Caldera eruptions leave behind large craters in the Earth – not what we think of when someone says volcano. Watch this animated video to see how Lake Rotorua could have formed from a caldera eruption.
Long ago, magma accumulated in the crust beneath the land around what is now Lake Rotorua. As the amount of magma increases, it heats the rock material in the crust above it, and pressure builds up. 240,000 years ago, the crust fractures, releasing pressure, and fissures kilometres long appear at the surface above the magma as the stressed crust cracks into large blocks.
Rotorua explodes violently, firing an ash column high into the atmosphere.
Widespread pyroclastic flows of ash and rocks sweep the landscape for hundreds of kilometres around. In places, the pyroclastic flows are so thick they change the river drainage pattern of the region. The erupted material partially empties the magma chamber below Rotorua. The weight of the fractured crust above makes it collapse, creating a large basin called a caldera.
In the following months and years, rain fills deeper parts of the caldera.
Dregs of magma continue to reach the surface, and explosive phreatomagmatic activity occurs as magma interacts with water. Rivers and streams find new courses within the caldera. Lake Rotorua begins to form from accumulated water but empties many times due to continued volcanic activity. Erosion starts to wear away at the caldera, forming rivers into and out of the lake. Vents at the bottom of the caldera stay active and continue to send out gas and squeeze out lava to form domes of glassy rock. Mount Ngongotahā and Mokoia Island are created in this way, although scientists are not sure exactly when they were formed. Pockets of hot crust continue to heat groundwater, causing geothermal activity such as geysers and boiling mud pools.
And now, many tens of thousands of years later, people make use of the lake and the geothermal resources that remain in this beautiful volcanic wonderland.