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WalesGeologyQuick questions
F3: Time and Change
Quick questions on The geological timescale - WJEC A-Level Geology
6short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is the hierarchy of the timescale?Show answer
The geological timescale divides Earth history into a nested hierarchy. From largest to smallest: eons (the longest, such as the Precambrian eons and the Phanerozoic), eras (such as the Palaeozoic, Mesozoic and Cenozoic), periods (such as the Cambrian, Jurassic and Cretaceous) and epochs (subdivisions of periods, such as the Pleistocene). The whole of Earth history spans about 4.6 billion years, but the abundant fossil record begins only at the start of the Phanerozoic, about 541 million years ago.
What are events that define the boundaries?Show answer
The major boundaries are placed at events that produced sharp, global, correlatable changes in the fossil record. Mass extinctions are the most important: the end-Permian extinction (the largest, removing most marine species) marks the Palaeozoic-Mesozoic boundary, and the end-Cretaceous extinction (linked to an asteroid impact and volcanism) marks the Mesozoic-Cenozoic boundary. The first appearance of major groups (such as the explosion of shelly animals at the base of the Cambrian) also defines boundaries. These events make practical markers because they appear in rocks worldwide.
What are cambrian named after Wales?Show answer
The Cambrian period takes its name from Cambria, the Roman name for Wales, where the rocks were first described, a reminder that the timescale was built from real rock successions. The Permian-Triassic boundary. The largest extinction in Earth history defines the Palaeozoic-Mesozoic boundary and is recognised globally by the collapse and slow recovery of marine faunas. The Cretaceous-Palaeogene boundary clay. A worldwide iridium-rich clay layer marks the end-Cretaceous impact, an instant of geological time used to correlate the Mesozoic-Cenozoic boundary across continents.
What is q1?Show answer
List the divisions of the timescale from largest to smallest. [2 marks]
What is q2?Show answer
Name the three eras of the Phanerozoic in order. [3 marks]
What is q3?Show answer
State the event that marks the boundary between the Mesozoic and Cenozoic eras. [1 mark]
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