#89 - Through the Guts of an Ancient Volcano

Click on photo to enlarge.

Lewis and Clark were here, and so was Kevin Costner . . .
This photo of the Missouri River, taken 30 miles southwest of Great Falls, shows the area where the Corp of Discovery left the Great Plains and entered the mountains in July of 1805. It is also where Paramount Pictures filmed a shootout scene for its 1987 movie titled The Untouchables, starring Kevin Costner and Sean Connery. Montana doubled as Canada in the film, with this bridge (Hardy Bridge) on the frontage road between Cascade and Wolf Creek serving as the Canadian border.

Leftovers from an ancient volcano. . . .
If the Lewis and Clark Expedition had passed through 75 million years earlier they would have encountered an area of volcanic activity that measured about 20 miles in diameter, and they would have probably seen more than one volcanic cone. The journey from Great Falls to Helena along Interstate-15 goes through this ancient volcanic center between Cascade and Craig. But, if you take this drive don’t expect to see any classic cone-shaped volcanoes - Millions of years of erosion have worn them away, leaving a mountainous area of igneous rocks that geologists refer to as a “volcanic pile”.

Right: This map from Northwest Exposures shows the extent of the volcanic center. A is the where the Hardy Bridge is located. The photo was taken from a “scenic turnout” along I-15 near - the place marked B on the map.

Clues in the igneous rocks . . .
Geologists classify all rocks into three categories: sedimentary, metamorphic, and igneous. Igneous rocks like the ones shown in the photos on this page, are formed as lava or magma cools. Although sedimentary and metamorphic rocks do their share in helping geologists learn about the past, igneous rocks provide several unique contributions to our understanding of Earth’s history.

1. For one, the age of igneous rocks can be determined using radiometric dating techniques. As the molten rock solidified, gases trapped within mineral crystals can be analyzed to determine when the magma hardened. Radiometric dating indicates that this part of Montana was volcanically active from about 81 to 71 million years ago. . . long before Lewis and Clark, and Kevin Costner stopped by.

2. Igneous rocks also help scientists figure out where a land mass was when the magma or lava cooled. Magma and lava consist of a mixture of minerals, some of which are sensitive to magnetism. As the molten material cools, these magnetic minerals orient themselves like compass needles, indicating where the magnetic north pole is. Then as the magma or lava hardens, those minerals are locked in that position, telling future geologists which direction it was to the magnetic north pole when that particular igneous rock formed. This can help geologists learn about the movement of crustal plates and the location of continents in Earth’s past.

3. Since the igneous rocks in this particular were formed as flows along the slopes of volcanoes, they could also be used to estimate the size and shape of the volcanic cones. Although only the bases of the cones remain, it would be possible to measure the angles of the tilted layers that are left, plot them on a map, and extrapolate to reconstruct the volcanic landscape. The outcropping behind the bridge (above photo) shows some of those tilted layers, which were flows on the slope of a volcano.

4. Another type of information that scientists can learn from igneous rock is whether the molten material cooled above the surface (lava) or beneath the surface (magma). When magma cools beneath the surface it cools slowly, giving the minerals time to arrange themselves in an orderly manner. As a result larger crystals (or grains) form. If you examine a sample of granite you will see several different minerals that started to form crystals before the magma hardened. In contrast, when lava erupts onto the surface, it cools quickly, giving little time for crystals to form. Igneous rocks formed in this way, such as basalt, tend to be fine-grained.

Sources:

1.Alt, David. "Adel Mountains" Profiles of Montana Geology; published by the Montana Bureau of Mines and Geology in cooperation with the Montana Magazine 1984

2. Alt, David, and Donald W. Hyndman, Northwest Exposures, Missoula, Montana: Mountain Press Publishing Company, 1995.

3. Sheriff, S.D., and Gunderson, J.A., 1990, "Age of the Adel Mountain volcanic field, west-central Montana": Isochron/West, no. 56, p 21-23.

Terms: volcanic pile, radiometric dating

#90 - Glacier Park's Key Bed

Above: Mt. Siyeh (sunlit peak on right) looms over an exposure the "magmatic sill."

