Colors of Rocks

The Monument Valley rocks are composed of sand, silt, and mud grains with finer grained material between the grains. Most grains are not red; it is the fine-grained material, which includes iron oxides, between the grains that is red. It turns out that only a little bit of iron (even less than 1%) may be enough to make the entire rock look red.

Here are some generalizations about the color of rocks:

• Rocks may be various colors, depending on what they are composed of and whether they were formed under oxidizing conditions.
• Iron minerals in rocks deposited in deep water, such as in the ocean or deep lakes, are less oxidized, and these rocks tend to be black or gray.
• If rocks are deposited in shallow water, they may be more greenish or brown.
• Rocks deposited on land instead of under water are more oxidized, especially if they were deposited in wet environments, and so tend to be reddish or tan.
• White rocks can be formed in sand dunes, beaches, lakes, and shallow sea.
• Rocks formed at high temperatures, such as from the solidification of molten rock, or at great depths tend not to be red because the iron minerals cannot oxidize under these conditions.
• If rocks sit at the surface under wet conditions, the iron minerals can be oxidized, turning the rock red.

Generally, most really red rocks were deposited on land, not beneath the water or at high temperatures or great depths.

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