Learn how geologists find out the ages of rocks and fossils to help explain how New Zealand’s structure – and the life it supports – has changed over millions of years.

This resource provides explanations of the key concepts encountered when exploring dating the past – the ‘basics’ that every student should understand when look at measuring the age of rocks and fossils.


The study of the Earth – the rocks that make it, its structure and the way it has changed over time.


A branch of geology that uses fossils to study the history of life on Earth.


Remains of once-living organisms preserved in rocks. Includes shells and bones, often replaced by other minerals, and traces such as tracks.

Igneous rock

Formed by the cooling of molten rock. Igneous rock may be volcanic (made above the surface) or plutonic (formed deep below ground).

Sedimentary rock

Rock made of material derived from other rocks by weathering or by chemical deposition. Most fossils are found in sedimentary rocks.

Ice ages

Times when ice sheets covered large parts of the Earth’s surface. Also called glacial periods, they occurred in cycles with warm interglacial periods in between.


To study rocks or sediments hidden below ground or below the sea, geologists drill out narrow columns of material called cores.

Relative dating

A technique used by geologists (and other scientists) to put material into a sequence from oldest to youngest.

Absolute dating

A range of methods for providing actual dates for material, including some types of rock.

Radiometric dating

Absolute dating based on measuring certain radioactive elements found in rocks. The amount they have decayed is related to how long ago the rock was formed.

Geological timescale

The arrangement of Earth’s 4.6 billion year history into time divisions, based mainly on the appearance and extinction of key living organisms.


A prefix, meaning ancient or prehistoric, which starts a number of words used by geologists. The standard spelling used by New Zealand scientists is ‘paleo’ but you might come across the spelling ‘palaeo’ used in some other countries.

    Published 11 May 2011