Markers in the layers . . .
Just as constellations help astronomers locate certain stars in the night sky, rock layers known as "key beds" can help geologists in the field identify the rock layers they are looking at, and compare the ages of rocks in different geographic areas where the key bed is present. In order to serve as a key bed, a layer of rock needs to be fairly widespread and easy to recognize. Quite often key beds, like the rocks above and below them, are sedimentary in origin, but they can also be layers of ash deposited over a large geographic area by ancient volcanic eruptions. One of the more famous key beds is a layer clay called the "K-T Clay" (K-T Boundary), which was deposited worldwide by the impact of asteroid roughly 65 million years ago. Geologists know that layers below the K-T Clay are older that 65 million years and those above are younger. Another that is useful in Montana is ash from the eruption of Mt. Mazama (Crater Lake, OR) 7,600 years ago.

Like a racing stripe . . . .
Although it is not nearly as widespread as the K-T Clay, the series of rock layers that make up the mountains of Glacier National Park (GNP) include what some might call a key bed. Unlike the layers above and below it, GNP's key bed is not a sedimentary rock, but rather a layer of igneous rock that used to be called the "Purcell Sill" (now referred to as the "magmatic sill"). Sills are formed when magma forces its way in between layers of sedimentary rock and then hardens. Throughout most of the park, the sill stands out as a 130 to 300 ft. dark layer of diorite sandwiched between strips of white, low-grade marble that formed as heat from the cooling magma baked the limestone. This heat also drove out organic material, bleaching the rock and causing more contrast between the darker diorite and the marble.

Unlike typical key beds . . .
Most key beds consist of a layer of material (sediment or ash) that was deposited on a surface at some point in the past. Over time, the key bed was covered by other sediments. According to the Principle of Superposition, layers below a typical key bed are older and layers above it are younger. However, since the sill was injected between sedimentary layers that had already formed, geologists know that the layers immediately above it are not younger than than the sill. Nonetheless, the sill serves as a marker - when geologists see it they know they are looking at the Helena (Siyeh) Formation, which lies just above the older rocks of the Empire Formation, and just below the younger rocks of the Snowslip Formation.

Below: The sill is labeled "magmatic sill" on the diagram below. The diagram illustrates the basic geology of the park (The Lewis Over-Thrust). The sill is not present in the Chief Mountain area because it has been eroded away from that region.

*NOTE: Geologists no longer refer to the sill as the "Purcell Sill" (photo) because there is a "Purcell Lava" found in the park, and the two (sill and lava) are not the same age.

Term: key bed

#91 - Cracker Lake in Glacier National Park

This photo was taken from near Many Glacier Lake in the east-central part of Glacier National Park. The view shows a 1-mile long lake named Cracker Lake, which was formed in a basin carved by a glacier during the last ice age. All of the glaciers from the last ice age melted completely, and then new valley glaciers such as Siyeh Glacier formed during a recent cold period that ended around 1850 ("The Little Ice Age"). Such lakes, referred to as "tarns" or "cirque lakes," are common in Glacier Park.

Got Milk?. . .
The beautiful turquoise color shown in the photo is the true color of the water. Sometimes called "glacial milk" (photo), the unusual color is due to the presence of "rock flour", which consists of tiny clay particles formed as rocks stuck to the bottom and sides of a glacier grind against bedrock. This abrasion reduces some of the bedrock to a fine powder that looks like the flour used to make bread. As the ice melts this rock flour is exposed and transported away by melt-water, often into a nearby tarn such as Cracker Lake.

They won't settle down! . . . .
In addition to the rock flour, melt-water also carries larger pieces of rock material - pebbles, sand, and silt. These larger rock particles reach the lake and quickly settle to the bottom as long as the water is flowing (spring/summer). In contrast, the much smaller rock flour remains suspended in the water until the fall and winter when the melt-water stops flowing or the lake freezes over. Only then does the water become calm enough to let rock flour settle to the bottom.

