Johns River

The Birth of the Three Brothers

A Geologic History

By  Dr. Mary E. White

The Three Brothers are mainly composed of microgranite dated at 205my (million years ago), in the Late Triassic Period. (Microgranite is a relatively fine-grained granite that had crystallised from extra-hot granitic magma at about 1000ºC.) The volcanic activity for their production occurred in the centre of the Lorne Basin,  whose sedimentary rocks, the Camden Haven Group, are of Early Triassic Age. (Mount Bago and Mount Juhle are part of the same volcanic episode as the Brothers, as is the small volcanic outcrop on Diamond Head, according to latest geological maps.)

The Triassic Lorne Basin

The Lorne Basin contains large areas of other volcanic rocks of Triassic age, largely unnamed and not forming such dramatic features arising out of flat landscapes like the brothers. The Basin and its Triassic volcanism are all part of the New England Fold belt geological activity that formed part our continent’s rising eastern sector as we know it now.

Also occurring within the mapped boundary of the Lorne Basin are much younger volcanic plugs, including Big Nellie (only 16my old) that are part of the Comboyne volcanic activity of Tertiary age. Their geological history is completely different, associated with the northwards drift of the Australian continent over a “hot spot” in the Earth’s crust, like the Glasshouse Mountains etc.

The “Big Brother” Volcano

So, what was it like in the Three Brothers Region about 200my ago, when the district was at 85'S, near the South Pole and Australia was part of the Super-continent Gondwana?

The accepted idea is of vast Mount Fuji-style volcanoes on the edge of the continent – and the Brothers and their tribe today are all that is left after 200my of erosion!

And the climate then in those high latitudes was mild was mild, no polar ice, and landscapes were  well vegetated with seed ferns, ferns, cycads, lycopods, early conifers etc. (no Flowering Plants yet) and with reptiles and amphibians and insects and other invertebrates comprising the land fauna (no mammals yet).

Present day remnants of the 205 my volcanoes – the Three Brothers and their changing surrounding landscapes (and the vast Continent of which they  are part) have been rafted  northwards in the last 45 million years since Australia became an island continent. The distance travelled has been more than 3000 km. They, and us, are still travelling northwards at over 7cm a year.

For the lucky residents of the Camden Haven region the Three Brothers are at very heart of our Sense of Place and Belonging.

Below: Over 200 million years ago, the vents of the Laurieton supervolcano, (the “Big Brother” to our Three Brothers and Diamond Head today), was over a kilometer high, looking much like Mt. Fuji in Japan today.