Chapter 7: Changing Continents and Climates

19th Century Geology

• Law of Superposition – 19th century geologists assumed that strata lower in the stratigraphic exposure were older than the overlying strata

• Law of Uniformitarianism – assumes that processes that weathered, eroded, and deposited rock in the past were the same as those operating today

• These laws used to develop geologic time scale

Alfred Wegener

• 1912 Theory of Horizontal Displacement of the Continents – similarities between Atlantic coastlines

• Theory of Continental Drift (1912-1929)– Crust of continents composed of relatively

light rocks. Ocean floors largely composed of dense basalts. Lighter continental rocks could float on viscous mantle of Earth and override oceanic crust

– Permian glacial deposits in Brazil, South Africa, India, and Australia – indicates that tropical areas once glaciated, so they must have been close to the poles at one time

– Similarities in the shapes of continents, their geology, and fossils suggest closer proximity in the past

– Geodetic measurements of latitude and longitude of Greenland indicated westward movement of the landmass

– Continental Drift led to Plate Tectonics

Alfred Wegener

• Pangaea

• Gondwanaland – Africa, South America, Australia, Antarctica, and India

• Laurasia – North America, Europe, and Asia

• Pangaea formed 195 mya – began to break up 180 mya with formation of Atlantic Midoceanic Ridge

Alfred Wegener

Figure 7.3

Vindication – Alfred Wegener

• Lystrosaurus – Triassic reptile –fossils found in Antarctica, Africa, and India – Antarctica must have been warmer in the past

• Galaxioidea – freshwater fish family – found in southern South America, South Africa, Australia, and New Zealand – must have been connected in the past

• WWII – sea floor mapping by submarines

Vindication – Alfred Wegener

• Paleomagnetism – ancient igneous rocks are

little compasses • iron or titanium point to their magnetic north• compare this to current magnetic north to

determine where continents used to lie

Figure 7.6

Glaciation• Radiocarbon dating – based on measuring

the amount of the radioactive isotope carbon 14 found in plant and animal remains

• Greenland ice cores extend 100,000 years• Antarctic ice cores extend 400,000 years• Ice cores provide record of atmospheric

chemistry• 18O isotopes deposited during warmer

times – 16O isotopes deposited during cooler times

Figure 7.7

Figure 7.9

Relationship between atmospheric temperature and 18O isotopes