Learning about a Rock's Cleavage

As part of Sophia and Olivia's multi-disciplinary geography study, they are learning about the state rocks for each state in the U.S.A.

One of the questions on the sheet they complete for each rock asks about the rock's cleavage. Up until this year, I did not know that rocks had cleavage.

So, what is cleavage...as it relates to rocks? According to the Rocks for Kids website, "Cleavage is when a mineral breaks with smooth flat surfaces. Cleavage can be described as perfect, good, imperfect, or poor."

Also, clevage can be described as:

• Perfect 1 way ~ breaks on one perfect cleavage plane, crystals break into slices, sheets peel off
• Perfect 2 ways ~ breaks into elongated boxy shapes, 90 degree angles
• Perfect 3 ways ~ breaks into perfect rhombs, pieces look like squished boxes
• No cleavage ~ does not break regularly

To demonstrate this, the girls put one sheet of paper toweling that was divided into two pieces on the table in front of themselves.

Olivia tearing the first piece of paper toweling.

It separated into a rather even line quite easily.

Then they tore each piece of paper toweling in half.  The first one they did they ripped from top to bottom. The second one they did they tried to tear it from side to side.

Trying to tear the second piece of paper toweling.

This piece appeared more jagged.

One of the pieces has a more even line while the other piece looks more jagged.

The top piece was tore from top to bottom.

The bottom piece was tore from side to side.

How did this activity relate to cleavage? According to Earth Science for Every Kid by Janice VanCleave, "Paper towels are made on a wire screen, creating a straight line in one direction. Pulling on the paper attracks the weakest point.

"The parallel lines on the paper made by the wire screen are thinner than the rest of the paper, and thus the paper rips easily down one of these lines.

"Jagged and irregular tears result when the paper is pulled in the opposite direction. This is like cutting minerals, such as diamonds, along cleavage lines. The mineral splits smoothly and easily along the lines where the molecules line up, but it can smash into irregular pieces if hit across the cleavage line."