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What does water have to do with geology?

A lot more on Earth than on any other planet in our solar system. Earth is sometimes referred to as the water planet, and at or near the earth’s surface water and geology are very tightly connected. For example, close to 75% of Montana’s residents depend on ground water for their water supply. This is water contained in the rocks; we drill wells that act like big straws for us to suck (pump) the water from below ground. The rest of our residents get their water from streams, but these streams also wear away at the rocks and move the rock particles downstream where they are eventually deposited as mud or sand (sometimes making rich farmland). Think of all the river valleys where running water, or the high mountains where glaciers (ice), have created scenery that we enjoy.

Water is important in many, many other ways. Without it, rocks would not break down chemically to form soils, and deep in the earth at high temperatures and pressures water promotes the change of crystals from one mineral to another (metamorphism). It is involved in most landslides, as water tends to lubricate soil or rocks and enable them to move under gravitational forces. These are just a few examples. On Earth, water and geology really cannot be separated.

What's morphing?

I dunno. Instead let’s try the word “metamorphism” which means a “change of form.” When rocks are buried deeply in the earth they are under high temperatures and pressures. This causes the mineral grains in some rocks to react and change to other minerals. The rock itself may be drastically changed. As one example, imagine the change that happens when a potter makes a pot out of clay and then puts it in a kiln at high temperatures. After a while, it comes out hard and brittle, quite unlike the soft clay that went in. How a rock will actually change depends on many factors, including the makeup of the original rock and the conditions it was subjected to, but the results can be outstanding.

Why does lava come out of volcanoes that have been dormant for so long?

Lava (or magma as it is called when it is still underground) rises from deep in the earth and commonly collects in pools (magma chambers) a few miles beneath the earth’s surface. Some of it may continue to the surface where it becomes a volcanic eruption, but the remainder can sit inside the earth for many years while remaining liquid. This allows a volcano to erupt, become dormant, and then erupt again from the same underlying magma chamber. Of course, there is always the chance that a new batch of magma may rise from great depths along the same path that an earlier batch did, thus reactivating the volcano again.


How deep is the earth's crust?

The earth’s crust is thinnest under the oceans, thicker under the continents, and thickest under large mountain ranges. Under the ocean, the crust may be only a couple of miles thick, but under the continents it is more likely to be on the order of 15-20 miles thick. Under a mountain range such as the Rockies it may be 40 miles or so thick, but it is probably not quite that thick under the Appalachians. Think of mountains like an iceberg; you see part of an iceberg floating above water, but most of it is below water, and the higher the part that you see, the deeper the bottom of the iceberg is.

Is the core really lava?

Because of the way earthquake waves behave as they pass through the core, we believe that the outer part is liquid. The inner part—well, many folks think it is solid. Isn’t it amazing what we think we can tell about something we’ve never seen and that is 3000 or 4000 miles deep?


What is the first volcano's name?

We don’t know. Very early civilizations in Africa and the Mediterranean regions must have witnessed volcanic eruptions and had names for them, but we don’t know what those names were. We do have good records for volcanoes in those parts of the world (particularly Italy, where Mt. Etna and Vesuvius are) that were scaring and killing people several thousand years ago. However, the earth’s first volcanoes were erupting billions of years ago early in the earth’s history, when there was no one around to see them or name them.


How is lava formed in the volcano?

Lava (or that magma stuff again) forms in the earth’s mantle or the lower part of the crust. A big chunk of the mantle or crust probably does not actually sit there and melt. Instead, melting probably occurs around the boundaries of the mineral grains that make up the rocks, and then it accumulates to form bigger globs of melt. When it gets big enough it may then rise to the surface. Think of it like a bowl of ice cubes. Water accumulates because all the ice cubes melt a little bit, rather than one ice cube melting completely while the others are unaffected. At first there is just a little bit of water, but as more ice melts, more water forms.


What's the difference between how a mountain forms and how a volcano forms? What happens differently between them that makes them different?

There are three main ways that mountains form. Volcanoes are probably the easiest to visualize, because we can see movies or TV coverage of lava coming out of the earth and piling up to form volcanic mountains. The two other main ways are folded mountains and fault-block mountains.

Folded mountains most often form from collisions between crustal plates. Imagine pushing the ends of a rug toward each other; the rug gets pushed into folds. Or two cars in a head-on wreck—the fenders get crumpled, bent, and broken. The same thing happens when plates, especially continents, collide. The rock layers get bent and crumpled and pushed up into mountains. The great example of this is the Himalayas—the highest mountains on earth.

