Weathering and Erosion
Grade 5 · Science · Study Guide
Discover how Earth's rocks slowly break down and move to create canyons, beaches, and valleys.
The Rocky Mountains are impressive, but they are constantly being torn down — just very slowly. Every rock exposed at Earth's surface is under constant attack from water, ice, wind, and chemical reactions. The process of breaking rocks down is called weathering. The process of moving those broken pieces is called erosion. Together they shape every landscape on Earth.
There are two main types of weathering. Physical (mechanical) weathering breaks rock into smaller pieces without changing its chemical composition. The most powerful physical weatherer is water. When water seeps into rock cracks and freezes, it expands about 9%, prying the crack wider with each freeze-thaw cycle — called frost wedging or ice wedging. Roots of plants growing into cracks can also pry rocks apart. Abrasion — rocks scraping against each other in rivers or glaciers — also breaks rocks down physically.
Chemical weathering changes the chemical composition of rocks. Acids are especially effective: carbon dioxide dissolves in rainwater to form weak carbonic acid, which dissolves limestone slowly. This is how limestone caves form — acidic groundwater hollows out the rock over millions of years. Oxygen also chemically weathers iron-rich rocks, turning them brown and crumbly through a process called oxidation (rusting).
Once rocks are broken down, agents of erosion transport the sediment. Water is the most powerful erosion agent — rivers carry enormous amounts of sediment (the Colorado River carved the Grand Canyon this way). Glaciers scrape and carry rocks as they move. Wind picks up sand and dust in deserts. When erosion slows, sediment is deposited — creating deltas, beaches, sand dunes, and eventually sedimentary rock.
Key terms
- Weathering
- The breakdown of rocks at or near Earth's surface by physical or chemical processes.
- Erosion
- The movement of weathered rock particles from one place to another by water, wind, ice, or gravity.
- Physical weathering
- Breaking rocks into smaller pieces without changing their chemical makeup; examples: frost wedging, abrasion.
- Chemical weathering
- Breaking down rocks by changing their chemical composition; examples: acid rain dissolving limestone, oxidation.
- Deposition
- The dropping of sediment when erosion agents slow down or stop.
- Frost wedging
- Physical weathering where water freezes in rock cracks, expands, and breaks the rock apart.
- Abrasion
- Physical weathering where rocks scrape and grind against each other, breaking into smaller pieces.
Flashcards
- Weathering
- Breaking down rocks at Earth's surface by physical or chemical means.
- Erosion
- Transport of weathered rock fragments by water, wind, ice, or gravity.
- Frost wedging
- Water enters rock cracks, freezes, expands, and widens the crack — powerful physical weathering.
- Chemical weathering
- Rocks break down through chemical reactions with water, oxygen, or acids.
- Deposition
- Sediment is dropped by an erosion agent when it slows down; creates beaches, deltas, sand dunes.
- Abrasion
- Rocks scraping against each other, wearing down into smaller, rounder pieces.
Practice questions
What type of weathering occurs when water freezes in rock cracks and breaks the rock apart?
- Chemical weathering
- Oxidation
- Frost wedging
- Abrasion
Show answer
Answer: Frost wedging
Frost wedging (ice wedging) works because water expands about 9% when it freezes, creating enormous pressure in cracks that pries rocks apart over repeated freeze-thaw cycles.
Erosion and weathering are the same process.
Show answer
Answer: False
Weathering breaks rocks down in place. Erosion transports the broken material somewhere else. Both processes work together but they are distinct steps.
The movement of broken rock pieces by water, wind, or ice is called ___.
Show answer
Answer: erosion
Erosion requires an agent of transport — water (rivers, rain), wind, glaciers, or gravity. Without erosion, weathered material just piles up where it formed.
Limestone caves form mainly as a result of:
- Physical weathering by frost wedging
- Chemical weathering by acidic water
- Erosion by glaciers
- Abrasion by wind
Show answer
Answer: Chemical weathering by acidic water
Rainwater absorbs CO2 to form carbonic acid. This weak acid slowly dissolves calcite in limestone, carving out passages and caves over millions of years.
Deposition occurs when erosion agents slow down and drop their sediment.
Show answer
Answer: True
When a river slows (entering the ocean or a lake), its water loses energy and drops sediment. Over time these deposits build deltas and alluvial fans.
Fun facts
- The Grand Canyon is up to 18 miles wide, 277 miles long, and over a mile deep — all carved by the Colorado River over about 5 to 6 million years.
- A single glacier can pick up and transport rocks the size of houses, sometimes depositing them hundreds of miles from where they originated.
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