Relative age dating principles

The age of the fossil must be determined so it can be compared to other fossil species from the same time period.

Understanding the ages of related fossil species helps scientists piece together the evolutionary history of a group of organisms.

The principle of superposition states that in an undeformed sequence of sedimentary rocks, each layer of rock is older than the one above it and younger than the one below it (Figures 1 and 2).

Accordingly, the oldest rocks in a sequence are at the bottom and the youngest rocks are at the top.

" First, the relative age of a fossil can be determined.

Relative dating puts geologic events in chronological order without requiring that a specific numerical age be assigned to each event.

Sometimes sedimentary rocks are disturbed by events, such as fault movements, that cut across layers after the rocks were deposited.

This is the principle of cross-cutting relationships.

Each time a new layer of sediment is deposited it is laid down horizontally on top of an older layer.

Despite seeming like a relatively stable place, the Earth's surface has changed dramatically over the past 4.6 billion years.

Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free.

Third, magnetism in rocks can be used to estimate the age of a fossil site.

This method uses the orientation of the Earth's magnetic field, which has changed through time, to determine ages for fossils and rocks.

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