Fire through time
Globally, fire was only possible once there was sufficient oxygen in the atmosphere. Fires on Earth were only possible with the production of oxygen by marine life in the Devonian period, which resulted in atmospheric oxygen exceeding 13%.
What evidence is there of fire in geological time?
There are a variety of sources of evidence for the presence of fires in geologic time. These include:
- Fusian – a fossil charcoal which occurs in coal-bearing strata
– in Australia the La Trobe valley is well known for the presence of fusian. - Baked clays – these have been found in coal measures between sediment layers.
- Fire holes – these formed particularly during the Pilocene (5 to 2 M yr BP) in brown coal.
- Burned bones.
Sources of ignition during this time would primarily have been lightning, and spontaneous combustion (especially in swampy vegetation), but also would have included ignitions from volcanic activity, rock falls and meteorite impacts.
Given the importance of lightning and the need for flammable fuel, fire activity and climatic conditions would also have been strongly related.
In northern Australia prior to human occupation, lightning-ignited fires would have been a key feature of the landscape. The savannas of Australia are particularly primed for regular fire as a consequence of the monsoonal climate. Grasses grow tall and thick in the wet season and then become highly flammable during the dry season when much of the countryside is on fire.
Since the mid-Pleistocene however, the story of fire in the landscape has been closely linked with hominids and human development; without doubt humans have played a key role in distributing and controlling fire across the globe.
Have a look at the following web site to familiarise yourself with the geological time scale.
