Erosion and weathering

Activities to try

1. 1

   Stream-table fair test: does plant cover change how much soil erodes?

   Why it works: Erosion is the transport of broken material, its rate can be measured and changed, and vegetation (root binding) generally reduces soil erosion rather than causing it — a conclusion backed by numbers from a controlled comparison.

   1. Build or use a sloped tray of soil/sand with a controllable water source (a real sand/soil stream-table, or a screen bench with sliders for slope and rainfall/flow and a plant-cover toggle).
   2. Run a baseline trial: tilt the tray, set a steady water flow, run it, and measure the soil washed into the catch tray (grams lost) and how far the sediment travelled.
   3. Change exactly ONE variable — add a mat of plant roots — keeping slope, flow, soil, and time identical, then rerun and measure again.
   4. Record both trials in a comparison table (variable changed → measured effect) and state which single variable differed.
   5. Repeat the trial at least once to check whether the result holds, then compare the plant-cover and bare totals.
2. 2

   Shake-jar abrasion + freeze-thaw: separating break-down from transport

   Why it works: Weathering (broken where it sits) and erosion (carried away) are distinct processes; freeze-thaw and abrasion both weather rock, and once material moves it is eventually deposited somewhere else.

   1. Shake rock chips or gravel in a jar (with some water in it if you want to see clouding) and look for the signs of abrasion — fine sediment produced, water clouding if water is present, and edges gradually dulling. Visible rounding is slow, so the fine material worn off is the clearest short-term evidence that abrasion (weathering) is happening. Sugar cubes can stand in to show the idea quickly, but they dissolve in water and break far faster than rock, so use them only to illustrate the process, not to judge real rounding or rates.
   2. Add water, swirl, then pour the water off into another container and watch where the broken bits settle — material carried away in the water and dropped elsewhere is erosion followed by deposition.
   3. Run a freeze-thaw demonstration: soak a porous block or fill a crack in it with water (keep it wet between cycles), then repeatedly freeze and thaw it — as the water freezes it turns to ice and expands, pushing on the crack walls. Over many cycles this can widen the crack, flake off bits or eventually split the block, though a few classroom cycles may show only small changes; nothing is carried away (weathering in place).
   4. For each observation, state out loud whether it shows weathering or erosion and what the evidence is.
   5. Record how rounded/dulled the particles became and roughly how much sediment/fines were produced.
3. 3

   Reading real landscapes: evidence of change over time

   Why it works: Much erosion is slow and gradual, not only dramatic floods; measured change over time is still evidence, and features in real landscapes record the agents (water, ice, wind, vegetation) that acted on them.

   1. Examine two photos of the same riverbank (or slope) taken about a year apart and describe what has changed.
   2. Measure or estimate the small change (e.g. how far the bank edge retreated) and argue whether water could still be eroding it even though the change looks minor.
   3. Classify features on a landscape image by the agent that shaped them (frost-cracked cliff, wind-carved rock, river-cut valley) and say whether each is mainly weathering or erosion.
   4. Explain, using your measured stream-table results, why a bare slope often gullies and loses more soil while a planted bank usually holds better — and say whether your own numbers showed this.