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    Geology, Gems, and Minerals

Geology, Gems, and Minerals

The first part of the hall is The Solar System through Time, showing the evolution of earth’s 8-planet solar system over the past 4.6 billion years. A 3-minute video here explains how the sun formed first, and then its gravity attracted the space dust and rocks that formed the other planets. Beyond this is a massive collection of meteorites collected from around the world, explaining where these come from in the universe, what they’re made of, and how you can tell whether a rock is a meteorite (spoiler alert: a good clue is if the rock has high concentrations of elements like iridium, generally rare on earth).

Another section of Geology deals with how earth’s moon was formed. Before men landed on the moon in 1969, there were three competing theories about the moon’s origin:

The co-formation theory said that the Earth and the moon were formed from the same collection of cosmic dust and rocks. Most of the material coalesced into the Earth, while a smaller portion formed the moon, and the moon stayed in orbit around the Earth because of our planet’s gravitational pull. While it’s true that the Earth and moon are composed of similar elements, but the moon is far less dense than the Earth, something the theory doesn’t explain.

The capture theory said that the moon was formed somewhere else in the solar system, passed by the Earth and got caught in its orbit. That would explain why the moon is less dense than the earth while still being composed of similar elements. However, the moon’s round shape and elliptical orbit around earth aren’t like those of other captured moons in the solar system.

The giant impact hypothesis says that a planet about the size of Mars crashed into Earth around 4.5 billion years ago, shortly after the Earth was formed. That kind of collision threw massive amounts of debris into space, which, because of gravity, stuck together and formed the moon.

When the first astronauts returned from the moon, they brought back rocks from the lunar surface. Analysis of these rocks, and more evidence from later missions, aligned closely with the giant impact hypothesis. Today, the giant impact hypothesis is generally accepted by most scientists.

After the solar system is an informative section on plate tectonics - definitely enough for some chit-chat during your next cocktail party. Prior to around 1900, the accepted theory of how the continents and mountains formed was that the Earth was once covered by a sea of molten lava. As the lava cooled and contracted, it formed the Earth’s land, mountains, and valleys, including under the sea. (The folks at Penn State liken it to a grape shrinking into a raisin, if that helps.)

That theory had its problems, though. For one thing, anyone who’s looked at a globe can see that the continents’ shapes kind of fit together like a giant jigsaw puzzle: the right side of South America fits into the southwest side of Africa, while Madagascar, Antarctica, India, and Australia fit on Africa’s southeast.

The other problem with the theory was that scientists kept finding similar species of plants and animals on different continents separated by thousands of miles of ocean. Some folks proposed the idea of trans-oceanic “land bridges” that once connected the continents and allowed the animals to migrate, but little evidence of these massive connections was found.

In 1912, a German named Alfred Wegener came up with the idea of plate tectonics, saying that all of the continents were once joined together into a single continent some 300 million years ago, and had broken apart and drifted into their current positions. Wegener couldn’t explain how the plates moved, though, and his theory was not widely accepted. Then in 1929, a Briton named Arthur Holmes proposed that Wegener’s theory would work if the Earth’s crust floated on an inner core of molten rock, with the earth’s rotation and thermal convection moving the crust around. It took until the mid-1960’s, when exploration of the deep ocean and sea-floor spreading, to confirm these theories.

Today, plate tectonics, continental drift, and a molten-core Earth are the accepted explanations for natural phenomena such as earthquakes, volcanoes, mountain ranges, and more. The gallery covers each of these in detail, showing how the earth’s huge plates come together and split apart.

Past the display on plate tectonics, volcanoes, and earthquakes is the Rocks Gallery, with a variety of short displays on the many features of rocks (we’re not making this up). For example, there are small sections of the room dedicated to how rocks erode and affect the flow of rivers and streams; how changes in rock formations can be linked to large geologic events; how rock columns form; and how rock like sandstone, found all around Washington DC, is used to decorate the outside of many buildings because it’s easy to carve.

One of the best sections of this hall is the Mine Gallery, built to resemble the inside of several important mines from around the United States. The Franklin-Sterling Hill mine in New Jersey, for example, contains more than 330 mineral species, many of which glow in the dark. The museum dims the lights in this part of the mine to show how these minerals fluoresce, and kids love it.

