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The geological timescale
20 May 1995
From New Scientist Print Edition.
Jeff Hecht
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Stratigraphic Timescale
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The Continents

Geologists can trace the history of the Earth back about 4.6 billion years, to its formation from a ring of gas and dust around the young Sun. They divide this vast span into intervals that form the basic yardsticks of geological time. Early geologists named these intervals on the basis of the rocks formed within them but without knowing how long they lasted. Succeeding generations have changed the names of some and calibrated them in years to produce a geological time scale - a means of measuring the history of the Earth.

Geologists measure this history using two scales - the chronological and the stratigraphic. Chronological history can be measured in years. For example, if you wanted to refer to the time when Victoria ruled Britain, you would specify the years between 1837 and 1901. Stratigraphic ages relate the order of events, so the Victorian age would follow the Georgian era but precede Edwardian times. This is the origin of the series of the geological time intervals shown in Figure, which are stratigraphic ages. These are the most widely used measures of geological time.

You might think it simpler to date events directly, for example by saying that the dinosaur Tyrannosaurus rex lived between 70 and 65 million years ago, rather than at the end of the Cretaceous period. But geologists find the system of named intervals both more practical and more accurate. Chronological dating requires special equipment and works for only a few types of rock. For example, if you see a rock that contains fossils of trilobites - sea creatures that look a bit like large woodlice - you know straight away that this rock formed at a time when trilobites were alive and well. All rocks around the world that contain fossils of trilobites must have formed during the many millions of years when the creatures existed - neither before they evolved, nor after they became extinct. Looking more closely at the fossils shows that there are many species of trilobites, some evolving before others, some dying out earlier. Any rock that contains a particular combination of fossils must have formed in the particular interval of time when those creatures lived. So a geologist who can identify the critical fossils can see how old the rock is.

But geologists are not able straight away to give a chronological age. What they do is match the interval of time to a sequence of named intervals. Geologists have established this sequence by looking at outcrops of rock, deciding which rocks are older and which ones are younger, then analysing the fossil record in detail. This sequence is the stratigraphic scale, the starting point for the geological timescale.

Ages and ages

Types of dating

EARLY geologists found that sedimentary rocks form in layers, where wind and water deposit new material on top of old. These sedimentary layers or beds are called strata. William Smith, a surveyor of coal deposits and canals around the turn of the 19th century, recognised that these layers - or strata - formed regular patterns in rocks. Smith and other early stratigraphers realised that the deeper the rock lay underground, the older it was. Imagine piling up the newspapers after you read them, day after day - today's paper is on top of yesterday's and the papers get older deeper into the pile.

Geological strata are more complex than piles of papers for several reasons, not least because rocks do not have dates printed on them. Sequences of sedimentary rocks can include breaks, or discontinuities, where rocks did not form or were eroded away - as if some newspapers were missing from the pile. And not all types of rocks form layers. Igneous rocks form when molten rock solidifies, and although they can occur in layers, such as lava flows, they are more often much less regular. Metamorphic rocks are formed when heat and pressure within the Earth modifies existing rocks so much that new minerals form. Movements in the Earth's crust can even fold old sedimentary layers onto younger ones. But although stratigraphers are able to tell the sequence of events from strata, they cannot tell how many years ago the rocks formed. Imagine a stack of newspapers with no dates on them. If you had another paper, you could find out where it fits into the pile by matching the headlines.

To date rocks chronologically, geologists need to pinpoint particular events within a strata and find out how many years ago they happened. If they can do this for several different times, they can calibrate the stratigraphic timescale and estimate the ages of rocks that formed between the known times. The best tool that they have to find the age of rocks is radiometric dating (see Radiometric dating), a technique based on the decay of radioactive isotopes in the rock. Because researchers know how long a particular isotope takes to decay, they can use the amount left in a rock at present to calculate when it formed. Results now cover the history of the Solar System. The oldest meteorites formed about 4.55 billion years ago, at the same time as the Earth and other planets. On Earth, the oldest rocks - in the Northwest Territories of Canada - are almost 4 billion years old and the oldest individual crystals, the mineral zircon, are almost 4.2 billion years old.

