Changes to Earth Over Time
Core ideas
Earth's surface is always changing. Inside the planet, heat drives convection in the mantle, which moves lithospheric plates that collide, separate, or slide past one another. These motions build mountains, volcanoes, and faults, and cause earthquakes. At the surface, weathering breaks rock into smaller pieces in place, erosion transports those pieces, and deposition drops them, building layers of sediment that harden into sedimentary rock.
Rock layers, fossils, and structures record the order of past events. The law of superposition states that in undisturbed sedimentary layers the oldest is at the bottom and the youngest at the top. A fault or a magma intrusion is younger than every layer it cuts across. An unconformity marks missing time created by erosion. Index fossils, from species that existed for a short time but were widely distributed, help match rock layers from place to place. Relative dating puts events in order; radioactive dating gives actual ages in years. Together, these tools build the geologic time scale, showing that most change is slow and gradual, though catastrophic events such as volcanic eruptions, floods, and asteroid impacts can cause rapid change.
Traps to avoid
- Applying superposition to layers that have been folded, tilted, or overturned.
- Forgetting that an intrusion cutting across layers is younger, not older, than those layers.
- Confusing weathering (breakdown in place) with erosion (moving material).
- Thinking fossils are usually found in igneous rock; most form in sedimentary rock.
- Treating a relative age as an exact number of years.
Rule of thumb and worked example
Ask "what cuts what?" Whatever cuts across a feature is younger than that feature. A flat sandstone layer is cut by a fault, so the fault is younger than the sandstone. If a lava flow then lies on top of both, the flow is younger still.