The next mine shown is the Copper Queen from Bisbee, Arizona, which produced more than 8 billion pounds of copper during its 94-year run. The thing the mine is known for, though, is the minerals found there. The simulated underground caves here are the big draw, and show how the caves were formed and the beautiful and unusual minerals found.

The final mine showcased is the Morefield Mine from Virginia. It’s small for a mine, around a thousand feet, but it has produced a lot of blue-green amazonite, used in ornamental jewelry. If you’re headed southwest on your way home from DC, it’s about 45 minutes on the other side of Richmond, just off US Highway 360.

The final part of the Mine Gallery shows native metals such as gold, silver, and platinum; how miners look for veins – concentrations of metals in a small area - that are easy to mine; and the difference between a mineral, metal, and ore.

Almost all of the remainder of this gallery is dedicated to exhibits on gems and minerals. Be forewarned that there are thousands of rocks on display here, and it would take a day or more to examine the details of each one. We’ve outlined the hall’s major subjects below, such as “diversity” and “how gems grow,” and we think the sections on crystal color and shape are not to be missed. If time is short, the middle of this hall forms a “Fast Track” summary of the displays on either side.

Unofficial Tip: Regardless of whether you take the short or long tour through gems and minerals, don’t miss the Hope Diamond, in its own room at the end of the hall to the right.

The first section of the hall covers pegmatites, including exotic minerals such as lithium and beryllium. These are metals, and what makes them special is their light weight. Beryllium, for example, weighs about 1/4th as much as iron or steel, making it excellent for everything from airplanes to golf clubs.

Next up is a section on how crystals grow. Two examples here, under the heading Amazing Gems, are diamonds and calcite. Both diamonds and calcite can be cut into gem shapes, but calcite is far softer, the result of it forming from decaying animals (such as mollusks) and from water interacting with calcium-rich rocks like limestone. Diamonds, on the other hand, are formed by immense pressure and heat, and are much harder.

The next section covers the diversity of minerals and how chemical composition is used to classify them: Silicates are formed between silicone and oxygen and form the vast majority of the Earth’s crust; oxides are pairs of metals and oxygen; carbonates combine carbon and oxygen; borates are boron and oxygen, and so on. An interesting display here is the asbestos case, because these minerals look like rocks with fur.

One of the most kid-friendly areas in Gems & Minerals is on color; its centerpiece is a huge display of minerals arranged to form all the colors of the rainbow. Minerals get their color in two ways: the base color of a pure mineral is always the same regardless of where it’s found, because it’s based on the wavelengths of light that are absorbed by the molecular structure of the mineral. For example, rubies are red because red light isn’t absorbed by the aluminum and oxygen atoms that make up all rubies.

The color of a mineral is also affected by impurities that are mixed in with its elements. The museum demonstrates how mixing a tiny amount of copper in with other minerals produces a green tint, because copper doesn’t absorb green light.

That said, the museum has displayed some minerals with very interesting color properties: Feldspar’s uncommon crystal structure reflects a rainbow of colors depending on the angle at which it’s viewed; and alexandrites, with a small amount of chromium in them, look red under a household incandescent light, but green under a fiber optic light.

The final major section of gem characteristics covers the shape of gems and minerals. The primary mechanism that determines a mineral’s shape is its atomic structure, since the atoms appear in regular patterns throughout the stone. Other things that influence shape are the relative concentration of those elements to each other; the level of impurities in the rock; and the temperature at which the rock was formed. The museum shows off dozens of different shapes for the same basic calcite crystals, showing how the impurities and forming process change its structure. A highlight here is a huge purple quartz you can touch.

The last part of Gems and Minerals is a jewelry exhibit, with earrings owned by Marie Antoinette, and other royalty. The highlight is the famous Hope Diamond, found in India in the 1600’s, and owned over the centuries by French and British aristocrats, before being donated to the museum in 1958 by jeweler Harry Winston.

These are all very pretty gems. The problem with seeing them is that they’re right off the high-traffic rotunda, and people coming in from the walkways stop to look at every detail. It can easily take 15 minutes to see the dozen or so jewels on display here, depending on the time of day you visit. By approaching these gems from the back, as we’ve done here, your wait is minimized, and you can leave the museum when you’re done.

Other Lands at Smithsonian National Museum of Natural History