Radiometric dating is a powerful technique but it has important limitations, notably that it cannot give a direct date for the deposition of a sedimentary rock. The radioactive isotopes used for dating must be trapped in the rock by crystallisation of minerals, as happens when an igneous rock cools and solidifies. Sedimentary rocks do not usually hold the right kind of isotopes.

But radiometric ages can be assigned to sedimentary rocks using the stratigraphic timescale. A lava flow will be younger than the rocks it erupts onto and older than the sedimentary rocks that settle on top of it, for example. Lava is an igneous rock, suitable for radiometric dating, so this layer can produce a chronological age. And if the sedimentary layers can be matched with others of the same age around the world, then the age of the lava flow can be applied to many sequences of rocks.

Careful choice of igneous and metamorphic rocks for radiometric dating at significant places in the stratigraphic scale has produced important new results in the past few years. An example is the duration of the Cambrian period - a crucial time slot over half a billion years ago when a rapid burst of evolution produced the first creatures with shells. For a long time geologists thought the Cambrian period lasted at least 65 million years, from about 570 to 505 million years ago. Now they believe it started only about 540 million years ago - cutting its length in half. So the initial burst of evolution that produced complex hard-shelled animals must have been even faster and more amazing than palaeontologists had thought.

Layer upon layer

Stratigraphic principles

STRATIGPAPHERS built the geological timescale by studying and correlating strata over wide areas. The task is difficult because nature does not deposit rocks as neatly as people pile newspapers. The sediments that eventually become rock collect in low areas, such as the sea, while erosion wears down mountains.

Even within the sea, different rocks form in different areas, depending on factors such as the depth of water and the strength of currents. Further complications come from changes in sea level. This is evident in the distribution of coal deposits that were forming in tropical swamps that lay along the coast. Sea level rose when periodic warm spells melted the polar ice, then fell as the ice thickened. As a result, the coastline advanced and retreated, and the swamps and coal deposits moved with them.

To make sense of such deposits, geologists have to match rocks in different areas by matching what was happening when they formed, for example, rising sea levels. This modification of classical stratigraphy is known as sequence stratigraphy. It is widely used in the search for oil and gas.

All these factors mean that rock of the same age can look quite different from one part of the world to another, depending on whether the region was land or sea, quiet lake or fast-flowing river. But stratigraphers are able to make the most of this variation. Teams of geologists compare sequences of rocks of the same age around the world and select the one that is the most complete, with the detailed layering and the most useful fossils. This becomes the type section defining that particular boundary. Other boundaries, at different times, will have their own type sections, chosen where the rocks contain the most detailed information. All other rocks of around the same age are compared to this section to find out their stratigraphic age. But this method depends on being able to correlate rocks that formed at exactly the same time.

To establish worldwide correlations, stratigraphers need to find markers that they know formed at exactly the same time and that they can recognise over large areas. The ideal would be some recognisable material spread over the entire globe in a geological instant. The only example of that is the iridium-rich dust from the asteroid impact at the end of the Cretaceous period 65 million years ago. Large volcanic eruptions, scattering ash over large areas, are more frequent and the ash can have a distinctive chemical signature. About 454 million years ago, a volcanic eruption along what is now the Carolina coast blew a thousand cubic kilometres of volcanic ash into the atmosphere. Today the deposit can be traced as far as Minnesota, some 2000 kilometres away, where the ash layer is 10 centimetres thick.

Fortunately for us, such disasters are rare. Stratigraphers use other widely distributed markers - fossils. The ideal index fossil for dating is one that is common, and evolved rapidly. Trilobites are widely used for dating the Cambrian period. For later periods, stratigraphers use many different fossils that also evolved rapidly, such as ammonites, that developed many subtle variations in the structure of their shells during the Jurassic period.

Eons, epochs and ages

The dating hierarchy

THE INTERVALS of the stratigraphic timescale have not altered significantly since the early geologists first named them because many of the dividing lines mark times of great change across the face of the Earth. Early geologists chose the first fossil evidence of life on Earth to mark the start of the Cambrian period. They called the entirety of earlier time "Precambrian". Later geologists found earlier signs of life, some fossils with tiny shells and some with soft bodies. The Precambrian-Cambrian boundary stayed in the same place on the timescale, but is now defined as marking the appearance of hard-shelled animals.