Now imagine that instead of a rug we have something like a concrete sidewalk that gets broken up. After a while, the pieces do not match smoothly; instead one piece will be higher and the piece next to it is lower. On the scale of continents, offsets between broken pieces like this may become mountains. The actual break between the pieces is a fault.

In reality, most large mountain chains include all three types of activity rather than being strictly volcanic mountains or folded mountains.



Does the earth need lava? If so, why?

I never thought about whether the earth needs lava! The earth contains radioactive elements, and when these atoms undergo radioactive decay they split into a couple of new (usually simpler) particles. This gives off heat. Although the amount of heat is small, with lots of these happening, it builds up, and one effect is that some rocks may begin to melt. If that molten rock rises to the surface to become lava, lots of heat is removed from the earth’s interior. Therefore, in a way, forming lavas helps to cool off an overheated earth.


I was wondering, is lava hotter than the sun?

I don’t have a number handy for the temperature of the sun, but it is thousands of times hotter than lava flows. The hottest lava flows are normally around 1400 degrees C (~2500 degrees Farenheit). Mighty hot, but not close to that of the sun.


How large does the hole have to be for the magma to emerge?

Is this a trick question? Actually, despite the way lava flows look in pictures and movies, it is a lot more like cold molasses than water when it moves. Therefore, it doesn’t really flow that easily. If you turn a ketchup bottle upside down, the ketchup will probably not run out, but if you squeeze it (apply pressure to it), you get ketchup, and the more you squeeze the more you get. Rather than squeezing, you could also get ketchup by cutting the top off the bottle.

Now what does this have to do with magma? Magma doesn’t flow easily, but it commonly is under high pressures—either from the weight of rocks around and above it, or from gases that build up inside it (think about what happens when you put your thumb over the top of a coke bottle and shake it up). That pressure can force magma into or through small cracks, or gas pressures can cause very large and explosive volcanic eruptions (like when you take your thumb off the top of that coke bottle you just shook up). Now, don’t go home and do it in the house!


How does the earth's core stay solid when it is so hot?

That does seem odd, doesn’t it. Actually, despite being so hot, the rocks in the earth’s mantle and core are under extremely high pressures from the weight of all that rock above pushing down. This pressure counteracts the tendency of rocks to melt from heat. If we could reach into the earth and grab a barrel of rock from the core or mantle and suddenly bring it to the surface without losing heat, it would surely melt instantly. However, we do think that at least the outer part of the earth’s core is molten, and only the center of the core may be solid. Part of this difference may be because the composition of those areas is different, not because of differences in temperature and pressure.


How do scientists know what's inside the earth if you've never been past
the crust?

We do lots of guessing. Actually, it is kind of like detective work. For example, we find rocks that used to be deeply buried and study them to see how they are different from rocks that formed at the earth’s surface. Geophysicists study things like earthquakes and how the waves pass through the earth. The waves will usually travel faster in denser rocks, and some waves will not travel through liquids—therefore the interpretation that the outer core is liquid, because some EQ waves do not travel there. We can also measure things like changes in the earth’s gravitational attraction, the amount of heat that flows out of the earth, and various electrical properties of the rocks—even though they may be very deeply buried. Then scientists put all this information together and try to make some sense out of it. Sometimes it works, but sometimes we just get more questions.

Do geologists and paleontologists work together on dinosaur digs?

Generally, no, these two groups of scientists do not work directly together on the digs. The relationship between the two sciences is that geologists make maps of the types of rocks that are present in an area, and paleontologists study these maps and decide what locations look promising for dinosaur remains. In looking for promising locations, paleontologists may already have seen dinosaur remains in a certain rock formation and are now looking for more places where this rock formation can be studied, or the paleontologists just suspect that a certain rock formation might have dinosaur remains because that rock was formed from sediments laid down during the time when dinosaurs lived.

For example, in Montana one of the most well-known dinosaur digs is in the Choteau area, at "Egg Mountain." Here, the early work began because dinosaur remains were found by local residents in the area. When paleontologists became involved, they noted two kinds of information on the available geologic maps – (1) the rock formation in which the remains were being found was of Cretaceous age, the height of the Age of Dinosaurs, and (2) the specific rock type was sandstones and mudstones of the Two Medicine Formation, once part of a broad low-lying alluvial plain that was good country for dinosaurs to live in. Thus, paleontologists knew that there was the possibility for finding extensive dinosaur remains in the area. As the digging at Egg Mountain proceeded, the paleontologists continued to explore within the Two Medicine Formation as mapped by the geologists.

