Landsat shows us the home planet, warts and all.
- By Linda Shiner
- Air & Space magazine, April 2013
(Page 4 of 4)
Some geologic structure is simply more comprehensible viewed from space than it is when you’re standing on top of it. John Spray, the director of the Planetary and Space Science Centre at Canada’s University of New Brunswick, is using the perspective offered by Landsat in a 10-year study of the Manicouagan impact crater in Quebec. “We’re a little bit like an ant walking on an elephant or a rhino,” he says. Though small by solar system standards—many impact craters on the moon and Mars dwarf it—Manicouagan is one of Earth’s largest; it hasn’t been deformed by erosion or squeezed by tectonics. “That huge crater was formed in seconds,” says Spray. “If you took the nuclear arsenal of the Soviet Union and the United States during the cold war and let them off in one place in one moment, it would not create as much energy as the energy that formed Manicouagan.
“What we think is totally weird is that the rocks at Manicouagan came up eight to 10 kilometers [five to six miles]—in a minute or two,” says Spray. The central uplift where those rocks now appear could have formed as the ground beneath the impact rebounded, he says , like a trampoline reacting to a jumper. He believes it was helped along by the almost simultaneous collapse inward of rocks and material around the sides of the excavated bowl. “We don’t really understand how rocks move that fast,” he says.
At the other end of the geologic time scale, the Brandberg Massif in central Namibia rose from the surrounding plain over a period of hundreds of thousands, if not millions, of years during Earth’s Cretaceous period, around 100 million years ago. Geologists call it a granite intrusion. It too is the result of almost unimaginable forces. The ancient surface bulged up because molten rock under pressure pushed through overlying layers into a zone of lower pressure. What drove that action was the movement of tectonic plates.
“Friction between two plates builds up a lot of heat that can’t be dissipated easily and actually gets hot enough to melt some of the overplate,” says USGS scientist Charles Trautwein, “and the melted rock comes up between structural cracks formed by the [movement of one plate beneath the other].” This particular intrusion covers 250 square miles and reaches almost 8,500 feet above the Namib Desert.
One of the things geologists look for in Landsat images are geometric shapes: circles and lines, which can indicate faults, or breaks in the crust. The ridges that ring the Brandberg Massif are an example of circular faults. Faults more often show up as straight lines; in either case, the scale may elude geologists working in the field. “It could continue on for maybe 10 or 20 kilometers,” says John Spray, “but when we’re down in the woods in that valley, we don’t know how big it is.” From space, scientists can see its full length and determine where to target ground studies.
One of Earth’s longest ancient fault lines stretches nearly 1,000 miles, from Flathead Lake, Montana, almost to the northern border of British Columbia in Canada. “The Rocky Mountain Trench gives us a picture of a historical event that started over 100 million years ago,” says Charles Trautwein. The first part of the action was the sliding of the Pacific tectonic plate. As it moved under the North American plate, it ruptured the overlying rocks. Over several ice ages, explains Trautwein, the repeated advance and retreat of glaciers scoured out canyons, scraping away structurally weakened rocks and forming the impressive Rocky Mountain Trench.
Linda Shiner is the editor of Air & Space.