Geology

sawtooth mountainsA lot has happened to the land in Idaho in just the last 12,000 years. Being a geologically "active" region gives an extra crispness to Idaho's landscapes. Glaciers have moved. Ice has melted. Volcanos have erupted. Earthquakes have adjusted the elevations of mountain peaks. All around Idaho, the evidence of these events is so fresh that it seems like they happened yesterday.

At Craters of the Moon in southern Idaho, for example, where magma poured from fissures in the earth's crust only 2,000 years ago, it is likely that Native Americans might have watched from the nearby hills. After the magma cooled, what they saw is what we see today--uneroded cinder cones, craters, endless fields of lava that look as if they rolled across the land yesterday.

craters of the moon

Lava flows at Craters of the Moon

Continental and alpine glaciers, the legacy of the last ice age, were grinding away on Idaho mountains as recently as 10,000 years ago. In fact, during the 1980s, Idaho geologist Monte Wilson made an annual pilgrimage to measure the small remnant patch of Idaho's last glacier, Otto Glacier, on the north side of Borah Peak.

Glaciers created over 500 cirques in the high mountains of Idaho. A cirque is a form remaining at the head of a glacial valley where glacial erosion removed big blocks of rock from a mountain and quarried it out. Shallow cirques appear as wet meadows today, while the deeper ones with bowl-shaped floors may contain lakes.

Above the cirques are the scoured and rugged remains of the rock left behind. Glaciers gave central Idaho the spectacular skyline of the Sawtooth Mountains. Not enough time has passed for gravity to pull down the horns and aretes (jagged peaks and ridges) or moderate their vertical walls, or round off their sharp edges.

cirque lake created by glaciers

Glaciers created this cirque lake

As the glaciers pushed their load of gravel and debris down the valleys, they piled up lateral and terminal moraines (ridges made up of boulders and gravel) along the sides or out in front. Lodgepole pine forests grew on the moraines (which contain a fair amount of clay and silt), but not enough time has passed to cover up the rocks and boulders completely. The glaciers shaped the valleys they moved through. Many Idaho children learn at an early age to observe the difference between a glaciated U-shaped valley and a V-shaped valley eroded by a stream.

The Borah Peak Earthquake of 1983 changed the elevation of Idaho's highest mountain. Before 8:06 am, October 28, 1983, the mountain was 12,655 feet high; a minute or two later, it was about eight inches higher. Along 26 miles on the west side of the Lost River Range, the valley floor was suddenly lowered ten feet relative to the Range itself. Several people out hunting elk at the time actually saw the hillside split apart before them. They reported feeling faint, hearing a noise like a sonic boom, and being knocked off their feet.

borah fault

Borah earthquake faultline

The swift violence of Idaho's magma eruptions, glacier carving, and earthquakes is a reflection of what is happening more slowly deep below the earth. Continental drift, plate tectonics, the hot spot — these theories become very convincing when you look around Idaho.

A heavy ocean plate collided with a continental plate about 65 million years ago, sinking below the lighter continental plate. The crustal material heated up to become molten magma. The magma formed into plumes that moved upwards towards the earth's surface. Plumes that arrived at the surface erupted as volcanoes; the ones that didn't are called batholiths.

batholith

A portion of central Idaho's batholith

The center of Idaho is a huge batholith complex called the Idaho Batholith — a huge mass of granite. The material that sat above the pluton as it rose toward the surface was lifted up like a turtle on the back of a hippopotamus emerging from the bottom of a river. Mountains formed. In central Idaho, these mountains are in varying stages of erosion, but mostly the exposed rock is the granite of the pluton. The "turtle" eroded away long ago.

As the plutons rose toward the surface, the outside edges cooled first. Hot gases and liquids became trapped inside and were forced to occupy less and less space as the cooled "rind" of the pluton became thicker and thicker. Finally, the pluton cracked from the pressure, and the gases and liquids shot out the cracks. These too cooled and became the veins so familiar to anyone visiting the mountains. The veins contain gold, silver, lead, zinc, cobalt, copper, and many other rare minerals.

raw & finished star garnet

Star garnet is Idaho's state gem

Among the minerals are "gems." The same geological processes that gave Idaho its mountains also gave it star garnets, the state gem. There's jasper from Bruneau, opal from Spencer, and jade, topaz, zircon, tourmaline... Gems from all around Idaho explain its nickname, the "Gem State." But they are most concentrated in the batholith.

South of the Idaho Batholith is a crescent-shaped flatness brought about by the passage of Idaho over a "hot spot." Imagine a candle sitting on a table. Pass a piece of white paper over the candle slowly enough to turn the paper brown. Now imagine a tectonic plate — a large piece of the earth's crust — passing slowly over an

polished bruneau jasper

An example of Bruneau jasper

intense source of heat coming from the mantle below. The heat melts the crust, burns it up, and weakens it. Magma squeezes up through fissures and spreads out over the land, filling up stream valleys and erasing the pre-existing topography.

The legacy of the hot spot is Idaho's "flat spot," more properly called the Snake River Plain, the brown streak on the piece of white paper. The "burned" crust is from 50 to 70 miles wide and 400 miles long. The lava rock at the western end of the crescent is the oldest. At the eastern end, Yellowstone boils and bubbles with hot water and gases taking their heat from that candle below. If the hot spot theory is right, the tectonic plate will keep moving westward a few millimeters a year, and its path of destruction will eat into Wyoming and Montana.

snake river plain

The Snake River is the Nile of Idaho

In economic terms, the Snake River Plain makes possible a vast aquifer containing 200 million acre feet of water, one of the largest fresh-water aquifers in the world. A resource of immeasurable value, the Snake River Plain Aquifer yields its water to thousands of wells and pumps that bring water from 600 feet below.

The Snake River is another crescent that crosses southern Idaho. It carved its way through the many layers of lava and interbedded sediments that accumulated during the quieter millennia of geologic time. The water from the aquifer, the Snake River, and the rich volcanic soil of the plain gave modern human settlers a rich heritage upon which to build "permanent" settlements.

snake river canyonRiver carving goes on all over Idaho, of course, one of the more constant and visible processes of geologic change. Whether geologic events are sudden like earthquakes or slow like the progress of tectonic plates, they have produced one of the best places in the world to see them all at work.




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