The sea of pumice - the size of 20,000 football fields - was first reported by Australian sailors earlier this month, the BBC reported.
Pumice is a lightweight, bubble-rich rock that can float in water. It is produced when magma is cooled rapidly.
Experts have said that the mass likely came from an underwater volcano near Tonga which erupted around August 7 according to satellite images.
Sailors have been warned to stay clear of the potential hazard.
An Australian couple sailing their catamaran to Fiji were the first to report the "pumice raft", after inadvertently entering the rubble at night.
They have since sent samples of the pumice stone - which range "from marble to basketball size" - to researchers at the Queensland University of Technology (QUT) in Australia.
Associate Professor Scott Bryan, a geologist studying the samples at QUT, said such pumice masses could be seen about once every five years in the region.
"It is a phenomenon reported over time, usually as islands in the middle of the ocean that people encounter but then can't find again," he told the BBC.
"It can be as if the whole surface (of the ocean) has turned to land."
The pumice is currently drifting westwards towards Fiji, and is likely to pass New Caledonia and Vanuatu before potentially reaching Australia in a year's time.
( With inputs from IANS )