The earth is estimated to be about 4.5 billion years old. rocks and rock formations found throughout earth’s crust give scientists clues to the past. they assume that the processes that acted on the crust millions of years ago are the same processes that act on and within earth today. this is called the principle of uniformity or uniformitarianism. the law of superposition us infer the age of rock layers, or strata. this law states that, if layers of rock are left undisturbed, the layers at the bottom are the oldest. the lower layers must be in place before younger layers can be deposited on top of them. scientists infer the relative age of the layers as well as any fossils found within them. the law of superposition does have exceptions. some processes within the earth cause younger layers to be moved below older. folding, faulting, and intrusion can cause these exceptions. regardless of position, any rock is always older than the process that changed it.

if an active volcano produced magma that eventually became igneous rock, geologists would assume that the processes producing this rock are the same as those processes that occurred millions of years ago. this assumption is referred to as

a. a law of superposition

b. uniformitarianism

c. plate tectonics

d. volcanism