Notes
Weathering, soil and erosion
- Mechanical weathering breaks rock apart (frost wedging, abrasion); chemical weathering changes minerals (oxidation, acid, water).
- Soil is weathered rock plus humus; the A horizon (topsoil) is richest in organic matter.
- Erosion moves material by water, wind, ice or gravity; deposition drops it. Glaciers carve U-shaped valleys and leave unsorted till.
Dating rock layers
- Relative dating: superposition, cross-cutting, inclusions, unconformities and index fossils.
- Absolute dating uses half-life: remaining fraction $\displaystyle = \left(\frac{1}{2}\right)^n$ where $\displaystyle n = \frac{\text{time}}{\text{half-life}}$.
Time scale
- Earth is about 4.6 billion years old. Eras: Paleozoic, Mesozoic, Cenozoic, all after the long Precambrian. Fossils record life and past environments.
Topics in this unit
Each topic has its own short notes and practice.
Practice
Chemical weathering changes or dissolves minerals.
Ice carries everything and drops it together.
A fault must be younger than what it cuts.
Two half-lives, so $2 \times 2 = 4$ million years.
Index fossils are widespread and short-lived.
Sediment is dropped as the current slows.
Free response
Write your answer first, then open the scoring guide.
Free-response question
A geologist studies a cliff with three sedimentary layers X (top), Y (middle) and Z (bottom). A dike of igneous rock cuts through Y and Z but not X. A volcanic ash bed in Z contains an unstable isotope with a half-life of 40 million years, and only 1/8 of the original amount remains.
(a) Put layers X, Y and Z and the dike in order from oldest to youngest.
Show the answer and scoring guide
Z, then Y, then the dike, then X. Z is at the bottom (oldest). The dike cuts Y and Z but not X, so it is younger than Y and Z and older than X.
Check off the points you earned:
(b) Calculate the age of the ash bed in layer Z.
Show the answer and scoring guide
$\displaystyle \frac{1}{8}$ means 3 half-lives. $3 \times 40 = 120$ million years.
Check off the points you earned:
(c) Name the law used to decide that layer Z is older than layer X and explain why carbon-14 would be a poor choice here.
Show the answer and scoring guide
Law of superposition. Carbon-14 has a half-life of 5,730 years, so nothing would be left in a sample that is millions of years old.
Check off the points you earned: