Activity 5: Stratigraphy
READING THE CHAPTERS OF TIME
By pretending that containers of sand are rock layers, students identify how old their container of sand is by keying out the plastic dinosaurs and other prehistoric animals the sand contains.
Instructional Method: Activity
Objectives: Students will be able to:
Setup: 60 min
Stratigraphy is the study of rock layers and their relative ages. In geology, the Law of Superposition suggests that rocks in the deepest layers are older than the rocks near the surface. Therefore rock layers can be thought of as pages in a history book that was written backwards with the most current events in the front and the most ancient history in the back. However, various geologic forces can disrupt rock sequence chronology.
When erosion removes layers near the surface, and later in time more layers are deposited over the exposed rock, an entire time period ends up missing from the sequence. These "missing pages" are called unconformities.
Tectonic forces like folding and faulting create especially confusing rock stratigraphy. Faults mix up rock sequences. Imagine taking a book, cutting it in half and gluing the right half on top of the left half. Now as you read down through the book you eventually encounter pages in the wrong order (first the right half and then much later the corresponding left half).
Folding occurs when rock layers are bent upwards and downwards like waves. When enough force squishes the folds, they can flop over on to each other in the same manner waves do as they approach a beach. When this happens entire portions of a rock sequence can be turned upside down. It's like cutting out the middle of your history book, flipping the cut section over and then gluing it back. Now if you were to read this history book you might think that Christopher Columbus sailed to the New World before the Pilgrims arrived in New England but after Neil Armstrong landed on the Moon!
If you think that is confusing, now imagine folding, faulting, and eroding the same history book several times. Only then would you truly have typical geologic stratigraphy!
One of the main reasons geologists study fossils is to help them sort out all the confusion that can occur in studying rock sequences. Long before radioactive dating was used to tell ages, geologist were using fossils to age rock layers. Even today a rock layer with a lot of different fossils can be more precisely aged (and less expensively) by a knowledgeable paleontologist than it can be by uranium dating.
In the last 3 billion years, billions of different species have come and gone - far too many for one person to know them all. This is why some paleontologists specialize on the plants and animals of a particular time period, while others study a single type of plant or animal and how it evolved and/or went extinct over several time periods. What makes fossils so useful to aging rock is that an individual species seldom lasts more than 2 million years; it either evolves into a new species or goes extinct. When paleontologists compare the temporal overlap of several fossils at once, looking for presence, abundance, and absence, they can more precisely pinpoint the exact age of a rock layer.
The best kinds of fossils for stratigraphy are those that are found world wide, are abundant within every location in which they exist, are easily preserved, and are formed from animals which lived in several different environments and evolved into new forms quickly or went extinct suddenly. These kinds of fossils are called index fossils.
In this activity we will use toy examples of real fossils to age imaginary rock layers. You will be provided a list of fossils and their ages to help you with the task.
Discuss with the kids which prehistoric animals were the most and least successful and why (based on the time periods they spanned). Discuss how finding a new fossil, never before discovered, is very exciting and helpful because it helps geologists more accurately go back and re-evaluate stratigraphies constructed before the discovery was made. Discuss what kinds of skills and subjects would be good to study in school for somebody interested in a career in geology or paleontology.
Instead of searching for and purchasing authentic small plastic dinosaurs, print, laminate, and cut out a complete set of fossils from the download file.
Included National Parks and other sites:
Agate Fossil Beds National Monument
Utah Science Core:
Kindergarten Standard 2 Objective 1