In the relatively flat Harman Valley, located between Wallacedale and Byaduk, south of Mount Napier in Victoria, Australia, are peculiar rocky mounds, like blisters on land. Some of them are up to 10 meters high and 20 meters in diameter. These mounds are known as tumuli or lava blisters.
Tumuli are formed in slow-moving lava fields. When lava flows, the surface often cools to form a thin crust, but underneath the lava is still viscous and molten. If the advancing lava underneath becomes restricted it may push up on the hardened crust, causing soft spots in the crust to rise up like a bubble. Generally, these structures grade into elongate forms called pressure ridges, but occasionally, they creates smaller, steep-sided domes called a tumulus. Usually, the dome is completely solid, but occasionally, part of the liquid core drains out and the top of the dome subsides to leave a central hollow or doughnut-shaped mound.
While tumuli aren’t a rare geological formation, the ones in Harman Valley are particularly notable for the exceptionally vesicular nature of the lava in the tumulus, suggesting that they developed in response to local gas pressure points. These tumuli formed about 32,000 years ago when Mount Napier, one of Australia’s youngest volcanoes, erupted and the lava flowed over a water-saturated swamp causing the ground water to vaporize and produce pockets of gas and high pressure.
In the Harman Valley today, there are dozens of tumuli consisting of bare rocks. The best examples are located on Old Crusher Road, in the town of Byaduk.
Other places where you can see tumuli are Iceland, Hawaii and Argentina.
A volcanic blister (tumulus) just off Old Crusher Road, near Byaduk, Victoria, Australia