Why so blue? . . .
Sunlight includes many different wavelengths of light ranging from the longer "reds" to the shorter "violets" (ROYGBIV). A white T-shirt is white because it reflects all of the wavelengths, a black shirt is colorless because it absorbs all of the wavelengths, and a red shirt is red because it absorbs the OYGBIV and reflects the R (red wavelengths). Apparently the tiny particles of rock flour suspended in the lake are just the right size to reflect more of the blues and some of the greens than any of the other wavelengths.

Seriously - "Cracker" Lake? . . .
Before the park was established, there was a copper mine at the head of the lake. According to legend the mine received its name when two prospectors, L. C. Emmonds and Hank Norris, after staking their claim, had a lunch of cheese and crackers on the site. Eventually the lake was named after the mine.

Below: In 2011 a team of researchers from Middlebury College in Vermont extracted a core sample from sediment in Cracker Lake. The core, which measured 3.19 meters, was analyzed along with cores from several other lakes in Glacier Park to better understand the advance and retreat of glaciers since the last glacial period ended about 10,000 years ago. Glaciers in the park completely melted as the last glacial period (sometimes referred to as the last "ice age") ended. Evidence from the lake sediment cores and along with other types of evidence indicate that glaciers reappeared in this area about 6,500 years ago, and then experienced several periods of advance and retreat. Periods of advance occurred from 6,900 to 5,700 years before present (BP), from 3,700 to 1,900 BP, from 1,600 to 1,300 BP, and then again during a recent cold period referred to as The Little Ice Age that ended around 1850 AD. Perhaps more importantly, evidence shows that the time period since the end of Little Ice Age is the most dramatic episode of ice retreat in the last 10,000 years.

Photo courtesy of Jeff Munroe, Chairman of the Geology Dept. at Middlebury

Term: tarn (cirque lake) - Be sure to tell how it is formed?

#92 - Montana's "Rock of Ages"

Above: Gates of Mountains Recreation Area 20 Miles North of Helena

If Montanans ever decided to select a state rock formation, a pretty good argument could be made for the Madison limestone. This "Madison Formation" (aka Madison Group) can be seen in many of our state's more recognizable landscapes (listed below). The photo above shows cliffs of Madison limestone near the southern end of the Gates of the Mountains Recreation Area. Below is a list of some other places where you can see outcroppings of Madison Limestone.

1. The Bridger Mountains (photo near Sacajawea Peak) and the Horse Shoe Hills (Bozeman area)
2. The Little Rockies in north-central Montana, including Mission Canyon
3. Lewis and Clark Caverns and Jefferson Canyon near Three Forks
4. The Sawtooth Range and Sun River Canyon between Helena and Glacier Park
5. The Little Belt Mountains, including Sluice Boxes State Park southeast of Great Falls and along the Smith River
6. Bighorn Canyon and the Pryor Mountains south of Billings
7. The mountains north and south of Lewistown (Judith and Snowy Mtns.)
8. The Jefferson Canyon south of Whitehall and the Headwaters State Park near Three Forks
9. The Castle Mountains southeast of White Sulphur Springs
10. Beaverhead Rock near Dillon

What is a "formation"? . . .
To a geologist a "formation" (or "group") is a thick layer (or series of layers) of a particular type of sedimentary rock covering a large geographic area. Some of Montana's more famous formations include the Hell Creek Formation, the Judith River Formation, and the Two Medicine Formation, which have all yielded significant dinosaur fossils. The familiar"Rimrocks" of Billings and the White Cliffs east of Ft. Benton are both exposures of the Eagle Formation. The Madison Formation consists of sediment laid down during the Mississippian Period of the Paleozoic Era (roughly 350 million years ago). It was laid down over most of Montana, eastern Idaho, northern Wyoming, and the Dakotas. The Madison is between 1,000 and 2,000 feet thick throughout most of the Montana.

