The Rock Cycle, Weathering, Erosion, and Geologic Time
Earth’s surface is always changing, though usually too slowly to notice. Rocks form, break down, move, and re-form in a never-ending loop, and the same processes have shaped the planet over billions of years. Understanding the rock cycle, weathering, erosion, and geologic time ties earth science together.
This lesson covers how rocks change and how we measure Earth’s long history.
The rock cycle is the continuous process by which rocks change from one type to another over time. Weathering breaks rocks into smaller pieces, and erosion moves those pieces to new places. Geologic time is the vast timescale, spanning billions of years, over which these processes have shaped Earth.
What is the rock cycle?
The rock cycle describes how the three rock types change into one another. Melting and cooling form igneous rock. Weathering, erosion, and compaction of sediments form sedimentary rock. Heat and pressure deep underground form metamorphic rock. Any rock type can become another given enough time and the right conditions, so the “cycle” has no fixed start or end.
What is the difference between weathering and erosion?
These two are often confused. Weathering is the breaking down of rock into smaller pieces, right where it sits, by water, ice, wind, or chemical reactions. Erosion is the moving of those broken pieces to a new location by water, wind, ice, or gravity. A simple way to remember it: weathering breaks it, erosion takes it. After erosion carries sediment away, it is deposited elsewhere, where it may eventually form new rock.
| Process | What it does |
|---|---|
| Weathering | Breaks rock into smaller pieces in place |
| Erosion | Moves the pieces to a new location |
| Deposition | Drops the pieces to build up new layers |
What is geologic time?
Geologic time is the enormous span of Earth’s history, about 4.6 billion years. Because rock and fossil layers build up slowly, scientists read them like pages in a book, with deeper layers being older. This lets them divide Earth’s past into eras and periods and understand changes, like the rise and fall of the dinosaurs, that took millions of years. The slow processes of the rock cycle make sense only against this vast timescale.
Watch: A Short Video Lesson
MooMooMath and Science walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:
A routine for rock cycle questions
- Recall the three rock types and how each forms.
- Separate weathering (breaks in place) from erosion (moves away).
- Remember the order: weathering, erosion, deposition.
- Know that deeper rock and fossil layers are older.
- Frame the processes against geologic time, billions of years.
Practice questions
- What is the difference between weathering and erosion?
- What forms igneous rock?
- What comes after erosion, when sediment is dropped?
- About how old is Earth?
- Why are deeper rock layers usually older?
- True or false: one rock type can change into another.
Answers:
- Weathering breaks rock into pieces in place; erosion moves those pieces away.
- The cooling and hardening of melted rock.
- Deposition.
- About 4.6 billion years.
- Because layers build up over time, so lower ones formed earlier.
- True.
Where this fits
This lesson expands on rocks and the rock cycle and on the minerals that make up rocks. Geologic time is also read from the fossil record. Find all topics on the ASVAB General Science Learning Hub.
Recommended Prep Books
These study guides and practice books help you keep building momentum as you prepare:
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