Relative dating diagram
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.
• Superposition, unconformities, and cross-cutting relationships allow geologists to determine the sequence of events in rock masses. The rocks eroded again, forming the upper unconformity.
For example, a xenolith in an igneous rock or a clast in sedimentary rock must be older than the rock that includes it (Figure 8.6).
Figure 8.6a A xenolith of diorite incorporated into a basalt lava flow, Mauna Kea volcano, Hawaii.
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