3. Evidence for Continental Drift

Transcription

1 YEAR 111 BIOLOGY EVOLUTION OF AUSTRALIAN BIOTA LESSON 1: GONDWANALAND SAMPLE RESOURCES

2 3. Evidence for Continental Drift Students learn to: Identify and describe evidence that supports the assertion that Australia was once part of a landmass called Gondwana, including: - Matching continental margins; - Position of mid-ocean ridges; - Spreading zones between continental plates; - Fossils in common on Gondwanan continents, including Glossopteris and Gangamopteris flora, and marsupials; - Similarities between present-day organisms on Gondwanan continents. Solve problems to identify the positions of mid-ocean ridges and spreading zones that infer a moving Australian continent. Development of the Plate Tectonics Theory Plate tectonics is a relatively new scientific concept, introduced some 30 years ago; but the belief that continents have not always been fixed in their present positions was suspected long before the 20th Century. This belief was seriously considered as a scientific idea in the early 1900s when Alfred Wegener introduced the idea of Continental Drift to explain the movement of continents after the break-up of Pangaea. Wegener s theory was essentially the forerunner to modern plate tectonics. His theory was based in part on what appeared to be the remarkable fit of the South American and African continents. He was also intrigued by the occurrences of unusual geological structures and of plant and animal fossils found on the matching coastlines of South America and Africa, which are now widely separated by the Atlantic Ocean. With Wegner's findings, scientists continued to seek evidence to support the idea that Australia, India, South America, Africa and Antarctica were part of a large landmass known as Gondwana, including: Matching continental margins, Positions of mid-ocean ridges or spreading zones between continental plates, Matching fossil types, and; Similarities in present day organisms. Page 15 of 219 this publication may be reproduced, stored in or introduced into a retrieval system, or transmitted, in any

3 Matching Continental Margins Looking at a map of the Earth, it appears that thee continental shorelines could fit together like a jigsaw puzzle this is what Wegener realised when he wrote his papers in Using this map, which continents show thee best fit of continental shores? 2 However, when using continental shorelines, the fit iss not perfect as rocks at some of thee continental shores aree younger than at other shores (e.g. Florida vs. Western Africa). What other factors may have altered shorelines such that they no longer match together? 3 Page 16 of 219

4 In the 1960s, scientists realised that fit could be improved iff continental shelves were used, instead of shorelines, as thee true boundaries of continents. The following diagram illustrates the difference between continental shorelines (or coasts), continental shelves and continental margins. The fit of South America and Africa is shown below,, including the continental shelf at a depth of 500 metres. Try piecing together the continental jigsaw puzzle using thiss LINK! Page 17 of 219

5 Wegener also noticed thatt when the continenta al margins were fit together: The mountain beltss lined up e.g. Appalachian Mountains off the coast of Newfoundland and mountains in Scotland and Scandinavia are comparable in age and structure. The rocks at the continental margins weree of matching ages and types, e.g. sedimentary, metamorphic or igneous. What does this evidence suggest about thee relative location of the continents in the past? 4 Page 18 of 219

6 Positions of Mid-Ocean Ridges ( Spreading Zones) The driving force behind the plate tectonics andd the movement of continents is heat generated deep inside the earth s core by radioactive r decay. This heat reaches the surface primarily along mid-ocean ridges. r One of the Earth s most dramatic topographical features, f the mid-ocean ridge, is a continuous range of f undersea mountainss more thann 3,600 metres high and 1,900 metres wide, winding through 64,000 metres of the world s oceans. It is here, at mid-ocean ridges, that new seafloor crust is produced and much of the Earth s internal heat is released. At these ridges, two plates are pulling apart from eachh other as hot magma (or liquid rock) emerges from the mantle m and oozes forth as lava to fill the crack continuously created by plate separation. The lava cools and attaches itself to the trailing edge of each plate, forming new ocean floor crust in a process commonly known as seaage of floor spreading. This is shown above. How would the age of the rocks near the ridges compare to the rocks further from the ridges? 5 The new crust formss in bands on either side of the mid-ocean ridge and each newly created band pushes the already-existingg crust a little farther away from the mid-ocean ridge. Page 19 of 219

7 The magnetic field f of the Earth reverses at regular intervals of approximately 5000 years. As a result, the rocks form bands off normal and reversed polarity, as indicated in the diagram below. The orientationn of the magnetic field is frozen into newly formed rock. Seafloor lavas have built-in magnetic clocks that reveal their age. Based on this information, we can work out at which points and in which directions the tectonic plates are moving. Tracing back these movements, scientists have concluded that the continents may have been joined at one point. Our geological plate, called the Australian-Indian Plate, is moving north at about 7 cm each year. How far have we moved since our separation from Gondwana 45 million years ago? 6 Page 20 of 219

8 As mentioned above, Australia is an island continent that lies on its own plate the Australian-Indian Plate. On the boundary of the plate south of Australia, the seafloor is spreading along mid-ocean ridges. From New Zealand to New Guinea, the eastern edgee of our plate is colliding with the Pacific Plate. Near Indonesia, the northern edge of our plate is sliding underr the Asian Plate. These boundaries are shown in the map of the continental plates below. Source: The existence of these mid-ocean ridges or spreading zones allows us to infer that the continental plates weree once joined together, and gradually moved into their current positions over time. If mid-ocean ridges are thee points on Earth where new crust is being synthesised, why is it that the Earth remains the same sizee rather than increase in size? Watch this ANIMATON (Length 1:13) to help you. 7 Page 21 of 219

9 Matching Fossil Types Additionally, Wegener found that the distribution of particular fossil types also supported the idea of continental drift. When he fitted together continental margins the fossil types found at the margins matched, including the following: Mesosaurus: a freshwater reptile with limited dispersal ability and geographic range. By Nobu Tamura ( (Own work) [GFDL ( CC-BY-SA-3.0 ( or CC BY 2.5 ( /licenses/by/2.5)], via Wikimedia Commons Page 22 of 219

10 The Mesosaurus is a significant piece of evidence for r the theory of continental drift, because its remains were found in southern Africa and eastern South America. How does this distribution of Mesosaurus fossils support the idea of continental drift? 8 Glossopteris: early fern-like tree that had large and heavy seeds that could not have floated or been blown across oceans and between continents. Fossils of Glossopteris have been found inn rocks of the same age in Australia, Antarctica, India, African and South America, providing evidence that these continents were once part of the great southern landmasss of Gondwana. Distribution of Glossopteris By Daderott - Daderot, CC0, By Petter Bøckman - Own work, Publicc Domain, /commons.wikimedia.org/w/index.php?curid= Page 23 of 219

11 Gangamopteris: ann early fern-like to be part of Gondwana. Fossilss have been found in tree with a wide distribution in many continents proposedd South America, Australia, India and South Africa. Diorit at the German languagee Wikipedia [GFDL ( or CC-BY-SA-3.0 ( via Wikimedia Commons These particula fossils (Mesosaurus, Glossopteris and Gangamopteris) have predominantly been foundd in Australia, Antarctica, India, Africa and South America, and because of their limited ability to disperse d by wind, water or land, they provide support the idea of a Gondwanan supercontis inent. Why would limited ability to disperse support the theory of continental drift? 9 When Gondwana existed as a single landmass aboutt 100 million years ago, many of the plant and animal speciess would have spread over the whole area. In fact, fossils of these particula species have h not been found in North America and Eurasia, because Laurasia had already separated from f Gondwana before the spread of these organisms. What was the name of the supercontinent made up off Gondwanaa and Laurasia?? 10 Page 24 of 219