Weathering and Erosion — Answer Key
Part A: Multiple Choice
Circle the best answer for each question.
1. A sinkhole suddenly opens in a neighborhood. What most likely caused it?
A) Wind erosion wore away the topsoil
B) Underground limestone dissolved and the ground collapsed
C) A glacier scraped away the bedrock
D) Tree roots pushed the surface apart
Sinkholes typically occur in areas with underground limestone. Acidic groundwater slowly dissolves the limestone, creating a hollow space. When the surface can no longer support its own weight, it collapses into the void.
2. Why do rivers deposit more sediment where they enter a lake or ocean?
A) The water gets deeper and hotter
B) The river water evaporates instantly
C) The river slows down and can no longer carry heavy particles
D) Salt in the ocean dissolves the sediment
A fast-moving river has enough energy to carry heavy particles. When it enters a lake or ocean, the current slows dramatically, and the water loses the energy needed to keep sediment suspended, so it drops to the bottom.
3. Which is the best example of physical weathering?
A) Iron in a rock turning reddish-brown from rust
B) Limestone dissolving in acidic rainwater
C) A rock splitting when water freezes in its cracks
D) Cave formations growing from mineral-rich drips
When water freezes in a rock crack, it expands and pushes the crack wider. This breaks the rock apart without changing its minerals, which is why frost wedging is physical weathering, not chemical.
4. How does vegetation help prevent erosion?
A) Leaves block all rainfall from reaching the ground
B) Roots hold soil in place and slow water runoff
C) Plants absorb rocks and break them into sand
D) Trees push soil uphill against gravity
Plant roots act like anchors that grip the soil and keep it from washing away. The leaves and stems above ground also slow rainwater, giving it more time to soak in rather than run off.
Part B: Fill in the Blank
Write the correct answer on each line.
1. A flood plain is flat land formed by layers of sediment deposited during river floods.
When a river overflows its banks, water spreads out and slows down, depositing layers of sediment on the flat land nearby. Over many floods, these layers build up the area known as a floodplain.
2. Rocks that tumble in a river become smooth and rounded over time from abrasion.
As rocks tumble downstream, they constantly bump and scrape against each other and the riverbed. This abrasion wears away sharp edges and rough surfaces, making the rocks smooth and rounded over time.
3. The process of hydrolysis turns feldspar minerals in granite into soft clay.
Hydrolysis is a chemical reaction between water and minerals. When water reacts with feldspar in granite, it breaks the mineral down and transforms it into soft clay, weakening the rock.
4. Barrier islands protect the mainland coast from strong ocean waves and storms.
Barrier islands are long, narrow strips of sand deposited by ocean currents along the coast. They absorb the energy of waves and storms before they reach the mainland, acting as a natural shield.
5. A glacier deposits a mix of unsorted rocks, sand, and clay called till.
Unlike water or wind, a glacier does not sort sediment by size. When the ice melts, it dumps everything together -- boulders, sand, and clay all mixed up -- and this unsorted deposit is called till.