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Superposition

Schoolyard
Geologist in ACTION
Layers of Road
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Location: Near South Hall, University of California, Berkeley
About: Zoom into the picture above and you can see four different layers of pavement on top of one another. The small white ruler (which is about 10 cm long) sits on the lowest of these layers. It is very common for layers of pavement to be placed on top one-another because it is cheaper and easier to simply cover up the old pavement than it would be to rip it out. Over time, the layers in this picture have eroded away so that layers that were once covered are now exposed.
Grand Canyon Layers
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Location: Grand Canyon National Park, Arizona
About: The classic "layer-cake" structure of the Grand Canyon. The oldest layers are deposited first. Over time, the layers are buried as new sediment gets deposited on top of the existing layers. Using radiometric dating, scientists have determined that the Kaibab limestone that forms the top layers is 270 million years old. The bottom layers are over 800 million years old. Here, a vertical mile of the Grand Canyon has been exposed by erosion. Now, we can see all of the older layers that were once buried.

Asphault 'flow'
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Location: Near McCone Hall, University of California, Berkeley
About: The darker material on the left lies on top of the lighter colored pavement. The darker pavement was added later to cover up the lighter pavement that was cracking (some cracks are visible directly behind the white ruler, which is shown for scale).
Lava Flow Over Road
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Location: Chain of Craters Road, Hawaiian Volcanoes National Park, Hawaii
About: Hot lava slowly flows over a road in Hawaii. The road was there for decades before the lava flowed over it. National Park rangers work in the background to move a building to rescue it from the flowing lava. Volcanoes can stay active for millions of years, with each new flow of lava covering up an older flow. The same thing happened to this road in 2003 -- the road in the picture was built on top of a lava flow that buried an older road in the 1960's that was built on top of a flow several hundred years old.
Key Concepts:
  • Newer rocks are deposited on top of older rocks. The newer rocks cover up the older rocks.
  • For sedimentary or volcanic rocks, the oldest layers are therefore on the bottom and the youngest layers are on the top. We call this "The Principle of Superposition" (super = top).
  • Erosion can help expose older layers that were once buried.
  • Geologists reconstruct the order in which layers were deposited by their relative position (which ones are on top of which). We call this a "relative dating" because we don't know the exact date and time of the event, but only what happened before or after it. Detectives also use relative dates when they reconstruct a sequence of events to help solve mysteries.
  • While the Principle of Superposition is generally true, think about cases when it might not be true. For example, plate tectonic forces can deform and contort rock sequences so much that they sometimes get turned upside down. Or, sometimes underground magma rises upward from the earth's interior and gets deposited beneath other rock layers (we call these rocks "intrusive igneous" rocks because they intrude into the existing rocks, and granite is a common example).
Classroom Activities:

GeoSleuth Murder Mystery

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