Final answer:
Radiopotassium dating measures the age of minerals in sedimentary rocks to determine the original crystallization time in the source rock, not when the sedimentary rock itself was formed.
Step-by-step explanation:
Radiopotassium dating is a technique used to determine the age of minerals in sedimentary rocks, which can provide valuable information about the sediment provenance. This form of dating measures the potassium-40 (K-40) to argon-40 (Ar-40) ratio to establish when the minerals being analyzed first crystallized in their source rocks. The key point is that radiopotassium dating provides the age of the sediment in which the minerals are found, not the direct date of the sedimentary rock itself.
For example, if a geologist radiometrically dates hornblende grains using the potassium-40 isotope system, the measured age will not reflect when the sedimentary rock formed, but rather when the hornblende grains crystallized in their original source rock. Similarly, dating zircon grains from an igneous utilizing the uranium-235 isotope system will give the age of the formation, which is then used to interpret the age-related events of the sedimentary rocks in the area.
By understanding the principles behind radiometric dating methods such as potassium-argon and uranium-lead, geologists can interpret a region's geologic history and pinpoint the origin of sediments. Potassium-argon dating in particular exploits the decay of K-40 into Ar-40 and, after measuring the escaping gas after a rock sample is crushed, the ratio can provide the age of the rock.