CHAPTER 10-1 CONTINENTAL DRIFT

Chapter 10-1: Continental Drift Chapter 10-1 Objectives E3.p3A – Describe geologic, paleontologic, and paleoclimatalogic evidence that indicates Africa and South America were once part of a single continent. E3.3A – Explain how plate tectonics accounts for the features and processes (sea floor spreading, mid-ocean ridges, subduction zones, earthquakes and volcanoes, mountain ranges) that occur on or near the Earth’s surface. Chapter 10-1 Vocabulary Continental drift – hypothesis proposed by Wegener that the continents once formed a single landmass, broke up, and drifted into their present locations Supercontinent – the single landmass that all the continents once formed called Pangea Mesosaurus – a small, extinct land reptile whose fossils had been found in both South America and western Africa lending credence to the single continent theory

Chapter 10-1 Vocabulary cont. Mid-ocean ridges – undersea mountain ranges through the center of which run steep, narrow valleys Rift – a crack in the Earth’s crust in the center of mid-ocean ridges Magma – molten rock from deep within the Earth that fills the rifts Sea-floor spreading – the process where magma rises through the rifts in the mid-ocean ridges and solidifies forming new oceanic lithosphere (sea floor) Paleomagnetism – the study of the alignment of magnetic minerals in rocks as it relates to the reversal of Earth’s magnetic poles, this alignment results when the magma in the rock solidifies as it is forming Normal polarity – rocks whose magnetic field points north Reversed polarity – rocks whose magnetic field points south Geomagnetic reversal time scale – using the normal and reversed polarity in rocks to create a pattern showing how Earth’s magnetic field reverses itself, over the last 10 million years the magnetic field has reversed 4 to 5 times every 1 million years

Wegener's Hypothesis In 1912 Alfred Wegener proposed that all continents were once a part of a single land mass and began to break up into smaller ones about 200 million years ago during the Mesozoic Era. He noticed that the eastern coast of South America and the western coast of Africa seemed to match up like a jigsaw puzzle. Other things that led Wegener to conclude this were similar plant and animal fossils found in these areas. He also speculated that the crumpling crust produced mountain ranges.

Wegener's Hypothesis cont. Geologic evidence also supported Wegener’s theory. The ages and types of rocks found along both coasts closely matched each other. The Appalachian Mountains extend northward along the eastern coast of North America while there are similar mountain ranges in Greenland, Scotland and northern Europe. Climatic evidence also exists. Areas that today show the remains of glaciers are in areas that have been too warm for millions of years for those to have existed if the area has always been where it is. Wegener died in 1930 and would never see his theory be proven true.

Mid-Ocean Ridges

Mid-Ocean Ridges cont. The previous slide shows the world’s mid-ocean ridges. The picture to the left is the Mid-Atlantic Ridge. These ridges were discovered in 1947. It was this final piece of evidence that proved Wegener’s theory. Scientists noticed two trends while studying these: 1. The sediment covering the sea floor nearest the ridges was thinner than the sediment farther away, it was also younger. 2. Scientists learned that the ocean floor is very young. On land rocks can be found to be 3.8 billion years old but on the sea floor no oceanic rocks were older than 190 million years old.

Sea-Floor Spreading It wasn’t until 1950 that a geologist named Hess suggested that in the center of these valleys in the mid-ocean ridges were actually cracks in the Earth’s crust that allowed magma to rise and fill the cracks. His hypothesis was that the sea floor was spreading apart, another mechanism that Wegener had failed to find.

Paleomagnetism In the mid 1960’s new evidence was uncovered that led more credence to Wegener’s theory, this came in the form of rocks of different layers having different polarities (the rocks’ magnetic fields faced different directions depending on when they formed). The picture at the left shows the polarity reversals of different strata of rock. The one on the right shows the polarity reversals in the lithosphere of the Pacific Ocean.