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Graveyard of Old Volcanoes
The first written reports from southern Idaho's Snake River plain were not complimentary. To the emigrants on the Oregon Trail, this was a desolate place. In time, this pathway of the pioneers came to be known as the world's longest graveyard.
To geologists who have since studied its mysteries, this immense basalt region is indeed a graveyard, a graveyard of dead volcanoes.
About 16 million years ago somewhere near the Idaho-Oregon-Nevada border, a plume of hot material worked its way up from maybe 50 miles underground. And as it did so, it melted the rocks closer to the surface. These rocks, which were relatively rich in silicon and aluminum, yielded rhyolite magma, which erupted explosively and spewed across the landscape in massive quantities. We catch present-day glimpses of the depth of the outpouring of this pale rock in some of the canyons of Owyhee county.
These large-scale eruptions extend northeastward from that location all the way to the present activity under Yellowstone National Park. Among geologists this is known as the "hot spot" theory. That plume or hot spot has not moved, even though right now it's directly under Yellowstone. Instead, it's the continental plate above the hot spot that has been moving, traveling westward at the speed of fingernail growth. But compound that over 16 million years, and what you have is the Snake River plain . . . in other words, a series of old Yellowstones.
"Just to go back to the basics," explains geologist Marty Godchaux, "it's the only continental plume track on earth today, so it's absolutely unique in that sense." Today, its approximate equivalent might be the oceanic plume track that is creating the Hawaiian Islands.
"Volcanoes have played an enormous role in the evolution of most of this state," says Godchaux, "particularly the volcanism in the Snake River plain that is the result of the so-called Yellowstone plume, or we think that's the best theory. So we have a snapshot in time, of that plume sitting under modern day Yellowstone. What we've done is to look at the ancient Yellowstones, the Yellowstones of five, eight, eleven, thirteen million years ago that stretch back across Idaho to the southwest."
Several popular geology books have argued that it was a meteor striking the earth millions of years ago that wounded the earth's mantle, causing the hot spot that created the Snake River plain. Neither Bill Bonnichsen nor Marty Godchaux buys that theory. "Superficially, it's a really attractive idea," says Bonnichsen, "but if one goes out and looks for specific evidence for that, the evidence is lacking."
He lists some of the missing evidence: the debris fields from the fallback of material that would have been blown into the atmosphere; the lack of shatter cones which occur when an impact hits; the lack of glass melts. "So, personally, I discount the idea. It's an interesting idea, but it's gone to the dust bin of interesting geologic ideas."
Bonnichsen has his own ideas about what started it all. "One of the theories is the 'hot spot' theory, that from somewhere very deep within the earth, some sort of a plume of hot material came up and caused melting in the upper part of the mantle.
"Another theory that has been put forth is that the Snake River plain lies astride a zone of disjuncture within the earth's crust. To me, that idea is a little bit more attractive. And then the process of melting got started at one end of it; and as the process of spreading to the south continued — and this didn't happen to the north — there developed a zone where there was upwelling within the mantle so that melting could occur, and that has progressed across the landscape from southwest to northeast.
"Now, one of the interesting things is that zone of melting lies approximately parallel to the direction in which the North American continent has been advancing across the interior of the earth as the earth's plates move.
"Beyond that, I don't think we have an exact explanation; but we're either looking at a model in which it was just something that started deep within the earth, or we're looking at a model that takes into account the plate motion and the variations of that as the earth's surface has evolved."
The latest basalt eruptions on the Snake River plain occurred approximately 2,000 years ago, in the Craters of the Moon area. Bonnichsen calls this the latest manifestation of a long term process in southern Idaho of the development of the Snake River Plain. "And in the case of the Craters of the Moon, there is a series of fissures that run across the plain known as the Great Rift; and along that series of fissures, basalt magma came up from deep in the earth's interior, came out on the surface and made a series of vents and a series of lava flows.
"So this is a process that was repeated several times in recent geologic history. The people who work here have determined ages of several of the flows. About every 2,000 years for the last 15,000 years there have been a series of basalt flows that came out and have built up the lava flows and the vents in the Craters of the Moon area."
It's not too far-fetched to imagine Craters of the Moon erupting again, much as Kilauea in Hawaii is doing now. "Both are fundamentally driven by the heat escaping from the interior of the earth," explains Marty Godchaux. "Heat is the engine that drives the earth. We geologists, those of us who consider ourselves volcanologists, are just waiting and hoping that maybe, maybe in our lifetimes, Craters of the Moon will see another basaltic eruption. It's not a given, but we're hoping."
We asked Bill Bonnichsen what some of the popular misconceptions are of this part of the state. "Many people see some rather drab, flat areas of sage brush and believe it's a dull place, and it really isn't. It's scenically interesting and geologically really quite interesting.
"Second, people think that a large volume of the Snake River is basalt. That's untrue. If you were to drill holes out in the middle of the plain, you'd find you went through a few hundred feet of basalt; then you would go into rhyolite, and that might be thousands of feet thick. So the volume of rhyolite in the evolution of the Snake River plain is much larger than the volume of basalt. It's just largely hidden or off to the sides. So those are a couple misconceptions."
Under the eastern Snake River plain is an aquifer that covers as much area as one of America's great lakes, Lake Erie. However, it holds considerably less water than Lake Erie because the water is stored in the cracks and crevices of the basalt flows that fill the eastern plain.
Pioneers on the Oregon Trail stopped at what they called Thousand Springs. It was a magical place to them, more so because they did not understand where all that water was coming from. Now we know.
"The present canyon of the Snake River, starting about Niagara Springs and Thousand Springs is at the lowest point of those porous basalts. The river is cut down deep enough to intersect the water table, so water just gushes out," explains Bonnichsen.
The aquifer flows mainly southwest, draining to the Snake River. The water at Thousand Springs comes from as far away as the southern end of Yellowstone National Park two hundred miles away, and from the Lost Rivers near Mackay, which flow into the Snake River plain and then just disappear into the basalt. Put a thermometer to this water, and it will read a constant 58 degrees.
Scientists are still mapping this underworld, but they do know that in some places, such as south of the Snake river near Twin Falls and Buhl, the aquifer is thin and not as robust as people once assumed.
"If you get into the lower reaches of the basalts, they get pretty tight. They don't have near as many fractures in them," says Bonnichsen. "The transmissible part of the flows is only about 100 feet thick. Below that the flows are very dense, and they won't transmit water."
Geologists differentiate between this eastern Snake River plain aquifer and other aquifers, like the one near Boise, or the Rathdrum prairie, both of which are filled with gravels. Because the eastern plain is a fracture controlled aquifer, it's more vulnerable to contamination.
Farmers have been drilling wells into this vast aquifer since the 1950's, allowing more land to be cultivated. But the groundwater and surface waters are intricately connected, in ways we are still discovering, "which says that we should be more careful with our aquifer than we have been in the past," says Bonnichsen, "because it has just been mined, and the assumption has been made by many that it is just infinite, which is really not true."
Text by Bruce Reichert