appalachians as a range in decline, dying away and becoming more of a muted topography,” Hancock says. “The idea has been that they once had this grandeur that has been effectively eroded away in the time period since they were truly tectonically active.”
The erosion is thought to be dominated by the activity of ice, even atop summits never carved by glaciers, Hancock says. Water seeps into rocks and expands into ice as it freezes, cracking and breaking down the rocks. Scientists thought such “periglacial” erosion occurs fairly rapidly, he says, because the rocks appeared jagged and sharp, rather than smoothed, he says. “Visually, it looks like it is coming apart quickly because everything looks so angular and fresh.”
But new evidence suggests that’s not the case: Using the isotope beryllium-10 to estimate erosion rates, Hancock and graduate student Matthew Kirwan found that periglacial erosion takes much longer than was previously thought, as they reported in January in Geology. Beryllium-10 forms in rocks when cosmic rays interact with atoms, such as oxygen, in the minerals of a rock’s surface. How much beryllium-10 is within a rock, therefore, is a measure of how long the rock’s surface has been exposed.
Hancock and Kirwan measured beryllium-10 in quartz minerals from rocks at the summits of mountains in Dolly Sods, W.Va. Previous estimates for the summits’ erosion were about 25 to 30 meters per million years, Hancock says. The new data suggest that the mountains are eroding away at a much slower rate of about 6 meters per million years.
Because rivers in the region are actually carving valleys faster than the summits are eroding, incising into the Appalachians at a rate from 30 to 100 meters per million years, the topography of the Appalachians is currently getting more, rather than less, dramatic, Hancock says.
“Right now, the relief is increasing, which is kind of counterintuitive,” Hancock says. That may not always have been the case, however — some studies have suggested that the rivers may have begun incising more rapidly 2 million to 4 million years ago, when the climate turned colder and more variable, and the summit erosion rates just have not yet caught up, he says.
In the grand scheme of mountain erosion, rates of 30 meters and 5 meters per million years “are still in the same ballpark,” says Frank Pazzaglia, a geomorphologist at Lehigh University. With either number, he says, Appalachian erosion is still occurring at rates orders of magnitude less than other major mountain ranges. For example, the young, still-growing Himalayas are eroding at a rapid pace of several thousand meters per million years, while the somewhat more mature Rockies are eroding at a rate of several hundred meters per million years. Comparatively, the Appalachians are “a very geriatric system,” Pazzaglia says.
What the new study shows “quite nicely,” however, is that the Appalachians — long thought to be the paradigm of a system in equilibrium — are anything but, Pazzaglia says. Furthermore, he says, “depending on how you define the temporal and spatial scale, all landscapes are always changing, and are in some state of disequilibrium.”
Links:
"New Appalachian tale," Geotimes, November 2005
Changes in sea-surface temperatures due to long-term climate patterns such as El Niño may have a big effect on where and when wildfires happen, a new study suggests. The wildfires that swept across the western United States in 2000 and 2002, amid a record dry spell that lasted from 1999 to 2004, followed closely on the heels of warming waters in the North Atlantic — and signs of ongoing warming in those waters could mean that fires in the West will become even more frequent in coming years, the authors say.
On a seasonal basis, links between wildfires and weather are well-understood, but scientists are less clear on how climate patterns over longer timescales are linked to fires. To better understand how these patterns might intersect, a team of researchers reconstructed a 500-year timeline of fires in the western United States from fire scars on tree rings from more than 4,500 trees in the region. The team then compared that timeline with a chronology of drought in the region, as well as with sea-surface temperatures from both the Pacific and Atlantic oceans throughout that time.
Reporting in the Dec. 25, 2006, Proceedings of the National Academy of Sciences, the team found that not only were the drought and fire timelines linked, but also that sea-surface temperature fluctuations due to three oceanic phenomena — the El Niño Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO) — played a role in determining patterns of wildfire occurrences. ENSO, which relates to temperature changes in the tropical Pacific Ocean, and PDO, marked by temperature oscillations in the northern Pacific, helped drive fires on scales of one to ten years.
ENSO brings warmer and drier conditions to the Pacific Northwest, causing snowpack in the mountains to melt sooner and lengthening the fire season. In the Southwest, the warm conditions during ENSO promote the growth of grasses, providing fuel for fires that can later occur during the drier La Niña events.
The Atlantic oscillation, which is marked by sea-surface temperature changes in the North Atlantic, had a more long-term effect on wildfire activity that spanned multiple decades, because it altered how strongly and where ENSO and PDO had their influences. Because AMO is currently trending positive, meaning ocean temperatures are growing warmer, it may also presage a rise in wildfires, the researchers say. Warming temperatures, earlier springs and longer fire seasons due to climate change, however, may ultimately overprint this signal, possibly enhancing the future hazard of wildfires in the region.
