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Wednesday, September 19, 2018

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)

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

Thursday, July 26, 2018

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.

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

Thursday, February 8, 2018

Strange Flow of Rocks is Remnant of Rock Glacier

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.

More about this hike -

More about rock glaciers from

Tuesday, January 30, 2018

Blue Moon, Super Moon, Lunar Eclipse of January 31, 2018

Here is a great resource for understanding/explaining the lunar eclipse that will happen early tomorrow morning (January 31). This will also be the second full moon of the month, therefore it is a "Blue Moon". . . AND it is a Super Moon as well! The graphic below illustrates the size difference between a smaller full moon and a super moon, which happens when a full moon occurs when the Moon is slightly closer to Earth. (Graphic courtesy of NASA/JPL - Caltech). Here is another resource - a blog post with several graphics and animations: CLICK HERE.

Friday, December 29, 2017

Amaze your students with this cloud demonstration!

Ever wonder why meteorologists make such a big deal out of pressure. Help your students understand by showing them this cloud demonstration. Also, be sure to check out the other videos (see links below).

Here are the basics
Anything in nature that makes air rise will help clouds form. Fronts, mountains, convection, and storms all move air upward. As air rises it cools by expansion, and if it cools below its dew point clouds will form. Clouds are made of tiny droplets (or ice crystals) that form as vapor condenses on particles (pollution, dust, smoke, etc.).

The unltimate cloud formation demonstration! from Rod Benson on Vimeo.

To see a longer version with more details (instructions), CLICK HERE.

To see my video of a student activity, CLICK HERE.

Here's another great video from MinuteEarth - CLICK HERE to see it.

Tuesday, December 12, 2017

Temperature Inversion - Air Pollution Montana-Style

This photo of a temperature inversion was taken from Bompart Ridge on the southern edge of Helena, Montana on Sunday, December 10. Cold air was sitting in the valley below. The temperature on the valley floor was ~15 F, and it was ~20 F on this ridge. The reason you can see the cold air is that it fills with fog and pollution.

Its a valley thing.
During winter months, the mountain valleys of western Montana are prone to inversions – called this because they really are "upside-down situations". NORMALLY the atmosphere gets colder as you get farther away from Earth's surface because the air is warmed from the bottom up by heat given off by the Earth. However during inversions, air at the surface is much colder than the air above. Local hikers know this means that it can be 5-10 degrees warmer on top of the mountain than it is down at the trailhead.

Recipe for a temperature inversion - Clear, Calm, Cold
Mountain valleys serve as “sinks” where cold, dense air may sit for several days. They develop during clear, calm, cold nights – especially in December and January. Clouds act like a blanket, keeping much of the heat given off by the Earth close to the surface. So, on clear nights this heat escapes quickly out to space, and air at the surface becomes cold (and heavy). The low angle of the Sun in December and January prevents this valley air from heating up during the days. Snow cover, which reflects sunlight, and the shortness of winter days also help prevent it from warming. If the inversion persists for several days, air quality worsens as the stagnant, cold air fills with pollutants such as smoke from wood-burning stoves or emissions from automobiles.

For more about this (blog with photo tour) CLICK HERE.

Monday, December 4, 2017

Ancient Sea-Floor Sediments Store Huge Amounts of Carbon.

Click on the photo to see higher resolution.

Got carbonates?
Scapegoat Mountain (elevation 9,186) is located in the Scapegoat Wilderness of western Montana. Both the cliff and peak are made of layers of carbonate rock deposited during the Cambrian Period (540-485 million years ago) when Montana was beneath a shallow part of the ocean as shown on this map. The cliff consists primarily of two limestone formations; the Pagoda formation lies beneath the Steamboat formation - and the peak is made of Devil's Glen dolomite. Limestone is mostly calcium carbonate (calcite, CaCO3), whereas dolomite is calcium magnesium carbonate. Dolomite can form as magnesium-rich groundwater moves through limestone, giving magnesium ions a chance to replace calcium ions.

Not that kind of reservoir.
Dolomites and limestones store huge amounts of carbon, hence they are an important reservoir of carbon and part of Earth's carbon cycle. It is estimated that Earth's carbonate rocks store over 60,000,000 gigatons of carbon, compared to the estimated 840 gigatons in the atmosphere (Source: Atmospheric carbon is mostly in the form of CO2, and experts believe that CO2 levels during the Cambrian Period were much higher than today. Movement of this carbon from the atmosphere to the rocks shown in the photo began with rain. Carbon dioxide combined with cloud droplets to form a weak carbonic acid that rained to the surface. This acidic rain "ate away" rock material (chemical weathering), releasing calcium, magnesium, and other ions that rivers carried to the ocean. In the ocean, the calcium ions combined with bicarbonate ions to form calcium carbonate, which precipitated to the sea-floor. These processes were certainly in high gear during the Cambrian Period as evidenced by the thickness of the carbonate layers shown in the photo. In fact the carbon in the cliffs of Scapegoat Mountain has been stuck there for 500 million years. So, what natural process(es) could put some of these carbon atoms back into the atmosphere? Read this to find out.

A couple "plot twists" in the carbon saga.
In modern oceans, most of the carbonate material being deposited originates from corals and shell-building organisms, including plankton (foraminifera, etc.). As these organisms die, they build up on the seafloor. Eventually layers of shells and sediment will be cemented together and become rock, storing the carbon in stone (limestone and dolomite). Another way that carbon is stored is as fossil fuels. Under certain circumstances dead plant matter or algae builds up faster than it can decay, Then as these deposits of organic carbon are buried and compressed they become oil, coal, or natural gas instead of sedimentary rock. (Source: NASA - The Slow Carbon Cycle)

There's been a disturbance.
Scapegoat Mountain sits near the southern end of the Lewis Overthrust Belt, which includes the spectacular mountains that extend northward through Glacier Park, into Canada. The overthrusting happened as the Pacific Plate pushed into North America starting in the Middle Jurassic Period (174 to 163 mya) and ending in the Early Eocene Epoch (56-34 mya). The super-slow collision forced slabs of crust (mostly sedimentary rock) to fault, and then slip up and over younger layers to the east, resulting in the unique north-south trending ridges and valleys found along the Rocky Mountain Front.

The "Bob" is a great Montana place.
The Scapegoat Wilderness is part of the Bob Marshall Wilderness Complex south of Glacier Park - The fifth largest wilderness area in the lower 48 and home to some of the most primitive back-country in the USA. Affectionately known as "The Bob," the complex consists of three wilderness areas, including the original Bob Marshall, which was established in 1964, along with the Scapegoat and the Great Bear Wildernesses, which were added in the 1970s.

To learn more about Scapegoat Mountain and see many more photos, CLICK HERE.