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Dating rocks and fossils

Lightly adapted from from Geological Process in NZ, by P. Keestra.

There are two primary ways of dating used by geologists: relative dating, radiometric dating and fossil dating.

Relative Dating

Consider this cross section, based on an area near Port Waikato.  (This is on the bank of the Waikato River, in New Zealand.)



The units on the key in the left are not shown in the correct order. Can you work them out? (answer down the page)

There are several rules we can use to work out the sequence:

The youngest is on the top:

1.  ‘Rule of superposition’.  Works even if the layers have been tilted or folded.
Applying this rule to the rocks above, the green sandstone ( only green on the diagram, not in real life) is the oldest rock, then the conglomerate, then the mudstone.

These rocks are folded (into an ‘anticline’), so you would not necessarily see the pattern at the surface we see above. Instead, you might see:


A geologist looks for bedding in the sedimentary rocks, which were originally horizontal.

Obscured rocks can be worked out. Example: What type of rock is under the landslide at ‘Q’? in the diagram? It is the siltstone, because it is above the mudstone and you can see it on the other side of the anticline. If you are walking along the road above from left to right, you would see the bedding go from dipping to the left on one side, to horizontal in the middle, to dipping to the right on the other side.

2. Unconformity 

If you look at the coal in the top sequence, you can see that it rests on the rocks below it at an angle. This ‘cutoff’ is called an unconformity.

It indicates a ‘gap’ in time – enough time for the sequence below it to be uplifted and eroded away to form the level surface you can see at the bottom of the coal.

This may mean that millions of years passed between the siltstone and the coal. The rocks that are in the bottom layers must eventually have sunk below the sea again before the next lot of sediments were laid down.

In fact, at Port Waikato where this section is based on, the sequence at the bottom was formed in the age of dinosaurs. The youngest rocks below the unconformity are about 140 million years old (from fossil evidence). The coal above the unconformity is about 100 million years younger – so this ‘gap’ represents a huge period of time in which New Zealand eroded away to a level, swampy plain (hence the coal) and then sank below the sea (forming the limestone).

There is a good picture of an actual unconformity in America here.

Unconformity between Popotosa Formation playa deposits and post-Santa Fe Group (Pleistocene or younger) alluvium, San Lorenzo Canyon area, north of Socorro, New Mexico.

{ http://geoinfo.nmt.edu/staff/scholle/graphics/Unconf.html }

Related articles

Carbon dating


Prentice Hall Earth Science, Tarbuck & Lutgens. Chapter 12: Geologic time

Learning Standards

A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas (2012), from the National Research Council of the National Academies.

By the end of grade 12. Radioactive decay lifetimes and isotopic content in rocks provide a way of dating rock formations and thereby fixing the scale of geological time.

College Board Standards for College Success: Science

ES.3 Earth’s History: Relative and Absolute dating. Students understand that various dating methods — relative and absolute — have been used to determine the age of Earth.

Suggested Connections. Between Earth Science and Other Disciplines: Evidence of Common Ancestry and Divergence (LS.1.1); Living Systems and the Physical Environment (LS.3.1); Nuclear Chemistry (C.1.6); Nuclear Interactions and the Conservation of Mass–Energy (P.2.3)

Benchmarks: American Association for the Advancement of Science.

Knowledge of radioactivity helps them understand how rocks can be dated, which helps them appreciate the scale of geologic time… Scientific evidence indicates that some rock layers are several billion years old. 4C/H6** (BSL)

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