Cows in the Valles Grande caldera. This section of the caldera is about 3 miles across and 5 miles long, the whole caldera has a diameter of over 12 miles. The hills in the back are resurgent domes that grew out of the caldera about 100,000 years after the collapse of the volcano about a million years ago. There is some controversy about the dates of the two main eruptions, we will go with the Smithsonian Institution's dates of 1.12 Million years ago and 1.54 million years ago. We have had much milder eruptions since, the latest being 57000 years ago or so.