How many eras on the geologic time scale

Seas covered Britain and there was dramatic volcanic activity as the ocean separating England and Wales from Scotland started to close. In the Cambrian Period, England and Wales lay near the south pole and experienced a cold climate. They were separate from Scotland, which was joined to North America. A shallow sea covered much of the area and animals such as trilobites, graptolites and molluscs first appeared.

The Precambrian is the name given to the span of time prior to the Cambrian. The Precambrian period accounts for 88 per cent of geological time. There are very varied deposits from the Neoproterozoic, including volcanic sequences, sedimentary rocks formed in environments from deep water to terrestrial, plutonic igneous rocks and metamorphic rocks. Multicellular life developed and diversified rapidly.

In the Mesoproterozoic Era, England and Scotland lay on different continents. Mudstones, sandstones and volcanic sediments were deposited in shallow seas. Multicellular life developed.

The only life on Earth was single celled. The principal chart shows the Phanerozoic Cambrian to Quaternary timescale. The names of the individual periods are links: each one leads to a more detailed chart showing the epochs and ages for that period. The charts for the individual periods are all drawn to the same scale. Where there is insufficient or contradictory data on the dating of age boundaries, the ages are given an average duration within the limits of the appropriate epoch boundaries.

These age boundaries appear as dashed lines on the charts. More detailed stratigraphical information is contained in the Stratigraphical Framework Committee reports. Intervals of geological time are given formal names and grouped into a hierarchy according to their length in decreasing time intervals :. Chronostratigraphy is similar, but is concerned with the relationships between time and the rocks deposited within those time intervals.

Chronostratigraphical terms are applied to rocks deposited during specific intervals of time. In order of decreasing time, their hierarchical grouping is:.

Although the age of rocks can sometimes be determined directly, by radiometric dating, in most instances rocks are dated indirectly, often by means of their fossil content. Historically, named divisions of geological time and their associated chronostratigraphical divisions have been developed independently in different parts of the world.

A major task for the world stratigraphical community has been to establish a single, universal scheme for the subdivision of geological time. There is agreement at the levels of eon, era and for the most part period, but regional terms continue to be widely used at the lower hierarchical levels.

Thus at any one time, deposits of different type and different fossil content were being deposited in different regions of the globe.

For some intervals and some regions, however, global correlations are poorly understood and the rocks are assigned to divisions of regional application. The oldest fossils are between 3 billion and 3. These are fossil bacteria, and for most of Earth history, life was simple. More complex animals appeared in the oceans about million years ago, and became much more common about million years ago. This last point in time is the start of a division of geological time called the Phanerozoic Eon.

These were named for the kinds of fossils that were present. This is because the fossils are similar to animals and plants that are common today. The Paleozoic era begins with the Cambrian radiation, a time of great growth in the number of different kinds of animals in the oceans.

It ends with the greatest extinction in the history of life. That means changes happen in spurts, rather than at some slow and steady pace. Take the Precambrian Era. That burst marked the beginning of the Paleozoic Era. Sea creatures like trilobites and fish emerged and came to dominate.

Then, million years ago, the Mesozoic Era burst into being. It marked the biggest mass extinction of all. It also kicked off the spread of life on land.

This era then ended abruptly — and famously — But they did understand relative ages, based on a simple, yet powerful principle. That principle is called the Law of Superposition. It states that in an undisturbed stack of rock layers, the oldest layers will always be on the bottom, and the youngest on top.

The Law of Superposition allows geologists to compare the age of one rock or fossil to another. It makes the sequence of geologic events more clear. After all, they lived millions of years apart. Still, how can we make sense of a calendar with no dates on it? To assign such absolute ages to the Geologic Time Scale, scientists had to wait until the s.

Certain isotopes — forms of elements — are unstable. Physicists refer to them as being radioactive. Over time, these elements shed energy. The process is called decay and will involve shedding one or more subatomic particles.

Eventually, this process will leave the element nonradioactive, or stable. And a radioactive isotope always decays at the same rate. Scientists measure how much of the parent element still exists in a rock or mineral.