Another example of using geologic maps to find dinosaur remains involves the Morrison Formation, made of sediments laid down on an alluvial plain in the Jurassic Period, about the middle of the Age of Dinosaurs. This rock unit, composed of red- and green-colored mudstones and yellowish sandstones, contained some of the earliest-discovered dinosaur remains in the Rocky Mountain region. Thus, whenever paleontologists study a geologic map of any western area and see the name Morrison Formation they wonder whether it would be useful to go to that locality and look around.

Places to look for more information on dinosaurs and paleontology
The Judith River Dinosaur Institute   Museum of the Rockies, Paleontology
Museum of Paleontology, Universit of California, Berkeley   National Geographic News

How much does petrified wood weigh?

OK, first, we've have to decide how large the piece of petrified wood is, because like anything else (cars, a sack of potatoes, kids) a big one will weigh more than a little one. To help get around this question of having to ask the size of the rock (or whatever) each time, scientists use a measurement called specific gravity, that allows us to compare different materials. To choose a few materials for comparison, water has a specific gravity of 1.0. Petrified wood (usually composed of the mineral quartz) would have a specific gravity of 2.65, so it is a little more than 2 1/2 times the weight of an equal amount of water. Gold has a specific gravity of 19.3.

To make this a little more useful, imagine that you have a plastic container the size of a common brick (2 1/4 inches x 4 inches x 8 inches) and fill it with water. It would weigh a little more than 2 1/2 pounds. A piece of petrified wood the size of the brick would weigh nearly 7 pounds. And a gold brick would weigh about 50 pounds. Don't you wish you had that!

Most common rocks will have a specific gravity fairly close to that of the petrified wood used as an example. Pumice, a type of volcanic rock that is full of air bubbles, has a specific gravity less than 1—it will float on water. Most metals, especially gold, have much higher specific gravities. That's what enables us to recover gold by panning.

How can a rock become parts a car?

Every chemical element that occurs naturally on earth can be found in a rock somewhere (there are a few man-made elements, but they don’t count here). Rocks that are rich in certain elements can be mined and the elements separated by various techniques. Sometimes this is fairly easy (and therefore relatively cheap); for example, rocks containing copper minerals can be mined, ground finely, and the copper-bearing minerals can be separated by a process call flotation. These mineral particles can then be smelted (heated until they are molten) and the copper (and a few other elements) are separated as metals. In this process, US mines will typically produce 6-10 pounds of copper for every ton of ore that is processed. Other elements are not so easily separated from their host minerals, and require much more extensive treatment to be recovered, making them more expensive, particularly if their concentrations in the original rock are very low.

So, once we mine and separate the stuff we need from rocks, how much of a car is made of rock materials? Nearly all of it! The most abundant metals will be things like steel (mostly iron with small amounts of things like carbon, tungsten, molybdenum, nickel, and others) for the frame, aluminum, copper for wiring, zinc for rust-proofing, chrome to make shiny trim.

The rest of the car is mostly plastics. Where do plastics come from? Most are made from petroleum products---more stuff that is recovered from rocks, although it is usually pumped and not mined. However, the materials to make the plastics are relatively expensive and by themselves do not always have the properties that are necessary to make a useful product. So, certain minerals may be ground up and added to the mixture. This serves two functions---the minerals are fairly cheap and they can change the properties of the plastic. In the industry, these minerals are known as fillers and extenders. Some of the more commonly used minerals are feldspar (the stuff that makes up about 70% of a typical granite), calcite (the mineral that makes limestone), clays, and talc.

So, without the materials produced by mining and petroleum geology, we would probably have to build our cars out of wood. This was common a hundred years ago; they were pulled by horses and were called wagons.

What causes an earthquake?

We feel an earthquake when seismic waves travel through the ground beneath our feet. Seismic waves travel through rocks in the earth’s crust much like ripples spread across the surface of a still pond after dropping in a rock. A cork floating on the pond will bob up and down when the spreading ripples pass beneath it. Although seismic waves in the earth travel much faster than ripples on a pond (and have some other fundamental differences also), it is the passage of seismic waves that causes the shaking motion we feel as an earthquake.

Sudden slippage along fractures in the earth’s crust (faults) causes seismic waves to form, which then radiate away in all directions. The greater the amount of slippage along a fault, the stronger the seismic waves. Like other forms of energy that travel as waves (sound and light for example), seismic waves lose energy as they spread out through the earth away from their source. This explains why damage caused by a large earthquake is generally greatest closer to the epicenter; the seismic waves are stronger closer to where they originate.


When do you think the next earthquake will happen in Montana?

