High Falls, Alabama
Red Rocks in Colorado and red dirt in Georgia are red for the same reason...iron.
Iron is one of the most common elements on Earth. Nobody's been to the Earth's core but everything we know about it tells us that it's a little less than 90% iron (about 10% is nickel with about 3% lighter elements like silicon, sulfur, and oxygen.) We figure that because our calculations for the density of the Earth from how it orbits the sun, it's magnetism, and the way shock waves pass through the core, and everything else points to iron.
And the crust is full of iron oxides...rust. you won't find much pure iron. If you do, it's probably a meteorite. Metallic iron rusts in short order in the oxygen atmosphere of Earth.
Why is iron so common on Earth? The planet is composed of the dust that came together to make up the whole solar system. The dust came from stars that had lived, grown old, and died before our Sun. Stars create energy by combining light atoms, under incredible pressures to form heavier atoms (It's called "fusion".) Once you get the reaction going (gravity packs the materials together under great pressures and temperatures), it will just keep going. The products are heavier atoms and energy. Chemists call such reactions that give off energy, "exothermic reactions".
First, hydrogen atoms are packed together to form helium, then carbon is formed, then silicon, and finally, iron. Why "finally"? Well, it turns out that iron is the heaviest element that can form in an exothermic fusion reaction. All the heavier elements have to have energy put into the reactions to keep them going. Elements like gold and uranium are created in exploding stars...novas and supernovas... there's plenty of extra energy there!
So, the commonest elements on Earth are also the commonest elements in stars. Hydrogen is common in our water, and there's plenty of oxygen, carbon, and silicon along with the iron.
But what about the helium? The only reason hydrogen is so common is that it's bound up in the heavier compound water, otherwise, gravity wouldn't be able to hold such a light substance to the Earth. It would have all leaked out into space long ago. Helium, though, doesn't bond very easily with other elements so...away it goes.
Iron, though, is a very reactive element and it does like to bind with other elements and one of the most available elements for it to bind with is oxygen. We know iron oxide by it's common name...rust.
Want to see rust in action?
Get two iron or steel nails (not chrome plated or stainless steel. If you buy nails for this demonstration, find a package that warns not to use them if surface rust will be a problem.) And place them in test tubes. Mark one test tube "Dry" and stopper it. Mark the other "Wet", push a wad of damp cotton ball in after the nail and stopper it. Let them sit for a couple of weeks or longer.
Iron oxide is iron and oxygen, and both nails were exposed to oxygen, so why did the damp nail rust and the dry one didn't?
Actually, the dry nail did rust, but it rusted so slowly that I couldn't tell. Over years, I would have been able to see the rust. In a humid environment, water takes part in the reaction to speed the rusting up. If there is salt in the water, it really speeds things up. That's why things rust much faster in Florida than in Alabama, and things rust faster in Alabama than in Arizona.
Rusting is a chemical reaction because the product of the reaction, iron oxide, is a different substance than the reactants, iron and oxygen.
Iron is a transition element. It can have several charged states and that means that it can combine with oxygen in different proportions to form different oxides. Most of the colors in rocks are from these different oxides of iron. Here are two of them.
The white plates above the rocks are called "streak plates". They're made of unglazed porcelain. You can rub a mineral across them to see what their powder looks like. Powder color is more reliable for identification than the color of a chunk of a mineral. The mineral to the right, hematite, can be red, black or brown, but it's streak is always red. In it, two atoms of iron combine with three atoms of oxygen. Hematite is what makes Red Rocks red.
The other mineral in the photograph is magnetite. It has three atoms of iron to four atoms of oxygen and it's streak is always black. It's also attracted to magnets and might even be magnetic. The ancients knew magnetite as "lodestone" and they magnetized other iron items, like compass needles by rubbing them against these natural magnets.
Another common iron oxide includes water in various amounts in its makeup. Because it has a yellow streak, it's called "limonite". It's what colors the ground at Englewood Reservoir.
These iron oxides are pretty but they're also important for another reason. They're the primary iron ores.
Iron has another charged stated that is green and most of the green stones in nature are also colored by iron. A few common green iron bearing minerals are olivine, amphibole, epidote, and serpentine.
In physical changes, no new substance is created. Here is some magnified sea salt.
It's a pretty, clear, blocky crystalline substance. When I dissolved it in water and let the water evaporate, I got this...
This physical reaction is arguably as important as the chemical reaction that produces rust. Rain dissolves salts from rocks and washes them into streams. The streams run into oceans and landlocked lakes like the Dead Sea between Israel and Jordan, the Salt Lake in Utah, and the Salton Sea in California. The water evaporates and the water gets salty. That's why the ocean is salty and that's how salt deposits form that can be mined for table salt.
It's not always easy to tell if a change is chemical or physical but if you pay attention, you can develop a feeling for it. The biggest difference is that chemical changes give you different substances than what you started with; physical changes don't. Ice and liquid water may look different but they're both water. When ice melts, you get your water back.
The next time you go on a hike, appreciate iron for most of the colorful rocks around you.
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