Fossils as Clues to Ancient Continents

Introduction

In the early 1900’s, a famous English explorer, Captain Robert Scott, made a surprising discovery when exploring the cold and forbidding continent of Antarctica.  Captain Scott describes the discovery of his diary in this entry for February 8, 1912:

“We found ourselves under perpendicular cliffs of Beacon sandstone, weathering rapidly and carrying veritable coal seams.  From the last, Wilson, with his sharp eyes, has picked several plant impressions, the best a pieces of coal with beautifully traced leaves in layers, also some excellently preserved impressions of thick stems, showing cellular structure...”

 

This was written upon Scott’s return from the South Pole. The coal seams and plant fossils had been found at the base of Mount Bowers, at the head of the Beardmore Glacier.

Geologists generally supposed the coal is formed in temperate or tropical regions.  How could it have entirely covered by glacial ice?

 

Objectives:

After you have completed these activities, you should be able to:

1.     Tell how rocks indicated the environment in which they were formed.

2.     Use superposition to find the relative ages of rock units.

3.     Use fossils to correlate rock unites

4.     Describe evidence which supports the theory that certain continents were once joined.

 

Procedure:

PART A: What can we learn from rocks about past environments?

Materials: six different rocks

 

A geologist is a scientist who studies rocks to learn about the history of the earth.  Your teacher will provide you with six rocks that have been identified and described be geologists.

 

1. Identify each of your rocks.  You or your group will have one of each of the following:

 

Basaltic lava: was formed from molten rock that erupted from volcanoes or from long cracks in the earth’s crust.  It is dark and so fine-grained that you will not be able to see individual grains.  It may have round holes formed by gases released from the molten rock and then trapped in the lava as it hardened.
Marine Sandstone: was formed from sediments deposited in a sea or ocean.  Usually it will contain only quartz grains.  Individual grains will all be of about the same size.  Any fossils it may contain usually will be of plants or animals that lived in salt water.
Fluvial sandstone: was formed from sediments deposited in the bed of a stream.  It may have minerals other than quartz, and the mineral grains will be of various sizes.  If there are any fossils , they will be of plants or animals from on land or in fresh water.
Shale: is a very fine-grained rock formed from clay or mud.  It will show some layering.
Tillite: is a rock formed from sediment deposited by a glacier.  These rocks may have a wide variety of grain seized and minerals.  Many of the grains will be somewhat angular.
Coal: is formed from the remains of trees and other plants that grew in swamps.  Coal will be black, brittle, and not very hard.  It may have fossilized leaves and plant stems.

 

2.     After you have identified all six rocks, place each one on Worksheet 3 on the correct symbol representing that rock.  Have your teacher check your rock identification before you continue with the activity. Notice that each of the symbols on Worksheet 3 is used in the rock column chart on Worksheet 1.  Remember what each symbol means and also the kind of conditions under which each type of sediment or rock material was deposited.

 

3.     Apply the principle of superposition to the South Africa rock column on Worksheet 1.  Which rock layer is the oldest?  Which rock layer is the youngest?

 

4.     What was the environment of Antarctica like when the marine sandstone sediment was being deposited?  What was the environment like when the coal material was being deposited?

 

5.     Does the present environment in Antarctica differ from either of those you describe in question 4 above?  If so, how does it differ?  Could coal be forming in Antarctica today? 

 

6.     Locate India on the world map.  What type of climate do you think India has today?

 

7.     Now examine the rock column from India.  Could rocks similar to those represented by the rock columns be forming in India today?  Which ones?  Could tillite be forming today in India?

 

8.     You have observed that both India and Antarctica have rocks that were formed in environments that were much different from the environments in those two places today.  List as many explanations as you can, to account for this situation.

 

9.     Examine the rock columns from all four areas.  List each similar order of rock layers that you can find.

 

10. One similarity that you will have noticed is the presence of basalt at the top of each of the rock columns.  How is basalt formed?

 

11. Can you tell weather the basalt in each of the areas was formed at the same time?  Explain.  How about the coal?  Explain.

 

Procedure:

PART B: How old are the rocks?

Materials: color pencils, political map of the world

 

            You found in step 11, Part A, that you really could not tell from the information provided, weather layers of similar rock type from different areas were formed at the same time.  If you could find out the ages of these rocks, then you could determine whether the same environment existed in all four areas at the same time.  You could also learn whether the environment changed in the same way in each of the four areas.  This would certainly tell us something interesting about the history of the areas.

