- Grade Level:
- Middle School: Sixth Grade through Eighth Grade
Using a simple three-or-four-generation family tree, students construct a relatives time tree that mimics the major divisions of the geologic time scale: Precambrian, Paleozoic, Mesozoic, and Cenozoic.
In any science, a common vocabulary is essential for ease of communication among peers. Geology is no exception; but being a historical as well as a descriptive science, it is important to have a universal understanding of time. The geologic time scale provides geologists across the world with a shared reference of time. You might say that the geologic time scale is to geoscientists what the periodic table of elements is to chemists.
The geologic time scale is divided into (from longest to shortest): eons, eras, periods, epochs and ages. Many of the period names have historical precedents dating back to the 18th and 19th centuries when outcrops of rock across Europe were being systematically mapped using the rules of relative timekeeping (original horizontality, original continuity, superposition, cross-cutting relationships, and faunal succession). In accordance with these rules, the geologic time scale is arranged so the oldest time divisions are at the bottom and the youngest are at the top. It was not until the twentieth century that absolute ages could be assigned to the geologic time scale using radiometric dating techniques. Even now these absolute dates occasionally undergo minor revision.
By contrast a family tree can be divided into generations with the grandparents representing the roots; parents, aunts and uncles the trunk; siblings and cousins the branches; and sons, daughters, nieces and nephews the leaves. Events like births and marriages can be used to further sub-divide the generations much as periods are a subdivision of eras. And just as radiometric dating techniques provide absolute ages for the geologic time scale, birth certificates and marriage licenses document our family trees.
Step 1 As a homework assignment, each student with the help of a parent should prepare a simple three or four generation maternal or paternal family tree. Provide students with Simpson (Dad) Family Tree, or Simpson (Mom) Family Tree.
Display a simplified geologic time scale (see materials section) showing only the Precambrian, Paleozoic, Mesozoic, and Cenozoic eras. Ask students if they notice anything in common between the era names (Answer: they all end in the suffix -zoic). Ask them if this suffix reminds them of any word they are familiar with (Most likely answer: zoo). Continue the discussion explaining that zoo and -zoic have the same word origin, the Greek word zoon meaning animal. Now explain that the prefixes paleo-, meso- and ceno- are also derived from the Greek language; palaios meaning ancient or old, messos meaning middle, and kainos meaning new or recent respectively.
Display a copy of your (teacher's) family tree.
Have students transform their family trees to mimic the simplified geologic time scale equating generations to eras. They should develop a scientific sounding name for each generation. See Simpson (Dad) Family Time Tree and Morgensen (Mom) Family Time Tree for examples.
Display a copy of the geologic time scale showing eons, eras, periods and epochs (see Geologic time Scale 2008* for example).
1. Geologists refer to the history of past events and life preserved in the rocks of Earth as the geologic record. Write a short family history, a family record, detailing the most significant events in the lives of your immediate relatives.
2. Take a closer look at your family time tree and the geologic time scale. Notice how a generation of your family time tree equates to an era, one of the longest divisions of geologic time, but that the geologic time scale also contains several smaller subdivisions of time called periods, epochs and ages. These individual time intervals often mark significant events in Earth history such as the appearance or disappearance of a biological organism (preserved as fossils) in the geologic record. Review what you have written about in your family record. How would you subdivide the generations (eras) of your family time scale into smaller time increments like the periods on the geologic time scale?
3. Prepare a My Relatives Time Scale of your extended family (grandparents, parents, brothers, sisters, nieces and nephews). Remember that the oldest event should be recorded at the bottom followed by progressively younger events. Remember relative time units often mark the first appearance of a biologic organism. This would be similar to what event in your life?
4. Take another look at the geologic time scale. Notice that an absolute (numerical) age has been assigned to the beginning of each eon, era, period, epoch and age. These have been determined by a variety of methods, the most common of which is the radiometric dating of igneous rocks. Think of radiometric dating methods as providing a birth certificate for the rock, much like the one issued to your parents when you were born. Geologic ages are recorded in years before present, so that the Precambrian began 4,567 million years ago. Assign an age to the beginning of each era and period of your family time scale. See My Relatives Time Scale for an example.
Last updated: March 22, 2022