Geology is Cool: Craters of the Moon


Craters of the Moon from U.S. Highway 20 Viewpoint

Craters of the Moon from U.S. Highway 20 Viewpoint

Think of it as a kind of “lava field light.” If you been to Hawai’i, especially to the Big Island, or hiked along the Pacific Crest Scenic trail through Mt. Washington and Three Sisters Wildernesses, Craters of the Moon is a pretty modest series of lava flows. But it is still an impressive display of volcanism. And it does offer, in one easy-to-navigate location, almost all of the forms of lava known to science.

Altogether, Craters of the Moon National Monument and Preserve has about 700 square miles of lava flows. (In comparison, the Big Island of Hawaii is about 4,000 square miles of lava flows.) Seriously mis-named, there’s nothing moon-like about Craters of the Moon. Perhaps it was someone’s idea of what the moon would like before we actually got there to see for ourselves.

Where Craters of the Moon really shines is in the amazing range of volcanic features. In eight major episodes from 2,000 to 15,000 years ago, the Great Rift has created textbook examples of volcanic features. There are spatter cones.

Spatter Cone, Craters of the Moon Nat'l Monument

Spatter Cone, Craters of the Moon Nat’l Monument

Spatter cones develop in the late stages of an eruption. The volcano vent throws out gobs of semi-moltern lava, which fall back around the vent, welding together. The throat of a spatter cone is usually still visible from the summit.

Spatter Cone Throat and Lichen, Craters of the Moon Nat'l Monument

Spatter Cone Throat and Lichen, Craters of the Moon Nat’l Monument

The throat of these vents is usually shaded from the blistering heat, so you commonly see more lichen, especially along the southerly side. As seen here.

You can find the two basic forms of lava, pahoehoe (pa-hoy-hoy) and aa, in many of their forms. Pahoehoe lays down in smoother layers, sometimes in ropes.

Ropey Pahoehoe, Craters of the Moon Nat'l Monument

Ropey Pahoehoe, Craters of the Moon Nat’l Monument

While the blocky, sharp-edged aa creates a jumble of shin-cutting, boot-eating rock chunks.

Field of Aa, Craters of the Moon Nat'l Monument

Field of Aa, Craters of the Moon Nat’l Monument

Aa and pahoehoe are chemically almost, but not quite, identical. The silica content, SiO4, is slightly higher in aa, making it stiffer and less flexible than Pahoehoe.

Basalt is a decent insulator, so you can get molten rock flowing under hardened rock. When the molten rock stops flowing, you are sometimes left with the emptied lava tube, a lava cave.

Buffalo Lava Cave, Craters of the Moon Nat'l Monument

Buffalo Lava Cave, Craters of the Moon Nat’l Monument

This lava cave is about five feet high and a couple hundred feet long. The bright light at the end is a reflector sign, telling would-be spelunkers to go no further.

You can also find cinder cones, lava rivers, lava bombs, tree molds and more lava rivers. There doesn’t seem to be any obsidian – basalt that is almost 100% SiO4 and is close to glass – but that may just be the National Park Service being coy. Some of the cinders are almost pumice, rock to full of vesicles of air that it floats.

Geology is messy. Different kinds of lava flows lie on top of each others, cinder fields covered some areas or were blown in afterwards. Over some 15,000 years, and countless eruptive vents in eight major groups, the result is a volcanic chaos.

Craters of the Moon also offers lava-adapted plants and animals, kipukas (areas that never had lava flows on them) and the harsh beauty and wonder that is the characteristic of volcanic fields.

Why are Craters of the Moon where they are? Current thinking is that as the western United States is pulled apart from the northwest, creating the basin and range topography, some of the faults those stresses created penetrated to the magma bulbs created by the Yellowstone hot spot as it passed under the region 6 million years earlier. And it’s probably not done. There could very well be further eruptions, perhaps as soon as in 900-1,000 years, if the frequency of past eruptions continues.

The most recent flows, the Broken Top flows, are only about 1,800-2,000 years old, based on Carbon 14 dating from tree molds. That practically yesterday, in geologic terms.

Definitely worth a visit. Sure, it’s a bit of a one trick pony, in comparison to the vastness of Hawai’i or the newer flows and spectacular destruction of more recent Cascade Mountains events. But still, pretty cool. Geology in real time.

Advertisements