Review It! Select the statement(s) that are true about gradual and catastrophic changes to Earth's surface. A geologist studies the Earth's physical structure and substance, its history, and the processes that act on it A paleontologist studies the fossils of once-living things. During an ice age, oxygen levels in the atmosphere drop up to 50%, causing most plants to decline in number. Scientists believe that the breaking apart of the supercontinent Pangaea supercharged the evolution of species on Earth Plate tectonics has been occurring for billions of years and causes gradual changes to Earth's surface.​