Question

Consider the two samples of sedimentary rock seen here. Predict which rock would be more likely to undergo physical weathering. Explain how and why.
Responses
A Sandstone. Sandstone has more obvious layers of sediments.Sandstone. Sandstone has more obvious layers of sediments.
B Shale. Shale is fine grained and is more likely to dissolve in water.Shale. Shale is fine grained and is more likely to dissolve in water.
C Sandstone. Sandstone is made of large sand-like grains and is easily eroded by wind.Sandstone. Sandstone is made of large sand-like grains and is easily eroded by wind.
D Shale. Shale contains iron which is an element that easily combines with the oxygen in the atmosphere to produce rust.Shale. Shale contains iron which is an element that easily combines with the oxygen in the atmosphere to produce rust.

Answer 1

C. Sandstone.

Why: physical (mechanical) weathering breaks rock into pieces without changing its chemistry. Sandstone is made of relatively large, loosely cemented sand grains and is often porous, so grains can be pried or knocked loose by abrasion (wind, water carrying particles) and by freeze–thaw cycles where water in pores/between grains freezes, expands and dislodges grains. Thermal expansion and salt-crystal growth also more easily break apart coarse‑grained, porous rock.

Shale, by contrast, is fine‑grained and tends to weather chemically (clay minerals altering, swelling) or to split along sheets (fissility), so it is less susceptible to the kind of abrasion and grain‑dislodgement that characterize physical weathering of sandstone.