地质中的时间尺度.pptx

Understanding Earth;Concepts you will need to know for the exams CHAPTER 8 Geologic Time Scale Relative age Absolute age Principle of original horizontality Principle of superposition Principle of cross-cutting relationahips Unconformity Angular unconformity Disconformity Half-life Geologic Time Scale;Geologists differ from other professional scientists in their view of time. Geologists deal with periods of time that go as far back as the origin of the universe.;Before we had the ability to read the “atomic clocks” of nature, geologists used the concept of relative ages. We can see changes in the landscape over time. We can see one type of organism replace another type of organism through the fossilized remains of these changes over time. ;With the advent of atomic/radioactive decay techniques we began to obtain absolute dates. Absolute dates or ages have specific numbers ;Relative dating has been done in the past through the study of layers of sedimentary rock (aka “the field of stratigraphy”) ;Principle of Original Horizontality sediments deposited innearly flat layers. Cross-bedded layers are contained within overall flat beds. If layers are faulted (broken) or folded it means that they were deformed AFTER sediments were deposited. ;Principle of Superposition Lowest layers are older If this is not so, then the layers have been overturned by tectonic activity: ;There are limitations for using stratigraphy to keep time because, (1) sedimentation rates are variable e.g. Mississippi can deposit 1m of sediment in 1000 y The deep ocean may deposit 1mm / 1000 years We need to estimate time in another way and be able to recognize when the representation of time via sediments is incomplete. Paleontology is the study of ancient life from fossilized remains. ;William Smith in the 19th century was an engineer who noted that similar rocks in different areas had similar fossils and that different rock types had different fossils. He made a composite (ideal, pieced-togethe