Rock Record and Relative Dating Notes PDF

The Rock Record  Organizing Time Fossils are the remains, traces, or imprints of ancient organisms that are often found embedded in rocks. ✣ By studying rock layers and the fossils within them, geologists can reconstruct different aspects of Earth history and interpret ancient en...

The Rock Record  Organizing Time Fossils are the remains, traces, or imprints of ancient organisms that are often found embedded in rocks. ✣ By studying rock layers and the fossils within them, geologists can reconstruct different aspects of Earth history and interpret ancient environments. 2  Organizing Time the geologic time scale, a graphical representation of Earth’s history from its origin 4.6 billion years ago (bya) to the present. 3 Geological Time Scale An eon is the largest time unit. ✣ The three earliest eons make up 90 percent of geologic time, known together as the Precambrian. ✣ During the Precambrian, Earth was formed and became hospitable to modern life. ✣ Fossil evidence suggests that simple life-forms appeared during the Archean Eon. 4 Geological Time Scale Eras are the next-largest unit of time and make up eons. ✣ Eras are usually tens to hundreds of millions of years in duration. 5 Geological Time Scale All eras are divided into periods. ✣ Periods are generally tens of millions of years in duration. 6 Geological Time Scale All periods of geologic time are divided into epochs. ✣ Epochs are generally hundreds of thousands to millions of years in duration. 7 Relative Age Dating  Relative Dating Relative-age dating allows scientists to place rock layers and geologic events in the order in which they formed or occurred. No numbers or dates are used with this method.  Principles of Relative Dating 1. 2. 3. Original Superposition Original Lateral Horizontality Continuity 4. 5. Cross-cutting Inclusions Relationships 10  1. Original Horizontality Original horizontality states that sedimentary rocks are deposited in horizontal or nearly horizontal layers. 11  2. Superposition Superposition states that in an undisturbed rock sequence, the oldest rocks are at the bottom and each consecutive layer is younger than the layer beneath it. 12  3. Lateral Continuity The principle of lateral continuity states that layers of rock extend in all directions until the environment in which the sediment was deposited changes. 13  4. Cross-Cutting Cross-cutting relationships states that an intrusion is younger than the rock it cuts across. ✣ This includes both faults and magma intrusions 14  5. Inclusions The principle of inclusions states that any rock fragments, called inclusions, in a rock layer must be older than the rock layer that contains them. Buried surfaces of erosion are called unconformities and does not form until the eroded surface is covered. 15  Correlation Correlation is the matching of rock outcrops exposed in one geographic region to similar outcrops exposed in other geographic regions. A key bed is a rock or sediment layer that serves as a time marker in the rock record.  Fossils Index fossils are fossils that are easily recognized, abundant, and widely distributed geographically. ✣ Fossils can be used to correlate rock layers that are the same age but not necessarily the same rock type. 17