A geologist says it is not uncommon for coal to wash up on Queensland beaches, and soon you could add pumice from Tonga's volcanic eruption to your collection of beach debris.
Senior lecturer of earth and environmental science at James Cook University, Maree Corkeron, said debris from the eruption of Tonga's Hunga Tonga-Hunga Ha'apai volcano in January is expected to arrive on Australian shores this year.
Dr Corkeron said it had to do with north Queensland's proximity to the volcanic hotspot known as the Pacific Ring of Fire, or the Circum-Pacific Belt — a 40,000-kilometre-long arc home to 75 per cent of the Earth's volcanoes.
Beaches along north Queensland that were downwind, or down current, from the Ring of Fire were more likely to see pumice.
Dr Corkeron said what washed up on the sand really depended upon activity in the Pacific Ocean.
"It's a combination of our location, tectonics, and global trade winds," she said.
"Local currents and wind conditions will determine exactly where the pumice ends up."
Stone that floats
Dr Corkeron said where the oceanic continental plates were crashing into each other and generating volcanoes, like the one that exploded off Tonga, "generate huge amounts of ash and rocky material".
Pumice is formed when volcanic matter hits the water and cools quickly.
The material often clumps together and forms huge rafts.
"Sometimes those rafts are hundreds of kilometres in diameter," Dr Corkeron said.
Waves break down the pumice, and by the time it reaches land it was often the size of a pebble.
But it could be much bigger.
"The whole beach was covered in great big pumice blocks."
But not all of the pumice would make it ashore.
"By the time pumice gets close to our shores it may have been floating for many months and picked up a lot of hitchhikers," she said.
Molluscs, bryozoa — aquatic invertebrate animals — and even coral is known to grow on the floating debris, adding to the weight of the pumice and causing some of it to sink.
"It also acts as a very important mechanism of rafting and dispersing these marine organisms," she said.
Dr Corkeron said she would expect to see debris from Hunga Tonga-Hunga Ha'apai in about six to seven months' time.
"As there is quite a lot of submarine volcanism in the western Pacific related to subducting oceanic plates, and associated with island arcs like Tonga and Vanuatu, we do get quite a lot of pumice on eastern Australian beaches," she said.
Compacted sea life
When Nicola Carlisle moved to north Queensland two years ago, she was surprised at what she discovered on Townsville's beaches.
Ms Carlisle said she regularly spotted black lumps on the beach and noticed others sorting through the debris to take it with them.
"I started to notice there was more and more in the last few weeks and some were getting quite big in size," she said.
On her last walk, Ms Carlisle picked up one up and was bewildered.
"It didn't smell like something the ocean had made."
Dr Corkeron said while coal can be found on our beaches, it was more likely to have been sediment.
As marine biota, seaweed, and organic material dies, it settles at the bottom of the ocean and is compressed by sand over millions of years.
"[It] accumulates in the bay over a long period of time. It gets buried for hundreds of thousands of years," Dr Corkeron said.
Dr Corkeron said the increase in sediment on beaches could also be due to a $232 million channel upgrade at the Port of Townsville.
Coal differentiates itself from compressed sediment by typically fracturing along its cleat lines rather than crumbling.
"[The coal] might be covered in barnacles and some have things growing on it. Anything floating in the ocean is a nice little host for biota to settle … so you might find there are little bits of mollusc," Dr Corkeron said.
Two-hundred kilometres south lies one of Australia's largest ports, Abbott Point, and coal from the boats frequently made its way to north Queensland's beaches.
"Technically you could burn it if you wanted to," Dr Corkeron said.
