Rock Reading Field Trip

Greg and Karin head into the field to practice their rock reading skills on a beach in Tasmania.

Summary

  • If we assume that the same processes operated in the past as now operate then we can read the rocks. This is the principle of uniformitarianism.
  • Sediments are laid down, one on top of the other in sub-horizontal layers.
  • In sedimentary rocks, the oldest rocks are at the bottom and get younger as you go up unless the rocks have later been turned over by folding or some other mechanism.
  • Fossils can be preserved in sedimentary rocks and, because they are the same age, they can help us work out how old the rocks are.
  • The size of grains in sedimentary rocks tell us about the environment in which they were laid down – small grains means that the sediments were laid down in calmer waters than large grains.
  • Some special features tell us more – like dropstones as evidence of icebergs (and therefore cold climates).
  • Features that cross-cut sedimentary rocks must have been formed after the sediments had turned to rock.
  • Rocks formed from magma (molten rock) often cross cut sedimentary rocks, and represent the intrusion of the magma in dykes through the sedimentary rocks.
  • The crystal size in igneous rocks (those formed from magma) tell us about conditions – small crystals indicate that the magma solidified quickly (for example basalt that solidified on or near the earth’s surface); large crystals occur in rocks that solidified deep in the earth.
  • This is just the beginning, if you look in more detail, then you can work out much more from rocks.
assessment

Rock Reading, it's all in the detail.

This quiz consists of 4 questions. The questions are about information on reading rocks. This task is to help you review the main ideas in the video and reading the rock section and help you prepare a bit for your own field trip in assessment task 1a.

  • In what part of the cliff shown in the “Reading the Rocks” video, would you go to find the youngest fossils?
    • Bottom of the cliff
      • Incorrect. Try again, consider the law of superposition.
    • Middle of the cliff
      • Incorrect.Try again, consider the law of superposition.
    • Near the dyke
      • Incorrect. Try again, consider the law of superposition.
    • Top of the cliff
      • Correct. Strata become younger up through the cliff. The biota that die and are preserved as fossils within beds will be youngest in the youngest strata at the top of the cliff. Yes the youngest fossils are at the top of the cliff because this is where the youngest of the sedimentary rocks are. The dyke is even younger, but won’t contain fossils.
  • How did dropstones shown in the Reading the Rocks video enter the original sediments?
    • They fell off the cliff
      • Incorrect. Review the video and try again.
    • They were delivered onto the beach by a river
      • Incorrect. Review the video and try again.
    • They dropped onto the seafloor from melting icebergs
      • Correct. The pebble dropstone at the field site was probably rounded in rivers before being carried by glaciers to the sea. As the drifting icebergs melted they dropped all the fragments that they carried, including the pebbles and the smaller particles such as sand. Yes this process can release a range of fragments all at once including very large and small grains and rain it down to form a deposit on the sea floor.
    • They were part of a magma
      • Incorret. Review the video and try again.
  • Which of these is an illustration of the principle of uniformitarianism?
    • What we see happening today on the beach, also happened throughout geological time
      • Correct. The principle of uniformitarianism is a fundamental principle in geology. It helps us to understand what has happened in the past by observing what we see today. This is correct. Geological processes in action today have always operated in a similar manner through time
    • Fossils in one stratum of sedimentary rocks will be younger than the fossil underneath them in the same set of sedimentary rocks.
      • Incorrect. Revisit the last few pages and try to identify the principles of rock reading.
    • Dykes cross-cut strata
      • Incorrect. Revisit the last few pages and try to identify the principles of rock reading.
    • Tasmania was once a cold place
      • Incorrect. Revisit the last few pages and try to identify the principles of rock reading.
  • Rocks that form from magma can contain fossils
    • True
    • False
      • Correct. This is not true, so it is the answer you are after. The rocks that contain the fossils are sedimentary rocks, formed from particles of other rocks and not by the cooling of a magma


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