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Name: Jack
Status: Student
Grade: 9-12
Location: MN
Country: United States
Date: September 2008


Question:
How do the different color stripes appear on rocks?



Replies:
Your question is too general for me to be able to provide a precise answer. Since I do not know which rock you are considering I may give an answer which is wildly inaccurate. GIVEN THAT PROVISO I will attempt to provide a general answer about MOST striped rocks.

In general stripes on a rock are indicative of parallel layers within a rock formation. Layers are representative of sediments laid down on the floor of a body of water, such as the sea or a lake. Given that, layered rocks are almost always sedimentary. (There can be layers of volcanic rock, but the layers are usually quite thick, and so we are not aware of stripes as you describe.)

The layers of sediment are built up from sand and silt and other materials carried into the lake or the sea by streams or rivers. As rainfall changes through the year, so the amount and the type of sediment may change. A stream swelled by winter rains can carry large amounts of coarse sediment with a lot of rotting leaves. The same river in summer flows slowly and only carries a small amount of fine silt. The changing nature of the sediment results in variations of the rock that is formed - the amount of sediment affects the depth of the layer, the size of the grains it contains affects the texture, and the amount of organic material can change the colour.

Sediments showing clear annual layering are not the norm, but they are cot rare. Almost all sedimentary rocks show some layering from variations resulting from changes in their sediment source.

On the other hand, there are striped rocks which show no sign of layering, and may not be sedimentary at all. Check out www.wollemigems.com.au/creations/zebra_stone.html for some really strange striped rocks!

Nigel Skelton
Tennant Creek AUSTRALIA


Jack,

The different colored rocks are basically different minerals, or different mixtures of minerals. For example, hematite (an ore of iron oxide) is mainly red but can go to nearly black depending on how much weathering and oxidation has taken place. Similarly, bauxite, which is an ore of aluminum can range in color from red to pink to white.

Moreover, mixing different types of minerals can cause changes in the colors of the rocks. Adding iron oxide to bauxite can make it appear blue.

The stripes or striation in rocks depend not only on the type of rocks or minerals, but also on the weather pattern that deposited or affected the rocks. For example, in the painted desert of Arizona, when the rocks or sediment were deposited slowly, high concentrations of oxidized iron and aluminum were deposited and this developed the red, orange and pink colors. But in sections where the rocks were deposited rapidly under low oxygen conditions (say, during a flood where sediment is rapidly formed underwater) this tended to deposit minerals that were less oxidized and formed the blue and gray colors.

In short, the stripes or striations in rocks, depend on the type of rock, how they were deposited, and how succeeding weather patterns affected those rocks.

Greg (Roberto Gregorius)


Jack,

Rocks can be divided into three big categories: (1) sedimentary, (2) igneous, and (3) metamorphic.

Sedimentary rocks form layer by layer of inorganic deposits such as sand and the exoskeletons of invertebrate animals. Sedimentary rocks form at the bottom of lakes and oceans. Over very long times, with the combination of heat and pressure the sand turns into sandstone (SiO2) and the exoskeletons turn into limestone (CaCO3). These two rock types often have stripes because for a relatively short time period other elements are present in the water. Iron and copper are two metals that are colored in solution and they would mix with the sand or limestone to form stripes of dark red (rust) from iron, and green to green-blue from copper.

Warren Young



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