Classic Chinook Arch over Helena, Montana

A Mountain Wave.
This photo was taken from the Helena High athletic fields, looking west toward the Continental Divide. It shows the classic Chinook arch that appeared on December 13, 2018. The clear area between the arch and the mountains exists because the air is down-sloping there. As air flows over the Rockies it may develop an up and down motion like water flowing over rocks in the rapids of a river. Although the air flows downward once it gets over the mountains, it may continue to oscillate up and down as it flows away from the mountains for several hundred miles. The upward flowing part of this "mountain wave" is what forms the long arch of clouds. (Click on the image to enlarge it or CLICK HERE to watch a 24-second video of the arch shown in the photo.)

Here's how it works.
As the air flows down-slope, it is warmed by compression. Then, as the wave action continues and the air begins to rise again, the air cools by expansion. If there is enough vapor in the air, the arch of clouds will form as vapor condenses to form cloud droplets (or cloud crystals). Typically, the long area of clouds will form near the crest (top) of the first wave and then get blown eastward by higher level winds. If the mountain wave continues, and another downward turn is taken, the arch (cloud) will evaporate farther downstream (east).

Same arch, different vantage point.
The G.O.E.S. East satellite image below shows what the same Chinook arch looked like from space at 9:47 am MST. It is called an "arch" because an observer standing below it sees a curved patch of clear sky between the band of clouds and the mountains below. In the satellite image, the Chinook arch is the distinct eastern edge of the bright white cloud that extends from north to south through western Montana.

Term to define: GOES East Satellite

Confused? - Check out this Great Falls Tribune article.

Make a cloud with your mouth!

This is the coolest activity I've seen in awhile - Lots of good science, and explained very well. Check it out.

Mazama Ash in Montana

This photo was taken 12 miles northeast of Helena, Montana. The white streak is a layer of ash from the explosive eruption of Mt. Mazama (Oregon) 7,700 years ago. Geologists can determine where the ash originated by comparing its chemical composition with the compositions of volcanoes found in the region. .

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!

Understanding Natural Climate Cycles

The connection between Milankovitch Cycles and Ice Ages was established decades ago. However, climatologists have long wondered how such small changes in Earth's tilt and orbit could cause such big changes in climate. This video looks at a possible explanation.

Check out this Slick Crossword Puzzle-Maker

I wanted to make a crossword puzzle to help my students review some terms that were included in our recent hydrology unit. I found this site: crosswordhobbyest.com - Although the puzzle did not include all of the terms that I entered*, I'm satisfied with the end product. To access the puzzle CLICK HERE, or (to see a different look) click on the "start puzzling" link below.

*Unfortunately the puzzle-maker was not able to include these words: watershed, septic system, recharge, aeration.

Marble formed by Contact Metamorphism

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.

Contact Metamorphism.
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.

Finishing touches.
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)

Term to define: contact metamorphism

To access a blog a photo tour of the hike to the summits of Elkhorn Peak and Crow Peak, go to www.BigSkyWalker.com.

Mountains of Conglomerate in Southern Montana

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.

Term to define: basin

To see more photos of Sphinx Mountain, The Helmet, and the Sphinx conglomerate, go to Bigskywalker.com.