Why are sedimentary rocks more useful than metamorphic or igneous rocks in establishing the relative ages of rock?
The definitions of those three terms have your answer.
Why are sedimentary rocks more useful than metamorphic or igneous rocks in establishing the relative ages of rock?
Sedimentary rocks are often more useful than metamorphic or igneous rocks in establishing the relative ages of rocks due to certain characteristics and features they possess. Here's why:
1. Superposition: Sedimentary rocks are formed through the deposition and gradual accumulation of sediment over time. This process occurs in horizontal layers, with the newest layers being deposited on top of older ones. This principle of superposition states that unless the rock layers have been overturned or folded, the deeper layers are generally older than the ones above them. By examining the position and order of sedimentary rock layers, geologists can determine their relative ages.
2. Fossils: Sedimentary rocks are great archives of ancient organic remains such as fossils. Fossils are the preserved remains or traces of past life, and they provide critical information for determining the relative ages of rock layers. Fossils are found in sedimentary rocks because organisms become buried in sedimentary layers when they die. By analyzing the types of fossils found in different rock layers, scientists can compare them to known fossil sequences and assign relative ages.
3. Index fossils: Some organisms existed only during specific geologic time periods, and their fossils are called index fossils. Index fossils are particularly useful for dating and correlating sedimentary rock layers across different locations. Since they were widespread but existed for relatively short time spans, finding an index fossil in a rock layer allows the determination of its approximate age. By matching the index fossils from different sedimentary rock layers, scientists can establish the relative ages of these rocks.
On the other hand, metamorphic rocks undergo intense heat and pressure, which can modify their original mineral composition and structure. This process can erase any features, including fossils, that could have provided clues about age. Similarly, igneous rocks form from molten material and often do not contain fossil records. While the absolute age of rocks can be determined through radiometric dating techniques applied to all rock types, the relative age relationships between different rock layers are often best understood using sedimentary rocks due to their characteristics and the valuable information they preserve.