Results from the Volcanic Concept Survey (VCS) indicated that many undergraduates do not fully understand volcanic systems and plate tectonics. During the 2006 academic year, a ten-item conceptual survey was distributed to undergraduate students enrolled in Earth science courses at five U.S. colleges and universities. A trained team of graders scored 672 completed surveys, coding responses to each item with a score, out of 3, based on accuracy and comprehensiveness. Questions requiring only basic content knowledge (e.g., terminology, volcano topology) received more high scoring responses than questions requiring higher thinking and deeper conceptual connections (association with plate tectonics, prediction of hazards and impacts on the environment). The mechanics of eruptions also appeared to be poorly understood. Special attention was paid to students' alternate conceptions about where volcanoes are likely to form. Male students, students highly interested in science, and students who lived in a volcanically active area received significantly higher total scores than other student groups. Science, technology, engineering, and mathematics (STEM) majors also performed significantly better than non-STEM majors. Understanding the nature of student comprehension and misconception may be useful for geoscience educators seeking to address student preconceptions and promote conceptual change.