Volcano Hazards ProgramSTEMList of large volcanic eruptions in the 21st centuryThis is a list of volcanic eruptions of the 21st century measuring a Volcanic Explosivity Index (VEI) of at least 4, as well as notable smaller eruptions. Note that the size of eruptions can be subject to considerable uncertainty. See also References External linksMagnetic stripes and isotopic clocks [This Dynamic Earth, USGS]Oceanographic exploration in the 1950s led to a much better understanding of the ocean floor. Among the new findings was the discovery of zebra stripe-like magnetic patterns for the rocks of the ocean floor. These patterns were unlike any seen for continental rocks. Obviously, the ocean floor had a story to tell, but what? In 1962, scientists of the U.S. An observed magnetic profile (blue) for the ocean floor across the East Pacific Rise is matched quite well by a calculated profile (red) based on the Earth's magnetic reversals for the past 4 million years and an assumed constant rate of movement of ocean floor away from a hypothetical spreading center (bottom). A team of U.S. Other commonly used isotopic clocks are based on radioactive decay of certain isotopes of the elements uranium, thorium, strontium, and rubidium. "Developing the theory"