Membur: Thank you, Dee. Scientific investigations of Yellowstone began late in the 19th century. Still, Ferdinand V. Hayden, who explored the area in 1871, stood on the summit of Mt. Washburn, a mountain in the central part of the park, and recognized immediately that the landscape before him appeared to be volcanic.

Inspector: I've memorized Hayden's words, Iva. Listen to this! "It might be called one vast crater, made up of thousands of smaller volcanic vents and fissures out of which the fluid interior of the earth, fragments of rock, and volcanic dust were poured in unlimited quantities. . ."

Iva: Shhhhh! Mike, I'm trying to listen.

Membur: However, members of the Hayden Expedition, did not recognize that there were two very different periods of volcanic activity in Yellowstone's past. The first occurred about 50 million years ago. Volcanoes, now seen as quiet mountains such as Mt. Washburn, periodically covered the landscape with ash and molten rock.

Today we know that the second period of volcanic activity is much more recent than the first, and is responsible for most of the present Yellowstone landscape, as well as the geysers and hot springs we see today.

Glossary: fissure - a long narrow opening; a crack or cleft