C Collapse Earthquakes: small earthquakes in underground caverns and mines that are caused by seismic waves produced from the explosion of rock on the surface. Compression: fractional decrease of volume due to pressure. Conservation: materials involved are neither created nor destroyed. Convergence: coming together or joining at a common point. Conversion Factors: ratios that allow variables to be interchanged. Cretaceous Period: the last period of the Mesazoic Era (approximately 70 million years ago) and its corresponding system of rocks. Cycle: interval of time during which one sequence of a regular succession of events is completed.