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Introduction

The Earth is just over 4.5 billion years old. It is difficult to appreciate exactly the scale of this number. If the entire history of the Earth were compressed into 1 year then the Earth was formed on Jan 1. During January the layers of the Earth developed into core, mantle and early crust. During February much of the oceans had been formed and toward the end of February life evolved in the form of simple organisms. In April the atmosphere gained free oxygen and by early May modern plate tectonic processes were operating. Inlate October organisms with hard shells had evolved. On December 7 reptiles were roaming the land and on December 25th the dinosaurs became extinct. Homo Sapiens made their appearance at 11pm on New Years Eve and Neil Armstrong landed on the Moon just a microsecond before the stroke of midnight.

Time is a fundamental concept in geology. Geologists need to understand the dating of events (how long ago they happened) and the order of events (that is, which event happened first and which was next).

The science of dating rocks in all its aspects is called "geochronology" and two methods are used:

a) Absolute time is gauged if a numerical age is given to a rock or an event, e.g. "this granite was intruded at 295 Ma" means the granite was intruded 295 million years ago.

b) Fossil evidence can be used to correlate with other rocks of known absolute age by comparing fossil assemblages. This is known as biostratigraphy.

Absolute Time

Absolute age is measured by using the radioactive decay of certain isotopes found in rocks and minerals.

The "half life" is the amount of time taken for the activity of a radioactive isotope to decay to half its original value; that is, for half of the parent atoms present to decay.

This process is not a linear decay but an exponential decay. During decay, the element is converted from the original "parent" element to a "daughter" element. The process by which it achieves this is called a "decay scheme".

It is indeed convenient that the rates of decay of all common radioactive isotopes can be measured on pure samples in the laboratory. If the ratio of parent and daughter isotopes present in a material when it was formed is known, the current ratio can be measured and, knowing the decay rate, the age of the material or rock can be calculated.

For example, one particular isotope of Uranium decays to an isotope of Thorium. The parent isotope (234)U is soluble in water, whereas the daughter isotope (230)Th is not soluble in water. When coral reefs grow by precipitating calcium carbonate (CaCO3) hard parts, the calcium carbonate contains a few parts per million of Uranium as a so-called trace element. This Uranium was dissolved in the sea water and precipitated within the calcium carbonate when the coral grew. However, the calcium carbonate contains no Tho- rium, as there was no Thorium in the seawater from which it precipitated.

The U/Th ratio when the coral formed is therefore known (some amount of Uranium divided by zero Thorium). This ratio will change through time as the (234)U decays to (230)Th. If we measure the (234)U / (230)Th ratio at the present day, and know the rate of decay, we can gain the age of the coral. The measurement is made using a mass spectrometer, a machine that can separate isotopes with different mass.

This technique is commonly used to date corals and the sediments which contain the corals. For example, corals from the Gulf of Corinth, Greece formed at 125 ka (125,000 years ago), constrained by U/Th dates. The deposits at the top of the 30 m high sea-cliff have been uplifted 30,000 mm in 125,000 years, implying that the land has risen relative to the sea at a rate of 0.24 mm/yr. Thus, radiometric ages can constrain the rates of geological processes.

Uranium series and Rubidium series decay schemes are used to date ancient rocks. In contrast, the carbon-14 system is used to date young organic material such as archaeological remains.

The Earth is thought to be 4567 million years (Ma) old. There are no known rocks as old as this on the Earth, but almost all meteorites are found to be of this age. Many meteorites are thought to be left over pieces of small planets which formed and broke up during the early history of the Solar System. Also, the oldest rocks from the Moon are older than 4200Ma. On the Earth, a few zircon crystals in an ancient sandstone in Australia have been dated at 4100 Ma, but these are detrital minerals and the igneous rocks in which they originally crystallised have not been found. The oldest rocks on the Earth are found in Canada (3900Ma). Any rocks which formed earlier than this were probably destroyed by either tectonic processes or by meteorite bombardment in the early history ot the planet. The Moon is a much smaller planet with no plate tectonic activity, and thus older rocks have not been destroyed.

