Relative dating of rock strata – cross-section puzzles, unconformities and the geological time scale
GeographyEarth's Structure & LandformsAges 15–16
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Read geological cross-sections like a geologist. Tap the lettered rock units and events from oldest to youngest using superposition, original horizontality, cross-cutting relationships (dikes, sills, granite, faults, erosion surfaces), inclusions and baked contacts; a wrong order is explained with the principle it breaks, and the solved section replays its history step by step. Six sections grow harder up to folded and faulted strata, and the last one places each unit on the geological time scale using index fossils and a radiometric date. Build your own section from layers, tilting, folding, faults, intrusions, lava and erosion, then quiz yourself, and watch angular unconformities, disconformities and nonconformities form with a time bar of the missing time.
Lesson: Relative dating: principles of stratigraphy, unconformities and the geological time scale
What it shows
Geologists reconstruct the history of an area from the order in which its rocks formed. Layers are laid down horizontally with the youngest on top; anything that cuts through rocks, such as a dike, sill, granite intrusion, fault or erosion surface, is younger than them, and pebbles of one rock inside another are older than the rock that holds them. Magma bakes the rock it touches, which tells a sill from a buried lava flow. Erosion surfaces are unconformities, gaps in the record. Index fossils and radiometric dates then place the events on the geological time scale. The sections are schematic and not to scale.
How to use
In Order the events, choose a Cross-section and tap the units in the picture or the cards from oldest to youngest, then press Check order; with Explain mistakes ticked, a wrong answer names the rule it breaks. After solving, press Watch the history. In section 6, tap a unit and then its period on the time scale and press Check time scale. In Build a section, add events with the buttons and press Quiz me. In Unconformities, choose a kind and press Play the history.
Parameters you can change
- Screen Order the events, Build a section, Unconformities
- Cross-section 1. Layers and a dike, 2. Tilted layers, 3. A fault and a dike, 4. Granite and pebbles, 5. Folds, a fault and lava, 6. Fossils and the time scale
- Unconformity Angular unconformity, Disconformity, Nonconformity, Mystery section
- Explain mistakes
Questions to explore
- How can you tell that a dike is younger than all the layers it cuts across?
- Why do tilted layers below flat layers show that time is missing at the boundary?
- How can index fossils and a dated dike together give the age of a sedimentary layer?