            To determine the age of a sedimentary rock, we must find fossils.  Certain fossil species are always the same age wherever they occur.  All rocks containing these fossils and the same age even if found in different places and even if they are different kinds of rocks.  Matching the ages of rocks is known as correlation.

            The plant fossils found by Scott’s expedition have been identified by geologists as Glossopteris.  These plants have been found in coal seams in many places around the world.  Glossopteris is of Permian age.

 

1.     The rock columns are from four different areas.  Antarctica, India, Brazil and South Africa.  Write the names of these areas in their proper places on the outline map of the world (Worksheet 2).

2.     Color in each time period in the time Scale, on the left edge of the rock columns chart (Worksheet 1), with a different color.  (These six distinctive colors will then be used later to mark the same ages on the four rock columns).  Color in the beds containing the Glossopteris fossils with the color you used for Late Carboniferous or Permian on the Time Scale.  Place a “G” on your world map (Worksheet 2) where Glossopteris has been found, next tot he names of the four areas located in step 1.

3.     How can you explain the presence of Glossopteris in four such widely separated areas?

4.     Fossilized pollen found in the tillites indicates that they are either Late Carboniferous or Early Permian.  Color the tillites with the color you used for that age in the Time Scale.

 

Figure 1: Leaf of the Glossopteris plant, which lived during the Permian age, 270 million years ago.  (Redrawn from Hurley, P.M., 1968, Scientific American).

 

In 1967, a geologist with the Ohio State University institute of Polar Studies found a jaw fragment belonging to an ancient amphibian, not far from where Scott’s party found the coal seems and the Glossopteris fossils.  A team from the institute of Polar Studies, encouraged by this find, returned to the same are in 1969 with the specialist in identifying fossil amphibians and reptiles.  On the first day of the expedition, the team’s leader, David Elliot, climbed a bluff near the base camp.  He found an ancient stream channel containing bones and teeth.  Edwin H. Colbert, the specialist in amphibian and reptile fossils, identified a jaw-bone he found there later as being that of Lystrosaurus, a reptile previously found in India and South Africa in rocks of Early Triassic age.  It was a land reptile, not adapted for swimming long distances. 

 

5.     In the rock columns, color those sediments containing Lystrosaurus with the color you used in the Time Scale indicating their age.

6.     Place an “L” on Worksheet 2 beside the names of those areas where Lystrosaurus has been found.  How can you explain the presence of a reptile, like Lystrosaurus, in such widely separated areas?

7.     Certain shells found in the marine sandstone’s of Brazil, South Africa and Antarctica have been determined to be of Devonian age.  Color in these portions of the rock columns with the appropriate color.  Dicroidium is a plant fossil restricted to the Late Triassic.  Color in the portions of the rock columns containing Dicroidium.

 

The age of igneous rock, such as the basalt at the top of each of the rock columns, can be determined through a process called radiometric dating. In this procedure, the amounts of certain radioactive elements in the rock are measured.

 

8.     The basalts in Brazil and India are Cretaceous age.  In South Africa and Antarctica they are from the Jurassic Period.  Color the basalt in the rock columns with the colors you used for these ages in the Time Scale.

9.     Now correlate the rock unites for each area.  Draw lines between the columns indicating the rock boundaries between each of the ages.  The line dividing the “Basement” rock (pre-Devonian) from the Devonian has already been drawn in for you.  The Early Triassic layers of Brazil and South Africa have also been correlated.  Have your teacher check your correlation’s before you continue.

10. During what age were glaciers present in all four areas?

11. When were three of the areas covered by the sea?

12. During what age did all four areas have extensive swamps?

13. How can you explain that in the past the environments of these four areas, as indicated by their rocks, were very similar when today their environments are so different?

 

In this activity you have studied some of the evidence that leads geologists to believe that at one time India, Antarctica, Africa, Australia, and South America were all part of one super-continent called Gondwanaland.

 

14. From your study of plant and animal fossils, determine the most recent time that Gondwanaland could have been a single continent.

15. When did extensive volcanic activity first occur in Gondwanaland?

16. When do you think Gondwanaland began to break up?

 

Conclusion:

1.     How do geologists determine the type of environment that existed in an area during the past?

2.     A geologist found several layers of sediment exposed in a river bank.  Which was the oldest layer?  The youngest?

3.     What is meant by correlation of rock layers?

4.     Describe the evidence for the former existence of the continent Gondwanaland.