Dense and vescicular in the one flow. 



(Updated 10/3/2018)

New evidence indicates that Mount Elephant erupted about 184,000 years ago. (Previously we thought it may be 10,000 to 20,000 years). It was just one of maybe several hundred eruptions across the western plains over several million years.

A project of Curtin University of WA is doing some research on a new way to date eruptions which eject a certain mineral called pyroxene. A description follows:

" The pyroxene samples we've acquired were of outstanding quality and are still being processed for analysis. I'll briefly explain our workflow to you:

1) we make use of the fact that upon eruption rocks are exposed to comogenic radiation. This radiation converts a calcium (40Ca) isotope in (to) an argon isotope (38Ar) at a certain rate. This is comparable to the changes of our skin during prolonged exposure to UV radiation (tan skin = long exposure).

2) this exposure age dating technique is especially interesting in dating 'events'; such as tsunamis, landslides, earthquakes etc, where rocks get suddenly exposed to the Earth's surface. A new generation (of) instruments makes it possible for the first time ever to measure the extremely small quantities of argon gas produced.

3) Most radiogenic dating techniques require separate analysis of the 'parent (40Ca)' and 'daughter (38Ar)' isotopes - however, we use a special trick. The Mount Elephant pyroxenes are currently being irradiated in the Oregon State TRIGA reactor, where 40Ca (the parent) is converted in 37Ar; another argon isotope that is not normally present in rocks. This means that we can measure the parent and daughter at the same time during our experiment - greatly improving accuracy and precision.

4) using an appropriate rate of conversion (see point 1), we can determine the exposure age. You might recall that we've also sampled rocks from the quarry: these will give us the eruption age. If no erosion took place, we expect the eruption age and exposure age to be the same."

This process resulted in an age of about 184,000 years.

Click here for an abstract of their work

Melbourne University researchers have used a different method to date Mount Widderin and arrived at 390,000 years.

Click here to read an abstract of their work.

Monash university have used a different method again for Mount Rouse at Penshurst and arrived at about 380,000 years.

Earlier work has found Mount Eccles to be younger at about 30,000 years.