Geological time

When you have been through this page, practics your skills here.

Dating: How do we know when things happened?

There are several sorts of dating used by geologists: relative dating, radiometric dating and fossil dating.

Relative Dating

Consider the example below of a cross section, based on the geology of an area near Port Waikato.
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 of geological events that would produce these rocks in this pattern.

The youngest is on the top:
This is the first rule we apply, and is called the 'rule of superposition'. It applies to rocks which are in layers like those above (even if the layers have been tilted or folded, as above). Applying this rule to the rocks above, it can be seen that the green sandstone (it is 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 se above. Instead, you might see:
A geologist looks for bedding in the sedimentary rocks, which were originally horizontal. The angle at which the rocks are tilted is measured and used to construct the cross section at the top. In this way, large vertical thicknesses of rock can be seen by moving horizontally.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 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. Geologists use compasses and clinometers to work this out.
Geologists usually check that the rocks are the 'right way up' by looking for clues in the bedding (for example, layered sandstones usually have larger grains on the bottom of the layer than at the top). Occasionally, rock layers get flipped upside down, but you won't be given any of these in the exam.

If you look at the coal in the top sequence, you can see that it rests on the rocks below it at an angle, and forms a 'cutoff' to them. 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, and 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.
Below is a photo of glacial deposits unconformably overlying a gabbro in a road cutting near Fox Glacier on the West Coast of the South Island (geologists love road cuttings, particularly when they are fresh; the road workers had stopped when I took the photo because it was raining too much).
The top of the gabbro is weathered into fossil soil. Geologists call this a paleosol. The gabbro (itself an intrusive rock) is intruded by a vertical sheet of granite (we call these ''dykes"). In this particular case, the gabbro and granite are both very old (about 350 million year; the granite is probably a few million years younger than the granite) and the glacial deposits probably come from the last 'glacial maximum' about 18000 years ago. That's why the top of the glacial deposits hasn't formed much of a soil horizon of its own yet - it's too young.
There are a few other rules you can use in working out a sequence of events like this:
  • if igneous rocks (like a volcanic pipe or an intrusion) 'cut across' other rocks, they are younger than anything they cut across. i.e. the granite above is younger than the gabbro (but in this case, probably not a lot)
    In the sequence at the top of the page, the basalt must be the youngest rock because it 'cuts across' everything else.
  • if rocks are offset (moved) by a fault, the fault is younger than the rocks that have been offset. (I'll try to produce a diagram to demonstrate this also)

Here is another photograph of an actual unconformity, at Matheson's Bay, north of Auckland


The Kawau gritstone was laid down on a very irregular, rocky coastline of greywacke that was rapidly sinking at the beginning of the Kaikoura orogenic period, about 20 million years ago. The greywacke 'pokes' up through the younger material, and is exposed at the present day coastline where the softer, younger material has been more easily eroded away. Below is a close-up of the grit:
As you can see, it consists mainly of fragments of the greywacke itself (this is right at the point of unconformity). This material would have been a gravel beach or deposited just below the low tide mark - it contains quite a few fossils.
As the landscape sank, this was quickly overlain by current-borne deposits of muddy sandstone from further away. You can see the slightly higher up material in the photo below right, with the older greywacke 'poking up' through it. When a younger landscape is eroded away to reveal an older one like this, geologists refer to it as an 'exhumed' landscape.

The sequence at the top of the page, from oldest to youngest then is: sandstone (1), conglomerate (2) mudstone (3), siltstone (4), coal (5), limestone (6), marl (7), basalt (8).

These rules only tell you how old the rocks are in relation to each other (older than..., younger than...).They DON'T tell you how old the rock actually is - that's why it is called 'relative dating' (it doesn't mean finding out how old your grandma is). . However, if I knew that the basalt was 2 million years old, I would know that the marl was older than that.
Have a look at this diagram and see if you can work out the relative ages of the different strata. Answer here

Radiometric dating:

This means using radioactive minerals, which change at a known rate. It is accurate for dating volcanic and plutonic rocks. For metamorphic rocks, it gives the age of metamorphism (when the new minerals were formed) which could have a variety of meanings. Using this method on sedimentary rocks will give the age of the original rocks that were eroded, not the time when the sediments were laid down.
Note that carbon dating is not used much in geology, because it has an maximum age limit of about 40,000 years which is much too young for most rocks. It also only works on things that were once living e.g. wood. It is sometimes useful for things like very young volcanoes - Mt Wellington in Auckland is 'dated' at 9000 years old from wood buried under some lava from it. However, Mt Taylor (which is nearby) is too old to carbon date, probably 50,000 to 100,000 years.

In the sequence above, only the basalt could be dated this way.
Geologists need to be quite careful when 'interpreting' this sort of dating because a variety of factors can make such a date appear to be younger than or (less often) older than it really is. Therefore, we look for supporting evidence e.g. is it consistent with what is worked out from relative dating.


Fossils change over time, so certain fossils are very useful indicators of time; this is termed biostratigraphy. They only occur in sedimentary rocks. The actual age of some 'key' fossils has been determined by matching sedimentary rocks with volcanic rocks that are known to be the same age (by careful fieldwork). For example, Inoceramus (shown right) is a common fossil in rocks of Mesozoic age in New Zealand, and is never found in rocks younger than 65 million years old. A species called Inoceramus galoi is found in the lowermost sandstone in the sequence above, and helps to date it as Jurassic (see table below).

Geological timescale: By using a range of the methods above, geologists have come up with a 'timescale' in which different periods of time are given different names. When it was first developed in the 1800s, this timescale was entirely biostratigraphic - based on fossils. Geologists at the time had no idea of the actual dates in millions of years. However, in the 20th century, starting with Ernest Rutherford, scientists were able to start pinning down dates on the various time periods.using radioactivity. The names of the timescale nevertheless continue to denote strata with distinctive fossils.
You don't need to memorize this timescale for any NCEA standards - it is for interest only. However, geologists use it a lot and you may want to refer to it if you come across terms in your reading. Note that the time periods are not all equal. "ka" means thousands of years and "Ma" means millions of years (geologists use "a" for "annum", meaning year, possibly because scientists tend to like Latin).
I have added a potted history of the rocks of NZ, which is the main part of the Year 12 Geology course.