The Precambrian-Cambrian boundary is the broadest division of the geological dating hierarchy, shown in Figure The hierarchy divides geological time into broad intervals, which in turn can be subdivided into smaller and smaller subintervals. The longest divisions are called eons. The first 2 billion years of the Earth's history are usually called the Archean eon; its end is set at 2.5 billion years ago, when the surface of the Earth took on the modern pattern of plate tectonics. The earliest part of the Archean eon - and of our planet's history - was Hadean times, when the Earth was so hot that there was no stable solid crust. The Proterozoic eon spans the time from 2.5 billion years ago until hard-shelled animals appeared. Precambrian time includes both Archean and Proterozoic eons. the following half billion years are the Phanerozoic eon, which is subdivided in greater detail because geologists know much more about it than about earlier eons.

Three eras make up the Phanerozoic eon: the Palaeozoic era of "early life", the Mesozoic era of "middle life", and the Cenozoic era of "recent life". Early geologists made those divisions based on changes they saw in fossils. We now know that the dividing lines mark two occasions when large numbers of species vanished in a geological instant - lasting perhaps a few million years, perhaps less. The worst mass extinction in the geological record came at the end of the Palaeozoic era; the victims included some reptiles and a host of marine animals that left abundant fossils. But the mass extinction made way for the dinosaurs that evolved to dominate the planet in the Mesozoic era, often called the Age of Reptiles. An asteroid impact contributed to the mass extinction that ended the Mesozoic era 65 million years ago. The mammals that survived were no larger than a cat, but in the early Cenozoic Era - the Age of Mammals - they rapidly evolved into a wider variety than we see today.

Eras, in turn, are broken into periods, which are mostly named after characteristic rocks or the areas in which they appear. For example, in 1835 the British geologists Adam Sedgwick and Roderick Murchison separately described Cambrian and Silurian rocks, with different and characteristic fossils from Wales. Hence the names of these two periods: Cambria is the Roman name for Wales and the Silures were an ancient Welsh tribe. As they continued to explore, both Segdwick and Murchison expanded their periods to include the same rocks. Their bitter argument was only resolved by adding a third period between the two to hold the contentious rocks. This was called the Ordovician period after the Ordovices, the last Welsh tribe to submit to the Romans.

Periods last for tens of millions of years, so geologists split them into smaller divisions called epochs, then divide these into ages. Most of these subdivisions are important only to specialists, except in the Cenozoic era, where epoch names are widely used because of the better resolution of dates in younger rocks. The epochs become shorter toward the present, reflecting greater knowledge of recent events. The current Holocene epoch spans only the past 10 000 years, essentially the time since the last of the great ice sheets melted in Europe and North America. Before that, the Pleistocene epoch spans the series of ice ages and interglacial periods from about 2 million years ago until Holocene times(see Figure).

Refining the record

Modern methods

CLASSICAL stratigraphy has become a thoroughly modern tool for oil and gas exploration through the use of modern correlation and dating methods. An especially powerful technique is magnetostratigraphy, based on changes in the weak magnetic field held by some rocks. The remanent magnetism of a rock shows the direction of the Earth's magnetic field at the time when the rock formed. At irregular intervals averaging every half-million years during the Cenozoic era, the north and south poles of the Earth's magnetic field swapped position. The periods of normal and reversed magnetism vary in length, so that they form a distinctive enough pattern to correlate sequences of rocks around the world, from land and sea. This is called the magnetostratigraphic scale. It is like dating wood on the basis of the widths of the growth rings formed each year. One tree ring alone is useless, but a sequence of them will form part of the overall pattern. And because many of the magnetised rocks are igneous, this scale can be calibrated directly by radiometric dating.

While rocks often contain information that can reveal when they formed, few give much indication of how rapidly they formed. However, there are a few important exceptions. The deposition of material on quiet lake bottoms can vary seasonally, leaving visible layers called varves which show annual cycles of deposition, with interruptions caused by winter freezing of the surface, or dry seasons. Glacial lake deposits show sequences of alternating coarse and fine layers deposited during summer and winter respectively. Some rocks contain thousands of these annual layers, making them a treasure trove for scientists interested in climate change in the past.

