Sedimentary Rocks Lab
Geologists classify rocks according to how they were formed. Using this simple idea, there are only three rock types:
Igneous = a rock
which forms by minerals crystallizing out from a melt.
Sedimentary = a rock which forms from the consolidation of
sediment.
Metamorphic = a rock which has undergone a change in temperature and
pressure.
Sedimentary rocks make up only a small percentage of the Earth's crust (by volume) and yet 75% of the rocks that most people see are sedimentary. This is because sediments and sedimentary rocks form at the Earth's surface. For instance, all the rocks exposed in Ohio are sedimentary (see the Ohio geology map).
Geologists recognize two types of sedimentary rocks: Here is the classification table you will use in this lab
Clastic rocks are derived from the disintegration of other rocks and therefore contains clasts (or pieces) of those rocks. Nonclastic rocks (chemical/biochemical) are those which precipitated out of water (during evaporation or through biological means.
Unlike igneous rocks which form solely by one process (crystallization from a melt), sedimentary rocks form by a number of surface processes which must occur in sequence: these include:
Sedimentary rocks generally lack interlocking crystals and instead consist of tiny fragments of other rocks that have been cemented together. Because of this they tend to break easily and are relatively soft and geologists who study them are often called soft-rock geologists (Barry Manilow types).
Also unlike igneous rocks, sedimentary rocks cannot be dated using standard radiometric techniques because they consists of many pieces of other pre-existing rocks. Instead, many sedimentary rocks are ascribed an age based on the fossils that are commonly found in them. The Earth History lab (Geol 11043) spends several weeks on the subject of fossils. For this lab you may just want to look at many of the excellent fossil specimens in the hall displays.
Sedimentary rocks often contain a number of interesting structures in them which formed when the rock formed. These structures (such as ripple marks, graded bedding, cross-bedding, mudcracks, raindrop imprints, and sole marks) tell geologists a lot about the environment in which that sediment was deposited (beach, lake, lagoon, river, glacial, etc.). The display cabinet upstairs outside room 331 shows some interesting sedimentary structures that you may want to look at. Besides providing clues to the environment in which the rock formed, these structures can often tell us what is the top of the bed and what is the bottom of the bed (which way the bed gets younger) and even which way the currents were moving in ancient times.
Textures of sedimentary rocks also provide important clues to the environment of deposition. In general, coarser grain sizes indicate higher energy environments (such as storm deposits, strong currents, and waves) whereas finer-grain sizes indicate quiet environments (i.e., offshore). Sorting (a measure of the mixture of grain sizes) tells us about the amount of reworking of the sediment. Good sorting indicates reworking say by wind to form sand dunes or by waves to form beach sands. Deposition of poorly sorted material occurs during flash floods and by glaciers. Roundness of grains can be an indicator of the transport distance (i.e. rounded grains probably traveled farther) or tranport agent (wind favors moving finer and rounded grains). The lab room has grain size folders available which nicely depict these textural variations in sedimentary rocks.
NOW, CLICK HERE TO VISIT THE PHOTO GALLERY