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Elementary Science Education

# Estimating how long processes of change have been going on?

Welcome to BFSU: Forums Volume Three Learning Progression D. Earth and Space Science (overview) Lesson D-19. Eons of Earth History Estimating how long processes of change have been going on?

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Bernard Nebel
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Pose the question: How long have these processes (erosion, sedimentation, movements of tectonic plates) been going on? A reasonable answer should not be in a number of years, but “since the Earth was formed? But how long ago was that?

Guide students in reasoning:

Rate of erosion (cm/yr) times number of years (yrs)  gives the distance (cm) erosion will proceed in that number of years. Stating this in mathematical terms

Erosion cm/yr x yr = distance in cm

Rearranging this equation, if we determine the distance erosion has proceeded, divide that by the number of centimeters erosion proceeds in a year, we get the number of years.

distance in cm/Erosion cm/yr   =  years

How much erosion has occurred?

Type into your browser: mesas and buttes images

Finding the same rock layers (note the identical banding) in adjacent buttes says that this area was once an  continuous high plateau; the intervening material has been gradually eroded away. Eroding at the rate of .005 cm/yr how old is this area? Don’t expect your kids to do the math, but have them estimate that the result is a big number.

Similarly, consider synclines and anticlines: Type into your browser: synclines and anticlines images

These “rumpled” rock layers are layers of sedimentary rock necessarily laid down in horizontal layers, but now rumpled by tectonic forces. Where is the mountain between adjacent synclines? It must have eroded away. How long did that that take? Where did the sediment from all that erosion go?

Seventy five percent of Earth’s surface is sedimentary rock overlying the igneous rock cooled from magma. See: http://www.classzone.com/vpg_ebooks/sci_sc_8/accessibility/sci_sc_8/page_80.pdf

Layers of sediment, gradually turning to sedimentary rock, can build up to tremendous thickness. The Grand Canyon shows us a slice through sedimentary rock layers, several thousand feet thick. Type into your browser: Grand Canyon depicting rock layers images

Movement of tectonic plates: The Mid Atlantic Ridge

Beyond witnessing earthquake and volcanic activity around the “Ring of Fire”, geologists have revealed a “crack”, known as the Mid-atlantic Ridge, down the middle of the Atlantic Ocean. This ridge is a place at which the continents on either side are spreading apart.

Type into your browser: mid atlantic ridge map images

Extrapolating the from the Midatlantic Ridge backwards leads to the conclusion that the continents of the Americas and Europe and Africa were joined at some time in the past and there is much additional evidence supporting this notion. See: https://www.classzone.com/books/earth_science/terc/content/investigations/es0802/es0802page02.cfm

The continents either side of the midatlantic ridge are presently moving apart at the rate of about one inch per year. Extrapolating backwards, how long ago were they together. See: https://www.nationalgeographic.org/encyclopedia/continental-drift/