Layers of Rock
|Students work in teams to understand a model of sedimentary layers.||Materials:
For each group of four students:
|Windows Original Activity|
|Preparation: 30 minutes; Class activity: two, 50-minute class periods|
Student Learning Outcomes:
Note: For a more straightforward exercise, make the models for groups beforehand, or, for a more open-ended exercise, have student groups make models for other groups. If students are making models themselves, allow an extra class period and make sure that they have a good understanding of the laws of stratigraphy (and, thus, the rules by which layers of sedimentary rock are formed and changed over time) so that the models they make reflect what may be found in nature.
Making models beforehand: This particular model leads to a challenging yet solvable puzzle of relative ages. Solving this puzzle requires understanding of the laws of stratigraphy such as superposition, original horizontality and angular unconformities.
- Assemble six colors of play dough, flour, and enough plastic containers so that there will be one for every four students.
- Sprinkle a small amount of flour at the bottom of each container. (This helps cores to be retrieved more easily during the first part of the exercise.)
- Create layers of dough according to the cross section below. The lower three layers are tilted. The top tilted layer (3 in the cross section below) goes all the way across the container, while the lowest two layers (1 and 2) do not. The upper three layers (4, 5, and 6) are horizontal. The top layer goes across the entire container, thus giving a uniform appearance from the top view.
- Affix painters tape to the outside of the container so that students can not see the layers from the side.
- Put the lids on the containers and store in a refrigerator until time to use. Refrigerating helps cores to come out smoothly during the class period #1 exercise.
Class Period #1:
- Explain to students that sedimentary rocks form layers that become buried under more layers over time. The layers above are younger than the layers below. This is called the Law of (or Principle of) Superposition and helps geologists deduce the relative ages of layers of rock (i.e., which ones are older than, or younger than, others).
- Explain to students that layers of sedimentary rocks are horizontal when they are formed. This is called the Principle of Original Horizontality.
- Look at examples in photographs to show superposition, horizontality, and angular unconformities.
- Explain that we can't see the layers of sedimentary rock if both the land surface and layers of rock are relatively flat. Geologists often make rock cores when wells are drilled to get detailed information about the layers of rock below the surface.
- Provide each group of four students with a plastic container than contains the dough layers. Explain that this is a model of layers of rock. The one layer that you can see is at the land surface.
- Supply each group with 15 clear straws, 15 pins, 30 labels (or roll of masking tape) and worksheets.
- Demonstrate how to take a core using the straw. Hold the straw vertically and push straight down. Make sure the straw touches the bottom of the box. Turn it around and wiggle it back and forth to make sure it has cut through the bottom layer and is free from the dough. Holding a finger over the top of the straw, pull the core up out of the tray.
- Student directions are on worksheets.
- Class discussion:
- Did you have unsuccessful cores? What percentage of cores were unsuccessful?
- Do you have enough data to know the relative ages of the rock layers?
- What is the color of the oldest layer? The youngest layer?
- Where in time did you place layers that are not found everywhere in the model.
Class Period #2
- Explain the concept of a cross section.
- Explain that when layers are tilted, the angle of tilt is called the dip.
- How do the four cross sections from the same box look different?
- Why are they different? Two of the cross sections will be along the strike of the rock layers and two will be along the dip of the layers (allowing you to see the tilt of layers).
- Which cross section provides the most information about relative ages? the tilt of the layers?
- Do the relative ages of rock layers that you deduced from your cores match what you see in the cross sections?
- From all the data (cores, cross sections) tell the complete story of the rock layers. (i.e., first layers 1, 2, and 3 were deposited, then they were tilted, and finally 4, 5, and 6 were deposited on top.)
Sedimentary rocks form in layers. These layers were once environments as the Earth's surface. For instance, a beach may form a layer of sandstone if the sand grains become cemented together and preserved as sedimentary rock. A swampy floodplain might become a layer of shale.
Layers of sedimentary rock are found at the Earth's surface and deep underground as well. The layers that are deep underground today were at the surface when they formed long ago. The layers of rock that are on top of them are younger (unless the whole stack of rock has been overturned by tectonic processes). This is called the Law of Superposition.
People drill wells deep through layers of sedimentary rock for a variety of purposes such as to find water, oil or natural gas. When a well is drilled, sometimes the geologists use a special drilling bit to take rock out of the hole in one piece. The cylinder of rock is called a core and tells what the layers of rock are like at that one point on Earth.
In this activity, students will work with a model of sedimentary layers. They will take core samples to understand their model as they would in a real-world situation. Based on their core samples, students will deduce the relative ages of the rock layers. To provide a different perspective, students will then make cross sections to help them understand the structural relationships between the sedimentary layers.
- Law of Superposition: In a stack of sedimentary rocks, the topmost layer is always youngest and lowermost always oldest (as long as the rocks have not been overturned).
- Law (Principle) of Original Horizontality: Sediments settling out from bodies of water are deposited in layers that are nearly horizontal and parallel to the Earth's surface.
- Relative Age Dating: Dating of events by place in chronological order of occurrence rather than in years.
- Core: A rock core is shaped like a cylinder and is cut from the Earth's crust by a drilling rig fitted with a coring bit that has a diamond blade.
- Cross Section: Construction of geologic cross-sections from maps helps to interpret structure in three dimensions.
- Strike: A cross section that is oriented parallel to the sloping layers of rock ("along strike") makes the sloping layers appear to be flat and doesn't give any information about the tilted layers.
- Dip: A cross section that is oriented perpendicular to the sloping layers of rock ("along dip") shows the maximum angle that layers of rock are tilted.
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