Geologic History

Rock layers are a history book with no page numbers. Relative dating is how geologists recover the order of events, older or younger, without knowing exact ages, using a handful of common-sense principles.

Superposition. In sedimentary layers that have not been disturbed, the bottom layer is oldest. Each layer above it is younger, because sediment piles up from the bottom. Original horizontality. Sediment settles in flat layers. So if layers are tilted or folded, something bent them after they formed. Cross-cutting relationships. Anything that cuts across rock is younger than the rock it cuts. Faults and igneous intrusions both do this. Inclusions. Pieces of one rock trapped inside another are older than the rock around them. The pieces had to exist first.

There is one more clue for intrusions. Hot magma bakes the rock it touches, leaving a baked zone along its edges. If the layers above an igneous rock show no baking, that rock is not an intrusion. It was a lava flow that hardened before those layers were laid down.

Sometimes the record has a gap. An unconformity is a buried erosion surface. Rock was pushed up, worn away, and then buried under new sediment. The time that erosion removed is missing from the stack. On a cross section an unconformity shows up as a wavy line, often with tilted layers below it and flat layers above it.

Every unconformity records the same four steps. Sediment is deposited. The rock is uplifted. Erosion strips it. Then new sediment is deposited on top. Spotting that order is a common Regents question.

No single cliff records all of Earth's history, so geologists correlate, matching rock layers from place to place to stitch local records into a global one. Layers can be matched by their rock type and sequence. They can be matched by a distinctive marker like a volcanic ash layer that fell everywhere at once. Most powerfully, they can be matched by their fossils.

The best fossils for this job are index fossils. They come from living things that spread over a wide area but lived for only a short stretch of geologic time. A wide range means they turn up in many places. A short lifespan means that finding one pins the layer to a narrow slice of time. New York's state fossil is the sea scorpion Eurypterus. It swam the shallow Silurian seas that once covered the state, and its remains help mark rocks of that age.

Stack all the world's correlated rock, order it, and you get the geologic time scale, printed across the middle of your Reference Tables. Its divisions, eons, eras, periods, and epochs, are not arbitrary. The boundaries mark major changes in the fossil record, usually mass extinctions followed by bursts of new life.

The single most important fact on that chart is its proportions. Earth is about 4.6 billion years old. Roughly the first four billion of those years is Precambrian time, before abundant fossils. That is nearly 88 percent of all of Earth history. All of visible life's story, trilobites to dinosaurs to people, is crammed into the last sliver. Humans appear so late that on a scaled 24-hour clock of Earth history we arrive in the final few seconds.

The rock record does more than preserve life. It shows living things changing the planet. Earth and its life have shaped each other from the beginning. Geologists call this coevolution. The clearest example involves iron and oxygen.

For the first half of Earth's history there was almost no oxygen in the air. Then, more than two billion years ago, tiny sea microbes called cyanobacteria started using photosynthesis, which gives off oxygen as waste. At first the iron dissolved in the ocean soaked that oxygen up. The iron rusted out of the water and settled in striped layers called banded iron formations. Some of those layers are the iron ore we mine today. Once the iron was used up, oxygen started to build in the air. This change is called the Great Oxygenation Event. It poisoned much of the life alive at the time, and it opened the door for everything that breathes oxygen now.

The effects spread in every direction. Oxygen let an ozone layer form high in the atmosphere. That layer blocks ultraviolet light, which eventually made dry land safe to live on. Oxygen also made possible the high-energy chemistry that complex animals need. Later, plants spread across the land, pulled carbon dioxide out of the air, and built soils, which changed weathering and the carbon cycle. Living things did not simply appear on a finished Earth. They helped build the Earth we know, and fossils and chemical traces in the rock let us read that history.

Relative dating puts events in order. Radioactive decay gives actual numbers. Some isotopes are unstable and decay into a different element at a perfectly steady rate. Heat, pressure, and chemistry do not change that rate. The half-life is the time it takes for half of the parent isotope in a sample to decay into its daughter product. After one half-life, 50 percent of the parent is left. After two, 25 percent is left. After three, 12.5 percent. Measure the ratio of parent to daughter in a rock, and the half-life turns that ratio into an age.

The Reference Tables list the isotopes you need. Carbon-14 has a half-life of 5,700 years. It dates recent organic remains such as wood, bone, and shell, back a few tens of thousands of years. For rock, geologists use slower clocks. Potassium-40 has a half-life of 1.3 billion years, uranium-238 has 4.5 billion years, and rubidium-87 has 48.8 billion years. The choice matters. A fast clock runs out on old samples, and a slow clock barely ticks on young ones.

Pick an isotope from the Reference Tables and step through half-lives. The lab tracks how much parent remains, how much daughter has built up, and how much time has passed. Notice how fast carbon-14 runs out compared to uranium-238.

Relative dating puts events in order. Bottom layers are oldest. Tilted layers were bent after they formed. Whatever cuts across rock is younger than the rock it cuts. Inclusions are older than the rock around them. Unconformities are buried erosion surfaces where time is missing from the record. Correlation, especially with index fossils, links local records into the geologic time scale. The big lesson of that scale is proportion. Earth is 4.6 billion years old, and most of that time is Precambrian. Radioactive decay adds real numbers, because half of the parent isotope becomes daughter every half-life. Use carbon-14 for recent organic remains, and uranium-238 or another slow clock for ancient rock.

Ten Regents-style questions, one at a time in a focused view, each with an instant explanation. The set reshuffles when you reach the end, so you can keep practicing as long as you like.