Down Under Volcanoes Down Under the Water
July 15, 2015
Australian scientists searching for shellfish breeding grounds made a lucky find when they discovered a cluster of underwater volcanoes (called seamounts) about 150 miles (240 kilometers) off the coast of Sydney. The volcanoes formed during the separation of what are now Australia and New Zealand, which occurred between about 80 million and 40 million years ago.
The scientists from Australia’s national science agency, CSIRO (Commonwealth Scientific and Industrial Research Organisation), came across a complex of four ancient volcanic calderas over 12 miles (19 kilometers) long and about 4 miles (6.5 kilometers) wide during a search for nursery grounds for lobster young. This region of the sea floor is largely unexplored. The scientists used sonar to map the sea floor. The base of the volcanoes is at a depth of about 16,000 feet (5,000 meters), and the highest peaks rise about 2,300 feet (700 meters) from the sea floor. The large and ancient volcanoes were not discovered earlier because previous research vessels could only map the sea floor to a depth of about 10,000 feet (3,000 meters).
Volcanoes form above regions where magma—molten, or liquid, rock—is produced in the mantle, the rocky layer beneath Earth’s crust. Plate tectonics demonstrates that Earth’s outer shell consists of rigid pieces called plates that slowly move over time. The volcanoes off the coast of Sydney formed along a divergent boundary between two plates. Divergent boundaries are areas where two plates are pulling apart. As the plates separate, hot rock from the mantle rises to fill the space between them. Divergent boundaries usually produce fluid magmas that contain few gases. Accordingly, eruptions at divergent boundaries tend to be less violent. Most divergent boundaries occur underwater between oceanic plates.
The newly discovered undersea volcanoes have not erupted for millions of years and are considered extinct. Geologists will target the region for further exploration. They hope to learn about the sequence of events that occurred as New Zealand separated from Australia and moved away from the divergent boundary between the two land masses. Features along the cluster of volcanoes will provide more precise information about when that process began and how long the process took.
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