How Do Rocks Tell Time?

Think of a pile of old newspapers in the corner of a room. Each day’s paper goes on top. Without reading a single date, you know that the paper at the bottom is the oldest and the one on top is the newest. Unless someone has shuffled the pile, the order of the papers is the order of the days.

Rock layers form in the same way. You learned in Grade 7 that sedimentary rock is made from bits of sand, mud and shells (sediment) that settle in layers, usually under water, and are slowly pressed and cemented into rock. And in Grade 9 you will read that fossils are found in these rocks Grade 9: Evidence from fossils. A river carries mud into a lake or the sea; the mud settles on the bottom; next year more settles on top of it. Each layer of rock is called a stratum; two or more are strata.

From this, two rules follow.

• a.Superposition: in layers that have not been turned over, each layer is younger than the one below it and older than the one above. Lower is older.
• b.Original horizontality: sediment settles in flat, level layers, because gravity spreads it out. So if you see layers that are tilted or bent, they were moved after they were made.

These rules give a relative age: whether one rock is older or younger than another. They don’t give an absolute age, a number of years. Your pile of newspapers tells you that the bottom one is older than the top one, but not by itself whether that bottom paper is from last week or from 1990. Strata are the same: on their own, they tell the order of events, not their dates.

Lower is older is only the first rule. Rocks hold other clues to the order of events.

Think of a wall. If a crack runs through the bricks, the crack must have come after the wall was built: you can’t crack a wall that isn’t there yet. And if a poster is pasted across the crack, unbroken, the poster came after the crack. Rocks work the same way.

• Cross-cutting: anything that cuts across a layer is younger than that layer. That includes a crack filled with once-molten rock (a dyke), a break along which the rock has slipped (a fault), and a valley cut by a river.
• Pieces inside: if a rock holds broken pieces of another rock, those pieces are older than the rock around them. They had to exist before they could be broken off and buried.
• Gaps: sometimes layers were lifted up, worn away by rain and rivers, then sunk again and buried under new layers. The worn surface between them is an unconformity, a gap in the record. The time when rock was being worn away, instead of laid down, left no layers at all.

Now you know enough to read a cliff. Geologists don’t just state the order: for each step they give the evidence, the clue in the rock that shows it.

One cliff tells only part of the story. Its layers start at some time and stop at another, with gaps where rock was worn away. To read the Earth’s whole history, geologists need to fit together the pages from many places. Matching layers from one place to another is called correlation.

Sometimes you can simply follow a layer along a valley wall. But layers in two mountains, a hundred kilometers apart, can’t be followed: valleys, rivers and plains lie between. How do you know which layer here matches which layer there?

Not by the kind of rock. Sandstone forms whenever sand settles, and that has happened at countless times in the Earth’s history. Two sandstones can be hundreds of millions of years apart. And at one moment, the same sea can lay down sand near the shore and lime mud farther out, so one time can be two different rocks.

By fossils. In the 1790s the English surveyor William Smith, laying out a canal for coal barges, noticed that each layer held its own particular fossils, and that the fossils always came in the same order, from the bottom up, wherever he went. This is the principle of faunal succession: fossils follow one another in the same order everywhere. Each kind of living thing appears, lives for a while, and dies out. Once a species has died out it never comes back, so its fossils mark one stretch of time and no other.

The best fossils for matching layers, called index fossils, come from living things that:

• lived for only a short time, before changing into something different or dying out, so they pin down a short stretch of time;
• were widespread, often floating or swimming in the seas, so they are found in many places;
• were common and had hard parts, so they are easy to find;
• are easy to recognize.

Ammonoids and graptolites (tiny floating animals that lived in colonies) are among the best. Some ammonoid species lasted less than a million years, so they can match layers to within that time. Some key layers work too: when a volcano erupts, its ash falls over a huge area within days, so a layer of ash marks the same moment everywhere it falls.

Fossils can do more than match layers. Since we know when each kind of living thing was around, a rock that holds two kinds must have formed when both were alive: where their time ranges overlap.

Strata give the order of events. Can they give the time as well? People tried, by asking how fast sediment piles up.

In some lakes fed by melting glaciers, each year leaves a pair of bands: a pale, coarse band of silt from the summer melt, and a thin, dark band of fine clay that settles slowly under the winter ice. One pair of bands, one year, is a varve. Count the varves, as you would count the rings of a tree, and you count the years.

Most layers are not varves. For them, geologists can only estimate how fast the sediment built up, from what is building up today in similar places.

The dates came later. From 1907, scientists learned to read the clocks inside radioactive atoms (How Old Is the Earth? in Going Further 9–12 explains them). Volcanic ash and lava contain crystals that can be dated this way, but sandstone and limestone usually can’t. So geologists date the ash beds and lava flows above and below the fossil layers: a layer lying between ash dated at 70 million years and ash dated at 68 million years formed between those two dates. The order came from the strata; the numbers were pinned onto it.

Now you can see how the geological time scale you met in Grade 8 was built. It isn’t a list someone made up; it is a summary of the evidence in the rocks.

1. Geologists in many countries worked out the order of the layers in their own region, by superposition and cross-cutting.
2. They matched the layers between regions by their fossils, and so built one long column, the order of all the strata known, far longer than any one cliff.
3. They divided that column where the fossils change most sharply. Many of these breaks are mass extinctions, when many kinds of living things vanish from the rocks at once and new kinds appear above. That is why the eras are named by their life: Paleozoic (“old life”), Mesozoic (“middle life”) and Cenozoic (“new life”). The smaller divisions were often named after the place where their rocks were first studied: the Cambrian after Cambria, an old name for Wales; the Jurassic after the Jura mountains of France and Switzerland; the Permian after Perm in Russia.
4. Last, radioactive dating of ash and lava put years on the boundaries. The first time scale with numbers was published by the English geologist Arthur Holmes in 1913, and the dates are still being refined.

So the time scale is a way to organize the Earth’s history using the evidence of the rocks. Take any big event: geologists can say what came before it and after it, and, with the dates, when.

• How Old Is the Earth? In Going Further 9–12: how the clocks inside radioactive atoms put years on the order the strata give, and date the Earth at 4.6 billion years.
• How Does One Rock Become Another? Sandstone and limestone are only one kind of rock. The rock cycle, and the heat and sunlight that drive it.
• Do Continents Really Move? The same fossils in rocks in India, Africa and Antarctica, matched as you matched the cliffs here, show that the continents were once joined.