I’m guessing that you want to know about the next big earthquake that will shake you out of bed. Now let me check my crystal ball. We really cannot predict when that may happen. From the late 1800s until 1959, Montana had a "big" earthquake that resulted in significant damage to buildings about every 10 years. Since the big one at Hebgen Lake in 1959 there have been no damaging quakes, but you might be surprised to find out that Montana has earthquakes every day. Nearly every one is too small to be felt, but the Montana Bureau of Mines and Geology has a network of 35 seismometers (mostly in the western third of the State) that detect several thousand earthquakes in an average year. Surprising how noisy things are when you have the right "ears" to listen with.

Despite brave predictions from some seismologists several decades ago that we'd soon be able to predict earthquakes, we're still not there. However, history says that another big one will come, and that when it does it will most likely be near Yellowstone National Park. For more information, take a look at the Earthquake Studies section of the Bureau's website.

If there were a big earthquake in Butte, would some of the abandoned mine tunnels collapse?


It is possible that a few unstable mine workings might collapse during a strong earthquake, but it seems unlikely this would be a serious problem to those of us living in Butte. First of all, many of the mine workings existed when the 1959 Hebgen Lake earthquake (magnitude 7.3) shook Butte but little if any damage was done to the mines. Natural underground openings, such as Lewis and Clark Caverns, have existed for millions of years and survived many earthquakes. Once the ceiling of the underground opening attains an arched shape, it is very stable and resists collapse.

Another reason that earthquake shaking may not damage mine works is explained by a phenomenon known as the free surface effect. The earth’s surface shakes more strongly during an earthquake than points within the earth below the surface. The difference in shaking intensity depends on both the frequency of the seismic waves and the depth below the surface. There are numerous reports from miners that were underground at the time of the 1959 earthquake and thought that it was a minor earthquake or a small blast, if they felt it at all, thus indicating it was not felt as strongly at depth as it was on the surface.

Given the number of old brick buildings, Butte is likely to face much greater problems than collapse of abandoned, underground workings during a big earthquake.

Are there any exposed faults near Butte?

There are literally thousands of faults in western Montana. Most of them are small and formed millions of years ago; only a few dozen faults show any evidence of having moved during geologically recent times (past 2 million years). The most significant fault near Butte trends north-south along the western front of the East Ridge. This fault—the Continental fault—allowed the East Ridge to move up to its present elevation relative to Summit Valley in which Butte sits. Nobody knows when the last earthquake along the Continental fault occurred but the available geological evidence suggests that it was at least several tens of thousands of years ago.

The Continental fault is exposed in Montana Resources’ Continental Mine. It is more difficult to see the fault in undisturbed areas along the base of the East Ridge because eons of erosion have washed soil and rocks off the steeper parts of the East Ridge and deposited these materials on top of the fault at the base of the ridge. Interstate 90 crosses the fault in the small dip just before the steady uphill grade of Homestake Pass begins. If you pay attention to the granite boulders that outcrop east of the highway as you approach the aforementioned dip, you can notice a fairly sharp line below which no boulders outcrop. A line marking the lower-most boulders (ignoring those few that have tumbled down the hill from above) traces out the Continental fault.

Why were some of the mines in Butte hotter than others?

The temperature in an underground mine depends partly on how deep it is, increasing something like 1 degree Fahrenheit per 200 feet of depth. But there are other causes too. Different mines in the same deposit may vary in temperature because of stresses within the rock from folding and rock deformation, decay from radioactive minerals, or geothermal heat from heat sources within the earth's crust—like intrusive bodies (molten rock) deep below the workings.

Some of the mines in Butte got pretty hot down there. Ask your family, or some of the ex-miners around town—they'll probably have some pretty good stories for you!


Is there any evidence of large meteors striking Montana like in Idaho or Arizona?

Evidence for at least one meteorite impact is known to exist in southwestern Montana. It is very interesting because the evidence is contained in rocks that are about 1.4 billion years old, and the impact is thought to have happened about 900 million years ago. Since that time, these rocks have been squeezed, pushed, broken, pulled apart, pushed together, and eroded, so that nothing that looks like a meteor crater is present. Interestingly, the actual impact may have happened in Idaho and then millions of years later tectonic forces pushed the rocks into Montana.

So, after all this, what is the actual evidence for an impact there? Large impacts sometimes create structures called shattercones in the rocks at the impact site. To the trained eye, these structures are distinctive, and look like those found around more modern and well-documented impact sites. So, no crater, but there is a smoking gun.

How big is the Berkeley Pit?

From rim to rim it is about 5000 feet by 7000 feet, so it covers a little more than 1 square mile. It is nearly 2000 feet deep.

Why is it filling with water?

