Three Cliffs, One Story: How Short Rock Records Become Deep Time

Three Cliffs, One Story

[1]No single cliff shows the whole of Earth's history. Even the walls of the Grand Canyon, more than a mile deep, hold rock that formed between about 1.8 billion and about 270 million years ago, with gaps inside that span where stretches of time left no rock at all. So how did geologists build a time scale reaching back 4.54 billion years? The way you might rebuild a torn-up book from pages found in different houses: find the overlaps and stack the pages in order.

[2]Reading one cliff starts with a rule you know: where sediment layers lie undisturbed, the lowest settled first and is the oldest, and each layer above is younger. One kind of layer is especially valuable. When a volcano erupts, ash can fall across thousands of square kilometers in a few days and settle as a thin, pale band. That band marks a single moment, and because its minerals formed as the eruption cooled, a laboratory can measure its age in years.

[3]Joining cliffs depends on a second rule. A sediment layer spreads sideways as a broad sheet until it thins out or meets a barrier, so a layer in one cliff may continue underground to another cliff far away; geologists call this lateral continuity. Imagine three cliffs. The lowest layer of Cliff A, a distinctive ash band, is also the top layer of Cliff B, which continues down through five more layers. The lowest of those, a limestone packed with a particular set of shells, is the top layer of Cliff C. Match the shared layers and you can stack A on B on C into one column longer than any single cliff. Matching works best with layers that are hard to confuse: an ash band with a unique chemistry, or fossils that always appear in the same order.

[4]The stacked column still has missing pages. A region may go millions of years without receiving sediment, or a layer may be worn away before the next one settles, so two layers that touch can be separated by a huge gap in time. Geologists notice these gaps where the fossils jump: the kinds of organisms change far more between two touching layers than they do anywhere else in the stack.

[5]Everything so far gives order, not years. Ages in years come from the datable layers, mainly ash bands and lava flows, and they are pinned into the stack like bookmarks. If an ash band near the bottom of a column measures 252 million years and another near the top measures 201 million years, every layer between them formed inside that window, even though none of them can be dated directly. The more bookmarks a column has, the tighter the ages of the pages between them.

[6]Chapter breaks came before the page numbers. By the middle of the 1800s, geologists had noticed that certain groups of fossils disappear together at particular levels, in rock from distant places. These were mass extinctions, when a large share of Earth's species died out in a geologically short time, and they became the natural boundaries of the time scale. The names of the eras and periods were in use for decades before anyone could attach a number to them. Absolute dating added the numbers later. The boundary at the end of the age of the dinosaurs is a thin clay layer found worldwide, rich in iridium, a metal common in asteroids; dating of nearby ash puts it at 66 million years ago. Because boundaries follow events, and events do not come at regular intervals, the units have very different lengths.

[7]One caution about the wall chart. The most recent eon, which began about 539 million years ago, usually fills most of the chart and is split into many named pieces. It is not most of Earth's history; it is about one-eighth of it. It gets the space because its rocks are widespread, well preserved and crowded with fossils of animals with shells and bones, which allows fine divisions. The three earlier eons, with scarce rocks and microscopic fossils, cover the other seven-eighths and are squeezed into a stub at the bottom. Drawn to scale, the chart would be almost all stub.