Reading the rocks . . .
One reason geologist get so excited about rock formations is that they provide clues about what the environment was like in that location when the sediments were being deposited. For example the Eagle Formation consists of sand deposited near the shore of a shallow inland sea, whereas parts of the Judith River Formation indicate the presence of deltas much like the one at the mouth of the Mississippi River. Based on the types of fossils contained in the Madison Formation, geologists think its sediments accumulated on the floor of a tropical shallow sea. The map below shows where geologists believe Montana was located during this time (the Mississippian Period). Click on the map to enlarge it.

A deposit of calcite sediment . . .
Although it comes in many forms, all limestone is primarily made up of the mineral called calcite (calcium carbonate; CaCO3). This mineral is produced as marine organisms draw calcium carbonate out of the water in order to build shells or other hard parts. As these organisms die their soft tissues decay, but the shells, etc. made of calcite build up as sediment on the sea floor. Tropical waters also support abundant seaweeds that excrete calcite, which precipitates onto the seafloor as a light-colored lime mud, especially during periods of evaporation. Judging by the thickness of the Madison Limestone, this tropical marine environment persisted for many millions of years.

Terms: excrete, precipitate

#93 - Slabs of Belt Rocks Tell of an Ancient Basin

Above: "Belt Rocks" can be seen throughout much of western Montana, but one of the most interesting exposures can be seen along the Pintler Scenic Highway between Philipsburg and Anaconda. Here (above photo) the layers have been tilted, making the layers look like books leaning against a wall. Sedimentary rocks are formed in horizontal layers, but in northwestern Montana the horizontal layers of Belt rocks were messed up as colliding plates formed the northern Rocky Mountains roughly 80 million years ago. The mountains of Glacier Park formed as a large portion of Belt layers were thrust up and over younger sedimentary rock. Not only did this collision form many of the mountains of western Montana, but it also exposed layers that were buried deep beneath the Earth, giving geologists a look farther back in time.

Map courtesy of Mountain Press Publishing in Missoula, Montana: This map was borrowed from Northwest Exposures, a great resource for anyone wishing to learn more about the geologic past of the Northwest. The yellow X indicates where the photo at the top of this page was taken. The authors David Alt and Donald Hyndman contend that the Belt Basin (a.k.a. the Belt Sea) was a roughly circular area found on the continent, and that it continued to subside as sediments accumulated. Recent radiometric measurements indicate that the sediment was deposited from 1.470 billion years ago to 1.400 billion years ago (Evans et al., 2000; Ross and Villeneuve, 2003). This is a far shorter time interval than previous interpretations from the late 1960s, which suggested that the sediment was deposited between 1.5 billion years ago and 800 million years ago.

Must have been quite the basin . . . .
Over 15,000 feet (about 3 miles) of sediment was deposited in the basin. Its layers of beach sands, clear water limestones, shallow water sands, fine sand flats, and mudstones indicate that a variety of environments existed in the basin during the Precambrian Era. The Belt Basin covers a large region in Montana, Idaho, Washington, British Columbia, and Alberta. It is one of the deepest, best exposed, most accessible, and well-studied Mesoproterozoic basins in the world. Geologists refer to its thick series of rock layers as the "Belt Supergroup".

Clues in the layers . . .
The Belt rocks exhibit great examples of some features typically found in sedimentary formations, but other characteristics are missing. These features provide more clues about the basin, and in some cases raise even more questions about the Belt Sedimentary Basin.

1. mudcracks: Abundant between layers of Belt rocks; These were caused by surfaces that dried in the sun before being flooded by water that brought more mud or silt, which preserved the delicate features.

2. ripple marks: These suggest the presence of gentle waves in shallow water.

3. no animals: Although the Belt Supergroup includes plenty of extremely primative plants and bacteria, the rocks offer no trace of animal life. This helps explain why features such as mudcracks and ripple marks are so well preserved. There were no burrowing organisms in the Precambrian Era as there was in the more recent Paleozoic Era when sea floor sediments where more likely to be "bioturbated" by mollusks and the like.

4. no windblown sand: Although the basin was dry at times, there is no windblown sand. This puzzles geologists and begs the question, "did the wind not blow hard enough to carry sand?"

5. diabase sills: In places molten rock from below forced its way between layers forming sills of hardened magma. Unlike sedimentary rock, scientists can determine the age of igneous rock using radiometric techniques. These dates can help establish the ages of various layers in the Belt Formation.

