The Big Sleep
Leading volcanologists believe that Monaro erupted some 630 cubic kilometers of lava over a 20 million year period, starting approximately 55 million years ago and ending about 37 million years ago. It has been dormant since then and is considered to be an extinct volcano.
The Monaro plateau's lava fields cover an area of about 4,200 square kilometers and contain 65 volcanic plugs. A plug, also called a lava neck, is formed when magma emerges from a vent in the Earth's surface and hardens to create a volcanic land form. As a whole, the Monaro region is defined by the Murrumbidgee River valley in north and the Errinundra Plateau to the south. The coast defines the boundary of the eastern part of this region and the Snowy Mountains to the west.
Australia has many volcanic regions, with the earliest eruptions dating back about 71 million years, and the majority occurring between 20 and 35 million years ago, though some occurred as recently as 3,000 years ago. Monaro is deemed to be one of the oldest, and its activity is thought to be associated with rifting of the continent and upwelling of the Earth's mantle.
The Monaro region is distinguished by sweeping plains separated by flat-topped mesas that were created by stacks of hardened lava and round-topped hills centered over volcanic plugs. Years of erosion have cut deep into the volcanic rock and in some areas 325 to 425 feet of rock have eroded away. The region is also home to very rugged peaks in the Tinderry Mountain Range.
Another interesting fact about the Monaro region is that, while most of the soil here contains granite and is infertile, a small area in the Monaro region near Cooma and Nimmitabel consists of basaltic bedrock that produces the only chernozems on the entire continent. Chernozems is a rich, black soil that contains a lot of humus, making the soil extremely fertile.
Unique rock walls were formed in the region when lava reacted with the area's water bodies. These walls are characterized by black pillow basalt deposits, a reddish layer of pyroclastic rock and sometimes even traces of fossilized wood. Today, the Monaro Volcanic region is home to the headwaters of both the Snowy and Murrumbidgee Rivers.