Changes at the Rainforest Cave Mouth

The Cave-Mouth Survey

A rainforest survey team studies changes outside a cave entrance. Students observe from a marked safe area. After heavy rain, brown water runs from a patch of exposed soil into a shallow channel. After several rainless days, a breeze lifts dry soil grains from that patch onto a nearby stone ledge. Breaking rock into smaller pieces is called weathering. Moving loose rock or soil from one place to another is called erosion.

The team also measures cracks in two similar boulders. Two years ago, both cracks were 2 millimeters wide. A root inside the first crack has grown thicker. That crack is now 5 millimeters wide. The second crack has no root and remains 2 millimeters wide. Small rock pieces beside the first crack have not moved from their earlier positions. The team records crack width and root thickness without pulling on roots or touching loose rocks.

Back at the field station, students test water moving over soil. They prepare three matching trays, each with 600 grams of the same dry soil. Tray size and slope stay the same. They pour 100, 200, or 300 milliliters of water from the same height. Each pour lasts 60 seconds. They collect soil carried out of each tray, let it dry, and measure its mass. The graph shows their results. Each water amount has been tested once.

The warm cave entrance does not experience freezing during the survey. To investigate ice, an adult helps students use a freezer at the station. They select two similar cracked sandstone rocks of the same size. Sandstone is a soft, porous rock. Both cracks start 1 millimeter wide. One crack is dampened before each cycle; the other stays dry. The adult moves both rocks through twenty matching freezing and room-temperature thawing cycles. Afterward, the damp crack measures 2 millimeters and the dry crack measures 1 millimeter. Water takes up more space when it freezes.