6. reds and greens: Many of the layers of mudstone in the Belt Supergroup are colored interesting shades of green or reddish purple. The red, purple, and maroon color is due to the a red mineral called "hematite" (Fe2O3, which is formed as iron reacts with oxygen from the atmosphere. It is believed that the green rocks were formed in deeper water where oxygen was less available. Instead the iron combined with silica to form iron silicate compounds that changed into a green mineral called chlorite as heat and pressure altered the rock. Alternating green and red layers suggest fluctuations in the depth of the Belt Sea (a.k.a. Belt Basin).

Below: All of the rocks in Glacier Park are part of the Belt Supergroup, including the colorful layers of the Grinnell formation shown in this photo by Jeff Kuhn of the Glacier Institute.

Term: subside, basin, Precambrian Era, bioturbated

Sources:

Alt, David, and Donald W. Hyndman. Northwest Exposures. Missoula, Montana: Mountain Press Publishing, 1995.

Evans, K.V., Aleinikoff, J.N., Obradovich, J.D., and Fanning, C.M., 2000, SHRIMP U-Pb geochronology of volcanic rocks, Belt Supergroup, western Montana; evidence for rapid deposition of sedimentary strata: Canadian Journal of Earth Sciences, v. 37, no. 9, p. 1287-1300.

Ross, G.M., and Villeneuve, M., 2003, Provenance of the Mesoproterozoic (1.45 Ga) Belt Basin (western North America): Another piece in the pre-Rodinia paleogeographic puzzle: Geological Society of America Bulletin, v. 115, p. 1191-1217.

For much more about the Belt Supergroup, CLICK HERE.

#94 - Where Cement Comes From

Limestone Quarry . . .
The photos on this page were both taken near Montana City, a community located 5 miles southeast of Helena. Here a light-colored rock, called Madison Limestone, can be found just beneath the soil, making this a convenient place for a quarry. Ash Grove Cement Company removes limestone from the quarry and hauls it to a plant just a few miles away where it is used to make cement. A quarry is sort of like a mine. However, at a mine the rock (called ore) is removed because it contains a valuable metal, which must then be removed by processes such as milling, leaching, and smelting. On the other hand, at a quarry it is the rock itself that the company is after.

A deposit of calcite sediment . . .
The Madison limestone is primarily made up of the mineral called calcite (calcium carbonate; CaCO3). This rock is made of sediment that was deposited on the floor of a shallow ocean about 340 million years ago. Marine organisms took calcium carbonate out of the water and built shells or other hard parts. As these organisms died their soft tissues decayed, but the shells, etc. made of calcite built up as sediment on the sea floor. Judging by the thickness of the Madison Limestone, found here and in other parts of Montana, this tropical marine environment persisted for many millions of years.

Why limestone . . .
Limestone is primarily made up of the mineral calcite whose chemical formula is CaCO3. As Ashgrove heats the limestone (CaCO3 aka calcite) to 1000 C lots of CO2 is given off as the calcite changes to CaO. This step releases so much CO2 into the atmosphere that cement production is one of the top five sources of the CO2 emissions in the USA. In the next step, the CaO is heated to 1500 C and it reacts with silica to form calcium silicate. The most common combination of materials used to make cement is limestone, clay and sand. These materials are crushed and then processed in a furnace called a kiln where temperatures reach 1500 C (2730 F). The intense heat causes chemical reactions that convert the partially molten raw materials into pellets of calcium silicate called clinker. After adding some gypsum and other key materials, the mixture is ground into the extremely fine gray powder that we call "cement".

The carbon cycle . . .
Limestone stores huge amounts of carbon for millions of years. As Ashgrove heats the limestone, this carbon returns to the atmosphere in the form of CO2. Another (natural) way that the carbon can return is through volcanism. If this limestone were ever to be melted into a magma, the carbon may be expelled as CO2 when the volcano erupts. Based on the current motion of Earth's tectonic plates, it doesn't look like this will be happening to the limestone in Ashgrove's quarry any time soon.

