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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.
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.
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!
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
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.
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.
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.
Local/Montana Area Questions and Answers
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.
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.
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.
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!
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.
These 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. Click postcard for larger image!
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.
Answers provided by our MBMG staff