Absolute vs relative dating methods
The lava flow took place some time after the diorite cooled, was uplifted, and then eroded.(Hammerhead for scale) [SE] Figure 8.6b Rip-up clasts of shale embedded in Gabriola Formation sandstone, Gabriola Island, B. The pieces of shale were eroded as the sandstone was deposited, so the shale is older than the sandstone.The boundary between the two represents a time gap of nearly 300 million years.[SE ] Figure 8.9 The four types of unconformities: (a) a nonconformity between non-sedimentary rock and sedimentary rock, (b) an angular unconformity, (c) a disconformity between layers of sedimentary rock, where the older rock has been eroded but not tilted, and (d) a paraconformity where there is a long period (millions of years) of non-deposition between two parallel layers.(The near-vertical stripes are blasting drill holes.The image is about 7 m across.) [SE photo] An unconformity represents an interruption in the process of deposition of sedimentary rocks.The Proterozoic rocks of the Grand Canyon Group have been tilted and then eroded to a flat surface prior to deposition of the younger Paleozoic rocks.
Figure 8.6a A xenolith of diorite incorporated into a basalt lava flow, Mauna Kea volcano, Hawaii.
The lower sandstone layer is disrupted by two faults, so we can infer that the faults are younger than that layer.
But the faults do not appear to continue into the coal seam, and they certainly do not continue into the upper sandstone.
New discoveries have filled in the gaps, and shown us in unimaginable detail the shape of the great ‘tree of life’.
Darwin and his contemporaries could never have imagined the improvements in resolution of stratigraphy that have come since 1859, nor guessed what fossils were to be found in the southern continents, nor predicted the huge increase in the number of amateur and professional paleontologists worldwide.
Current understanding of the history of life is probably close to the truth because it is based on repeated and careful testing and consideration of data.