Weathering, Erosion, and Deposition

Weathering: Breaking Down Rock#

Weathering is the process by which rocks and minerals at Earth's surface are broken down into smaller pieces or chemically altered over time. Weathering dös NOT move the material — it simply breaks it apart in place.

Types of Weathering#

1. Mechanical (Physical) Weathering

Mechanical weathering breaks rocks into smaller pieces without changing their chemical composition. The pieces are still the same kind of rock — just smaller.

Causes of mechanical weathering:

• Freeze-thaw (frost wedging): Water seeps into rock cracks. When it freezes, it expands, widening the crack. Over many cycles, the rock splits apart. This is very common in mountain and tundra climates.

• Abrasion: Rocks grind against each other (like sandpaper), wearing each other down. Occurs in rivers (rocks knocked together) and deserts (sand carried by wind scratches rocks).

• Root wedging: Plant roots grow into cracks in rocks. As roots grow and thicken, they push rock apart. You can see this in sidewalks where tree roots lift concrete.

• Thermal expansion and contraction: In deserts, rocks heat up during the day and cool at night. This repeated expansion and contraction can cause surface flaking (exfoliation).

2. Chemical Weathering

Chemical weathering changes the mineral composition of rocks, creating new minerals or dissolving the rock entirely.

• Oxidation: Iron in rocks reacts with oxygen and water → rust. Reddish-brown rust weakens rock structure.

• Carbonation: Rainwater + carbon dioxide = weak carbonic acid. This acid dissolves limestone, forming caves (karst landscapes).

• Hydrolysis: Water reacts directly with rock minerals, changing their chemical structure.