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When Rocks Tell Stories: Describing Rock Properties

Look at the three images of rocks shown below. What are the similarities? What are the differences?

Image of rounded grains Image of rounded cobbles Image of angular asphalt grains
Click on an image to enlarge it.


Close your eyes and picture "a rock". Did you picture a boring, grey stone? Not all rocks look the same, and the things that make rocks look a little different from each other give clues about each rock's "story." By making careful observations of a rock, geologists can tell where a rock came from and what has happened to it since that time. Since every rock has a slightly different story, it's important to notice the differences in the rocks. You can do it, too! Once you learn a little about the processes that shaped one rock, you can find other rocks that had similar experiences by looking for rocks that look similar.

Individual pieces of a rock are called GRAINS

Many rocks are made up of smaller pieces. Sometimes the grains of a rock are all the same and sometimes a rock is made up of all sorts of different grains.

The individual grains from different classes of rocks (i.e., igneous, metamorphic, and sedimentary) develop by different processes. For example, a sedimentary rock might have individual grains of sand held together by a natural cement. Grains of an igneous rock are individual crystals that have grown as a magma cooled. Observations of grains help tell a story about the entire rock, but the observations must be interpreted in different ways for these different rock classes.

This lesson focuses on sedimentary rocks because most rocks you find in your schoolyard are sedimentary rocks (often made by humans, such as cement, asphalt, etc).

COLOR gives clues about composition

What is your rock made out of? You can't tell just by looking, but grains with different chemical compositions are often different colors. Knowing a rock's composition can tell you if the rock came from deep within the earth, formed at the bottom of the ocean, or was exposed to wind and rain.

For example, some grains are the same color as rusted metal (reddish-brown). The similarity in color is also related to similar history -- the rust-colored grains might be a mineral called "hematite," which is composed entirely of iron and oxygen. Just like rusted metal, iron in the grains rusts when exposed to oxygen in air and water. We call this process chemical weathering. Not all red grains are made from the same elements, but color does give clues about composition and is the easiest thing to notice.

Photograph of gold Photograph of Silver

Color is a clue about composition: Gold v. Silver

GRAIN SHAPE tells us about the mechanical weathering history

People rub wood with sandpaper to smooth it, and the same thing happens to rocks. When rocks first break apart, they are split into sharp, angular pieces. So most grains start out angular. Over time, they get worn smooth by wind, water, and other rocks. We classify grains based on their "Roundness." Rounded grains have very few sharp edges and corners. The opposite of "Round" is "Angular." The process of breaking rocks apart and smoothing them down is called mechanical weathering and usually happens when rocks are moved (transported) by forces like wind, water, glaciers, or in a landslide. Rounded rocks have been exposed to more mechanical weathering; they have traveled further, they have been around for longer, or existed in areas with more rubbing and abrasion.

Graphic comparing weathering amounts to grain shapes

GRAIN SIZE tells us about how much energy it took to move the grains

Some grains are big and some are small. To move a giant boulder takes a lot more energy than to move a tiny sand particle. Which is more likely to move a giant boulder: a trickling stream or a raging river? So if you see a massive boulder in the middle of a dry creek bed, you know that there had to be a whole lot of water rushing through it at one point. In addition to the amount of water, gravity also gives streams energy to move boulders. Streams on steeper hills can be higher energy than streams on gentle slopes.

Photograph of a boulder Photograph of small grains

Big grains take a lot of energy to move.
Click to enlarge.

Photograph of large stream
Photograph of small stream
Click to enlarge.

A high energy river system is able to move bigger rocks. Look at the boulders in the white water of this stream. The largest rocks moved last when the river was moving even faster than it is in this photograph.

Click to enlarge.

This is a very low energy creek. The water moves more slowly and there is less of it, so there are no boulders here. Looking closely at the shadows beneath the water surface, you can see the shape of individual rocks that are all smaller than about 10 cm. That means that not only is it slow now, but that it has probably never had enough energy to move large boulders (otherwise, they would have been left behind in the creek).

The man-made materials like cement, concrete, asphalt, and brick that you find around your schoolyard are not that different from natural sedimentary rocks. They are all made from natural rocks: Cement comes from ground up rock called "limestone", concrete usually has sand or small pebbles in it, asphalt has crushed rock mixed with tar, and brick is made from baked mud. You can learn about where these natural rocks came from by looking at the rock-like materials in your schoolyard.

Above, we said rust-colored rocks are often the product of chemical weathering. Can you find any rust colored building materials in your schoolyard? How about bricks? Bricks are clay that has been baked at very high temperatures. The color of a brick is related to chemical weathering of iron and other elements that are commonly found in clays. The chemical weathering process happens much faster when the temperature is really hot (like 1500 degrees!), so brick manufacturers are able to cause weathering that might take thousands of years at the earth's surface in just a few hours in their hot brick kilns. The color of a brick depends on the original clay composition and the temperature to which it is heated.

You can sometimes even learn something about the history of your city from looking at schoolyard building materials. For example, some older concrete is made with very rounded grains. Because they are rounded, we know that they came from an environment that smoothed them out -- probably the bottom of river bed. However, a lot of newer cement has very angular grains. Why the change? It turns out that most cities and towns have grown so much over the years that they have used up all of the easily accessible river gravels nearby. It is often cheaper to dig huge chunks of rock out of the ground and crush them into smaller pieces than to transport the naturally small gravel from far away places. In other words, many of the angular grains we see in concrete today are the product of instant "mechanical weathering" caused by machines. The smoother, rounded grains from older concrete are the product of lots of mechanical weathering over thousands of years by nature.

Rock quarry in San Rafael, California
A crushed rock quarry in San Rafael, California. Picture taken from a small airplane, and you can see nearby roads and a golf course in the background to get a sense of the size of the operation. The pit is about 200 feet deep.
Image From: County of Marin. Click to enlarge.

Related Student Activity

Schoolyard Geology Home  •  Lesson 1  •  Lesson 2  •  Lesson 3  •  Downloads

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