Wednesday, December 5, 2018
Wednesday, November 7, 2018
The volume of ash produced by Mazama was forty-two times greater than the amount produced by St. Helens in 1980. Prevailing winds caused the ash to spread eastward. Initially the ash covered much of the ground in the Northwest. But in the months following the eruption, wind and runoff transported the ash to low places (lakes, valleys), where it was eventually buried beneath layers of sediment. Today it can be seen in places where it has been exposed in road-cuts, cut banks, archeological digs, etc. The sediment above the ash layer in the photo to the right was deposited in the centuries after the ash settled here, then the exposed when road construction cut into the slope.
The Mazama ash layer has been found at many other places in the Northwest as well. In fact, Mazama ash serves as a good "key bed" for the region. Key beds help determine relative age - For example, several years ago archeologists came upon Mazama ash while excavating a Paleo-Indian campsite near Helmville, Montana. Mazama ash was exposed at the dig site ABOVE the evidence, indicating that the Indians used the site before the big eruption (at least 7,700 years ago).
The Mazama eruption also emptied significant amounts of magma from the chamber beneath the volcano. As a result, after the eruption the remaining cone collapsed into the chamber, forming a huge crater known as a "caldera". Today, Crater Lake (Oregon) fills the caldera.
For much more about Mazama Ash, CLICK HERE
Also, check out my Montana hiking blog at www.bigskywalker.com - Lots of geology!
Tuesday, October 23, 2018
Saturday, October 13, 2018
*Unfortunately the puzzle-maker was not able to include these words: watershed, septic system, recharge, aeration.
Wednesday, September 19, 2018
The Elkhorn Mountain Volcanics.
The Elkhorn Mountains south of Helena, Montana are the remnants of volcanoes that were active in this area 74 to 81 million years ago. During that period, a tectonic plate was subducting beneath western North America, allowing magma to rise to the surface. As a result, the Elkhorns are made up primarily of extrusive igneous rocks, but are related to plutonic rocks of the nearby Boulder Batholith. The volcanic rocks that make up the Elkhorns (lots of andesite) formed when lava poured onto the surface and cooled, whereas the plutonic rocks (granite, etc.) of the batholith formed as magma beneath the volcanoes cooled underground.
Despite the volcanic origin of the Elkhorns, the outcropping shown in the photo is made of marble - a metamorphic rock formed as limestone was changed by heat and/or pressure. Sometime during the late Cretaceous, magma melted its way into the area, coming close enough its heat to change the limestone into marble - a process known as “contact metamorphism”. Evidence for this is the presence of granite (formed as that magma cooled), located not far below the marble.
Ancient hot springs?
Limestone is usually formed by sediment deposited in a shallow tropical sea, so how did limestone form in a center of volcanic activity? One possibility is that hot springs existed here when the area was volcanically active. An unusual variety of limestone called "travertine" can form on the surface around hot springs by the rapid precipitation of calcium carbonate. This is what is happening today at Mammoth Hot Springs in Yellowstone Park where thick terraces of travertine continue to form as hot water comes to the surface.
In the millions of years that followed, the igneous rocks and marble were deformed (folded, faulted) by tectonic forces that built the Rocky Mountains (80 to 55 mya). As the formations were pushed up, they were also shaped by erosion, including glaciation. In fact, the marble in the photo marks the top of a cirque formed by a glacier that once (or multiple times) flowed from Elkhorn Peak toward the present-day location of the town of Elkhorn. More significant cirques lie on the northeast sides of Elkhorn Peak and Crow Peak - the two high points in the range. (Map)
To access a blog a photo tour of the hike to the summits of Elkhorn Peak and Crow Peak, go to www.BigSkyWalker.com.
Thursday, July 26, 2018
This unusual peak, called “The Helmet”, is located in the Madison Range of southern Montana, 22 miles northwest of Yellowstone Park. The photo was taken as friends and I descended from a neighboring peak called Sphinx Mountain. The Helmet is so-named because it resembles the comb on a Spartan’s helmet, and Sphinx Mountain was so-named because it looks like Egypt’s famous Sphinx when viewed from the north. Besides their unusual shapes, the two peaks share another strange feature - Both are made of a fairly uncommon rock called “conglomerate”.
Conglomerate is a sedimentary rock (sandstone, shale, and limestone are others). With conglomerate, the sediment that became rock was gravel. It is unusual to find a whole mountain made of layer upon layer of conglomerate, but that is the case with Sphinx Mountain and The Helmet. Both mountains are composed entirely of thick layers of a “limestone conglomerate” - pebbles, cobbles, and boulders of limestone embedded in a reddish sandstone matrix. All total, the beds of conglomerate are over 2,000 feet thick.
It is believed that the gravel was deposited here during the Eocene period (56-34 mya), when the area was a basin. The basin, which was probably much more extensive during the Eocene, presently occupies an area of only 2 square miles - and it’s not a basin any more. Over millions of years the layers of gravel became stone, and then were pushed up as the Rockies formed. Now Sphinx Mountain (10,876 ft.), one of Montana’s most iconic peaks, stands as a remnant of this gravelly basin. The Sphinx conglomerate is found only on Sphinx Mountain and The Helmet.
To see more photos of Sphinx Mountain, The Helmet, and the Sphinx conglomerate, go to Bigskywalker.com.
Thursday, February 8, 2018
This photo of Mt. Powell (10,168 ft.) in western Montana shows an impressive cirque, shaped by a glacier that once flowed from Powell’s northeast slope, down toward the valley of the Clark Fork River. According to geology maps, the strange flow-shaped mass of rocks near the bottom of the cirque was left by a “rock glacier”. Apparently during the final decades of Mt. Powell’s glacier, there were more rocks than ice in the mix. Eventually even the ice between the rocks melted away, and the rocks were left without a “ride”. From the summit, the deposit looks like a fluid blob of rocks, but without the matrix of ice the rocks are no longer flowing.