Igneous rocks, such as the Wall Mountain Tuff and the Pikes Peak Granite, are formed by geologic processes beginning deep within the Earth. An igneous rock begins developing as molten magma deep within the Earth. Magma can take one of two paths: it can remain deep underground and cool slowly to become an intrusive igneous rock such as granite, or it can be erupted onto the Earth's surface either as lava or more explosively as pyroclastic eruptions of tuff, ash, and pumice (as seen at Mt. St. Helens).
A complex of stratovolcanoes near the modern-day town of Guffey, Colorado, about 15 - 18 miles southwest of Florissant played an important role in the formation of ancient Lake Florissant, as well as the deposition of the Florissant Formation.