What's the difference between cement and concrete? . . .
Although the terms cement and concrete often are used interchangeably, cement is actually one of the ingredients used to make concrete. The other ingredients are sand and/or gravel (aggregate), and water. Typically, concrete is about 10 to 15 percent cement, 60 to 75 percent aggregate and 15 to 20 percent water.

If it can't be grown it has to be mined . . .
No one likes to see the land torn up, but we all benefit from buildings, roads, and sidewalks made of concrete. Although 93 % of U.S. highways are paved with asphalt, 40 % of our interstate highways are made of concrete.

Above: Here is a closer view of Ash Grove's limestone quarry at Montana City.

Term: aggregate

#95 - Shock Wave Frozen in Stone on Rim of Impact Crater

A blast from the past . . .
In 1990 geologists discovered evidence of an ancient asteroid impact in the mountains of southwestern Montana. The team found rocks with distinct cone-shaped fracture patterns resembling horsetails. They recognized that these were “shatter cones”; the unique fracture patterns formed as intense shock waves generated by an asteroid impact travel through bedrock.

Right: This photo, which was provided by Mike Plautz, Science teacher at Hellgate Elementary in Missoula, shows a classic shatter cone at a location scientists refer to as the "Beaverhead Site”. The presence of shatter cones here puzzled scientists because the area lacked visible evidence and other characteristics typically associated with impact craters.

Solving the mystery . . .
Scientists have identified over 160 impact craters on Earth. Unlike the surface of the Moon where impact craters dominate the surface, most of Earth’s craters are much more difficult to identify. On Earth there are many processes that wear away or hide the craters, including erosion, vegetation, seafloor sediments, lava, and plate tectonics. So, in order to find impact craters scientists search for the following clues.

1. Evidence of rocks that have been changed by shock waves (shatter cones)

2. Craters, or geology that indicates the presence of a crater

3. Geophysical anomalies: variations in gravity and/or magnetism that stand out as “unusual”

4. The presence of meteorites: fragments from the asteroid (or comet)

The Beaverhead Impact Structure . . .
Although the shatter cones found in southwestern Montana were the first clue that there had been an impact, scientists eventually determined that the actual crater is centered about 50 miles southwest of the Beaverhead Site around the Challis, Idaho area. The mystery of the crater’s location was solved when scientists found that some of the rocks in the area around Challis had different properties when it came to magnetism and gravity readings. As it turns out these geophysical anomalies revealed a crater-shaped pattern indicating that the crater was probably about 100 km in diameter (65 miles). Radiometric dating done in 1999 suggested that the impact happened about 900 million years ago. . . . No wonder it was so hard to find! The crater, which was named the Beaverhead Impact Structure, is one of only eight known impact craters over 50 km in (32 miles) diameter.

Below: These diagrams are from a masters thesis paper by A. E. McCafferty, Colorado School of Mines. The top one shows a (proposed) cross-section of what the geology of the crater area might have been like soon after the impact 900 million years ago. The bottom one shows a cross-section of the area today. Compare the location of the points labeled “Grouse Peak” and “Beaverhead Site” on the two diagrams. Geologists believe that, since the impact happened, the top part of the crust moved several miles to the east causing these two points to be offset from their original position on oposite sides of the crater. This type of "thrust faulting" was common in the area during the formation of the Rocky Mountains. The yellow area indicates location of bedrock that provided the unusual magnetic and gravity readings.

Sources:

McCafferty. A.E. , 1995, Assessing the presence of a buried meteor impact crater using geophysical data, south-central Idaho: Masters Thesis, Colorado School of Mines, 88p.

Carr, J., and Link, P.K., 1999, Neoproterozoic conglomerate and breccia in the formation of Leaton Gulch, Grouse Peak, northern Lost River Range, Idaho: Relation to Beaverhead Impact Structure, in Hughes, S.S., and Thackray, G.D., eds., Guidebook to the Geology of Eastern Idaho: Pocatello, Idaho Museum of natural History, p. 21-29.

Term: anomaly