Scale, Proportion, and Quantity- Geologic Time Scale

I recently added a unit on the evolution of life and the early atmosphere to my 10th-grade Integrated Science Curriculum. I needed to tie in my initial Characteristics of Life Unit and Popcorn Growing Project with the later units of Forces and Motion on Earth and Earth and Human Impact in a more cohesive manner. I focused on NGSS Earth and Space Standards ESS-2-5 and ESS-2-7 to make this bridge. I also needed to add more activities for students to apply their knowledge of Scale, Proportion, and Quantity because as a district we are seeing that the application of these concepts is a weakness.

What do you notice? What do you wonder? What does it remind you of?

I began having students make observations about the Geologic Time Scale. Students were asked what they noticed, what they wondered, and if the image reminded them of anything.

Student responses were gathered using padlet.com and ranged from surface-level observations (lots of colors, lots of words, etc) to much more in-depth observations and questions (This is version 5.0, what did the previous versions look like? Eras are bigger than periods and the eras sometimes group periods together, As you go further down each time scale the age gets older and some of the ages are grouped together in a certain period, etc)

After our initial observations, we began a class discussion about what we thought the different groups represented and what different titles might mean. We made connections to some of the prefixes and suffixes they had previously learned in middle school ELA and looked up others to determine that -zoic means life and students were able to decipher that the four category labels meant: Cenozoic - "New Life", Mesozoic - "Middle Life", Paleozoic - "Old/Ancient Life" and Pre-Cambrian - "Before Life". Students also recognize that all the age names (with an exception of a few) end in the suffix -ian and that the largest unit is the eon, which is further divided into smaller units such as eras (10), periods, and epochs.

We followed this up with a class activity from Robin Kropholler, to build a scaled model of the timeline on the whiteboards. Groups were tasked with looking up the dates of specific major events in Earth's history and then plotting them on the timeline. Students begin to see that the Geologic Time Scale as printed is not at the same scale. We then compared the two timelines, and I give them this link to a virtual timeline from the University of Utah. One pixel on the timeline equals 100-years, and 100-pixels equal 10,000-years. Students realize how long they need to scroll just to get out of the Precambrian! Students identified that most of the events relating to life as we know it on earth occurred in the last end of the timeline however on the GSA Time Scale, they appear to take up three-quarters of the timeline. This opens up conversations about different number scales, specifically logarithmic scales. ‘What is a log scale and how do we make one?’ On a whiteboard I draw two lines 1 m long, about 20 cm apart, and make a mark at each 10 cm to create a scale. Label the top scale Linear and label every other mark 0, 10, 20 … 50. I Label the second scale Log or Logarithmic. Label every other mark 1, 10, 100. Stop and ask: · What do you think I should write by the one after the next mark? Can you explain how I am constructing this scale? What is different between this log scale and the linear scale? Key points to note are that a log scale is multiplicative, so each increment represents multiplying by 10 not adding 10 and cannot start from zero.

I follow this activity and mini scale lesson up with having students create a smaller version of the timeline for their notebooks with the major events notated on it.

Download copies of the Geologic Time Scale from the Geological Society of America. Download other translations from the International Commission on Stratigraphy

Math Challenge

Here is a quick math challenge for your students.

" If the entire history of Earth were compressed into a single year, with the formation of Earth on January 1 and today as December 31, on what date would humans first appear? (Hint: Do some research and calculations!)" Scroll down for the answer.

Solution: Earth is 4.6 billion years old, so our yearly calendar is equivalent to 4.6 billion years. Divide 4.6 billion by 365 calendar days to find out how many years per calendar day and you find 12,602,739.73 years per calendar day. Humans have been on the planet for approximately 6 million years, which is half a day, so humans arrived on December 31st, around noon!