The spaces in the rocks under the Berkeley Pit were originally filled with ground water. In order to keep the underground mines (that lie even deeper than the Berkeley Pit) from being really deep swimming pools, the Anaconda Company installed pumps to move the water to the surface and keep the mines dry. Over years of pumping, the water in the vicinity of the mines was removed, but surrounding rocks still contained water, in effect creating a deep dry hole in otherwise wet rocks (something called a cone of depression). When underground mining stopped and the pumps were no longer necessary, guess what? Water started flowing in from the surrounding rocks to fill this dry hole. At first, the deep mines were filled up, but as the water level rose it eventually intersected the bottom of the Berkeley Pit and a pit lake formed. Water continues to flow in from the surrounding rocks, and surface water from precipitation and drainage also adds to the inflow. The water level in the pit is currently rising at about a foot per month.

Will it ever overflow? If water flows into the Berkeley Pit, could conditions ever change so that it would flow out the other way?

No. Left on its own, the water level in the Berkeley Pit would rise only to a level equal to that of the original water table—still below the rim of the Pit. After that, general ground-water movement would be to flow through rock fractures and the water would eventually end up in Silver Bow Creek. However, under government regulations, the water level will be controlled by pumping and treating water to a quality suitable for release into Silver Bow Creek. This level will be lower than if left to seek its natural level, so that the Berkeley Pit will continue to be the “drain” for surrounding rocks. If a river were flowing into the Berkeley Pit, it is possible that it could overflow, but that is not the case.

If the minerals in the water are naturally occurring, why are they toxic?

Interesting question. Our bodies need small amounts of many minerals (really chemical elements), but in large (or sometimes not so large) amounts these can overwhelm the body systems and become poisonous. For example, many communities add very small amounts of fluorine to our drinking water to provide necessary stuff to strengthen our teeth and prevent tooth decay, but at higher concentrations fluorine starts to harm us. Even pure water itself could be toxic if you managed to drink so much that the body could not cope with it. In the Berkeley Pit, many metals concentrations are high enough that the water certainly would harm us if we drank it. On the other hand, there is much talk about how acid the water is. In reality, the acidity is about the same as a carbonated soft drink.

Why don't plants and trees grow in the soil on the terraces of the Berkley pit?

This is probably due to a combination of reasons rather than any single reason. First, there is little or no soil on those terraces. They are mostly rock and small rock fragments that have not sufficiently weathered to produce a soil that larger plants need to survive. Additonally, the terraces are directly exposed to extreme temperatures and wind and lack consistent water that plants need, particularly for young plants to get started. Undoubtedly there are small plants such as lichens that grow on the terraces; they are not visible unless you can look closely, but in time they will help to break down the raw rock materials and form soils.

Is that red volcanic rock alongside the road to Fairmont?

Yep. Those red rocks are part of a large volcanic field that developed north and west of Butte about 50 million years ago. Big Butte (or “M” mountain) above Tech is part of this pile of volcanic rocks. Erosion has ensured that this area does not look like a volcano any more, but the rocks that are left give us some clues as to what it may have looked like.

Is the East Ridge above Butte geologically different from the Butte Hill? Why wasn't mining done on the East Ridge?

The East Ridge and Butte Hill are both underlain by basically the same granitic rock (called the Butte Quartz Monzonite by geologists). However, under the Butte Hill hot fluids (mostly water) moved through fractures and altered the minerals in the granite. Immense quantities of metals, including copper, molybdenum, zinc, lead, manganese, and others were brought in and deposited in fractures, or in some cases actually replaced previous mineral grains in the granite, creating an orebody. However, all good things must come to an end. The orebody exposed at the surface on the Butte Hill has limits, and the eastern limit mostly stops short of extending under the East Ridge.


Were/are there volcanoes in Montana?

If you take a look at the geologic map of Montana depicted on the Bureau’s post card, you will see areas of rocks labeled as “igneous extrusive”. click for larger view of postcardThese are volcanic rocks and most of them are concentrated in southwestern or north-central Montana. The volcanoes that produced most of these rocks have not been active for 50 million years or so, and because of erosion they do not look like volcanoes any more. Some of the better exposures of these volcanic rocks are preserved southwest of Great Falls where I-15 traverses the Missouri River Canyon between Craig and Cascade. Here, peculiar structures that look like rock walls extend for miles across the country side; geologists call these structures dikes, and they represent magma that rose in fissures toward the earth’s surface but froze before it actually made it there.

Yellowstone National Park itself is a volcano (caldera) that produced huge eruptions of volcanic ash only 600,000 years ago. Although the caldera does not extend into Montana, large amounts of the ash it produced certainly came across the border.


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