At the opposite chronological extreme are thick layers deposited very quickly by some catastrophic event such as a volcanic eruption, tsunami, or submarine or surface landslide. Such unusual happenings can turn out to have geological significance. When an underwater slope collapses, it can preserve unusual communities, such as the strange soft-bodied Cambrian animals preserved in the Burgess Shale in Canada.

As the dating tools improve, geologists continue to refine the geological timescale. While most adjustments are minor, some have important implications. For example, the shrinking of the Cambrian raises new questions about how animals managed to evolve so much diversity in so little time.

Conversely, other research often raises questions that require improved dating, especially better correlation of deposits from different areas. Among the hottest issues are the timing and pattern of widespread extinctions at the end of Permian times. Research indicates that there may have been two peaks of extinction, separated by a few million years. Big questions remain, yet an improved timescale may be the key that unlocks the mystery behind the greatest extinctions of all time.

Radiometric dating

THE natural clock that lets geologists measure the age of certain rocks is based on the decay of unstable atomic nuclei. Scientists do not know when any single nucleus will decay, only the probability that it will decay, which they usually express as the helf-life. One half-life is the time it would take half the nuclei in a sample to decay (meaning that each nucleus has a 50 per cent chance of decay in that time). If you start with a million atoms of an isotope with half-life of 100 years, half of them will remain after 100 years, and half of that half, or one quarter, will remain after another hundred years. Thus, as time passes, a rock will contain fewer of the radioactive nuclei and more of the isotopes produced.

Geological dating requires radioactive isotopes that are reasonably common and have very long half-lives, comparable to the ages of the rocks themselves. One choice is the family of long-lived uranium and thorium isotopes that decay to lead through a series of short-lived intermediate nuclei. The most common is uranium-238, with a 4.5 billion year half-life, which decays to lead-206. Others are uranium-235, with 710 million year half-life, which produces lead-207, and thorium-232, with a 13.9 billion year half-life, which yields lead-208. Although uranium-lead dating is complicated by the need to account for lead compounds originally present in the rock, the three isotopic systems provide crosschecks. Uranium-lead dates give the age of the Solar System, and of the oldest rocks on Earth.

An alternative that works better for some rocks is the decay of potassium-40 nuclei to argon-40 with a half-life of 1.3 billion years. Potassium is common in rocks, and argon is a gas, so it normally escapes from molten rock. This means that most argon in older volcanic rocks should come from the decay of potassium-40 in the time since the rock solidified. So comparison of the levels of potassium-40 and argon-40 can indicate the age of a rock. But errors can occur with this method; for example, separate samples are needed to measure argon and potassium, and their compositions may differ slightly even if they come from the same rock. Better results come from a more elaborate process that runs a single sample through a nuclear reactor, where the common potassium-39 nucleus absorbs a neutron and emits a proton to become the short-lived isotope argon-39. Heating the sample then drives off both argon-39 and argon-40 and, with proper calibration, the ratio of the two isotopes gives the age of the rock.

Zircon is a mineral widely used for dating; it holds atoms even at high temperatures, and contains both uranium and potassium. However, old zircon crystals can be deposited in younger rocks, so merely dating the zircon in this case would not give the age of the sedimentary rock. Geologists also must watch for signs of chemical alteration that would give spurious dates, perhaps by allowing either the original isotope or its decay products to escape from the rock.

Good radiometric dating gives ages to within a quarter of 1 per cent, a margin of 250 000 years for a date 100 million years ago. These dates have given geologists a solid chronological calibration for points on the geological timescale where the right rocks are available. Radiometric dates can also point out serious errors in stratigraphic dating, such as the Alaskan rocks that turned out to be 6 million years old rather than 16 million. However, radiometric dating is not easy, and it is not good at telling the sequence of closely spaced events, such as those before and shortly after the impact which ended the Cretaceous period 65 million years ago.

From issue 1978 of New Scientist magazine, 20 May 1995, page 1

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