Rock layers preserve evidence of environments, events, and transformations from Earth’s distant past. Explore geological time, stratigraphy, dating methods, and the clues scientists use to reconstruct sequences extending across immense intervals.
Many rocks preserve a faint memory of the magnetic field that existed when they formed. Because Earth’s magnetic polarity has reversed repeatedly through geologic time, a sequence of rocks can contain alternating intervals of normal and reversed magnetization. Magnetostratigraphy turns that pattern...
At the bottom of some glacial lakes, a year can leave a visible layer of sediment. Meltwater delivers silt and sand during the warmer season, while quieter winter water allows finer clay to settle. When that seasonal pair is preserved without being churned up, it forms a varve: an annual sediment...
A tree ring can mark one growing season, but a stack of rings is not automatically a calendar. Trees sometimes form extremely narrow rings, fail to produce a complete ring around the trunk, or develop bands that resemble extra annual boundaries. Dendrochronologists therefore do more than count...
Sand rarely contains organic remains suitable for radiocarbon dating, and its individual grains may be far older than the deposit in which they rest. Luminescence dating solves a different chronological problem: it estimates when quartz or feldspar was last exposed to enough light to empty a...
A volcanic ash bed can settle across valleys, lakes, and distant basins within hours or days, creating a thin time marker inside the geologic record. Potassium-argon dating can assign an eruption age to that marker when the ash contains suitable potassium-bearing minerals. Once identified in...
A zircon crystal may be smaller than a grain of sand yet preserve several chapters of a rock’s history. Its crystal lattice readily accepts uranium when it grows but usually incorporates very little lead. Over time, radioactive uranium produces lead at known rates. That combination makes zircon one...
Radiocarbon dating occupies a narrow but unusually detailed part of geological time. It does not measure the age of most rocks, nor can it reach back to the dinosaurs or the formation of continents. Its strength lies in younger deposits containing carbon that once circulated through a living...