One step further back in time from today’s active volcanoes are people who picnic in the White Mountains of New Hampshire, enjoy the autumn colors in the Blue Ridge of Shenandoah National Park, and hike in the rugged Big Bend National Park of Texas. Many of the rocks in these areas were formed by eruptions or by intrusion of magma into the Earth’s crust many millions of years ago. Because volcanic activity has been so important in shaping the Earth, watching active volcanoes today provides a window through which we can glimpse and reconstruct the early volcanic history of our planet. As we increase our knowledge about volcanic processes, by studying volcanoes erupting today as well as those that have lain dormant for hundreds to thousands of years, we increase our ability to predict when and how volcanoes will erupt. Accurate predictions, presented in terms that are meaningful to public officials, will minimize the number of lives lost and the social and economic upheaval that an eruption can cause. [Illustration: Thick layers of volcanic rocks form the Superstition Mountains, located about 60 kilometers east of Phoenix, Arizona. The consolidated deposits of pyroclastic flows, lava flows and domes, and lahars in the Superstition Mountains and adjacent areas testify to a period of intense volcanism about 17 to 25 million years ago in central Arizona. (_Photograph by D.W. Peterson._)] Glossary Andesite A volcanic rock containing 53-63% silica with a moderate viscosity when in a molten state. Ash Fragments less than 2 millimeters in diameter of lava or rock blasted into the air by volcanic explosions. Basalt A volcanic rock consisting of less than 53% silica with a low viscosity when in a molten state. Caldera A large volcanic depression, commonly circular or elliptical when seen from above. Composite A steep-sided volcano composed of many layers of volcano volcanic rocks, usually of high-viscosity lava and fragmented debris such as lahar and pyroclastic deposits. Dacite A volcanic rock containing 63-68% silica with a high viscosity when in a molten state. Dome A steep-sided mound that forms when viscous lava piles up near a volcanic vent. Domes are formed by andesite, dacite, and rhyolite lavas. Fumarole A vent that releases volcanic gases, including water vapor (steam). Lahar A flowing mixture of water and rock debris that forms on the slopes of a volcano, sometimes referred to as debris flow or mudflow. The term comes from Indonesia.