Radioactive decay schemes such as the U and Rb systems mentioned above can only be used to date the time at which an igneous rock crystallised at high temperatures or a metamorphic rock was recrystallised. Sedimentary rocks cannot be dated in the same way. However, about 570Ma ago, many animal groups began to evolve hard skeletons or shells, and thus when the animals died, their skeletons or shells began to be preserved as fossils. Fossils in sedimentary rocks enable us to date these rocks according to the relative timescale.

Relative Time

The science of stratigraphy deals with the order in which rocks were deposited. It breaks down the rocks into rock units, which were deposited in time units. The largest time unit is the Eon and is illustrated in the diagram to scale.

Image source: BGS geological timechart http://www.bgs.ac.uk/discoveringGeology/time/timechart/phanerozoic/

There are three Eons:

The Archaean Eon (Greek archaios = ancient), represented by ancient metamorphic rocks which may pre-date plate tectonics as they are known today.

This was followed by the Proterozoic Eon (earlier life), which represents the time between the beginning of modern plate tectonics and the onset of common visible hard fossils.

The third is the Phanerozoic Eon (the eon of visible life). In terms of the absolute time scale, the Archaean eon lasted from 4600 Ma to 2500 Ma, the Proterozoic lasted from 2500 to 570 Ma, and the Phanerozoic began 570 Ma ago and is still continuing.

Each Eon is divided into smaller time units called Eras, and each Era is subdivided into Periods. Stratigraphers use even smaller time units called "epochs" and these in turn are divided into stages. The Phanerozoic Eon is divided into three Eras: the Palaeozoic (old life), the Mesozoic (middle life) and the Cenozoic (young life). The Phanerozoic Eon is also divided into 11 major periods of time. The order in which these periods occurred is of fundamental importance to an understanding of geology.

Palaeontologists are experts in identifying fossils and can ascribe them to a genus and species, and can say which period, epoch and even zone (a smaller unit of time) a fossil is found in. Good zonal fossils which are useful for accurately identifying the age of a rock are formed from animals which evolved rapidly and which could distribute themselves widely around the Earth. Examples include graptolites in the Silurian and Ordovician and ammonites in the Jurassic. Each Period (and smaller time units) has its own characteristic assemblage of fossils, and thus a rock-unit if it contains fossils can usually be dated very accurately.

Combining Absolute and Relative Time

Calibrating the stratigraphic column may be carried out by radiometric dating of igneous rocks such as lava flows. A lava flow will post-date the surface over which it flowed and predate the sediments which cover it. An igneous rock may occur between sediments that contain fossils so that crucial date points may be applied. Revisions of the geological time scale appear from time to time. Important ages include 570 Ma (the beginning of the Palaeozoic Era), 245 Ma (the end of the Palaeozoic and beginning of the Mesozoic) and 65 Ma (the boundary between the Mesozoic and Cenozoic Eras). Note that human remains are only a maximum of 4-5 Ma old and that the last major Ice Age, which has shaped so much of our topography, is confined to the last 1.6 Ma.

Geomagnetism and Magnetic Field Reversals

Many studies of magnetism in rocks over geological time (Palaeomagnetism) have revealed that at various times the Earth's magnetic field has been the reverse of the present field, that is, the magnetic poles have changed places. Such periods are known as field reversals.

Field reversals are particularly well recorded in the igneous rocks which form the ocean floors. The progessive growth of the ocean floors at sites of asthenospheric upwelling has produced a series of linear magnetic anomalies. Each anomaly records either a "normal" or "reversed" geomagnetic field. Magnetic field reversals are also recorded in vertical successions of lava, which Epochs named after scientists who discovered them.

Image Credit: http://www.coolgeography.co.uk/