Earth Science Chapter 17: Plate Tectonics

17.1 Review – Drifting Continents

17.1 – Essential Questions What are the lines of evidence that led Wegener to suggest that Earth’s continents have moved? How does evidence of ancient climates support continental drift? Why was continental drift not accepted when it was first proposed?

17.1 - Main Idea The shape and geology of the continents suggests that they were once joined together.

Early Observations With the exception of events such as earthquakes, volcanic eruptions, and landslides, most of Earth’s surface appears to remain relatively unchanged during the course of a human lifetime. On the geologic time scale, however, Earth’s surface has changed dramatically.

Early Observations In the late 1500s, Abraham Ortelius, a Dutch cartographer (map maker), noticed the apparent fit of continents on either side of the Atlantic Ocean. He proposed that North America and South America had been separated from Europe and Africa by earthquakes and floods.

Early Observations The first time that the idea of moving continents was proposed as a scientific hypothesis was in the early 1900s. In 1912, German meteorologist Alfred Wegener presented his ideas about continental movement to the scientific community.

Continental Drift Wegener developed a hypothesis that he called continental drift. He proposed that Earth’s continents had once been joined in a single landmass, a supercontinent called Pangaea (Meaning All Lands), that broke apart about 200 mya (million years ago) and sent the continents adrift.

Continental Drift – Evidence from Rock Formations Wegener observed that many layers of rocks in the Appalachian Mountains in the United States were identical to layers of rocks in similar mountains in Greenland and Europe. These similar groups of rocks, older than 200 million years, supported Wegener’s idea that the continents had once been joined.

Continental Drift – Evidence from Fossils Wegener gathered evidence of the existence of Pangaea from fossils. Similar fossils of animals and plants that once lived on or near land had been found on widely separated continents.

Evidence from Fossils

Continental Drift – Climatic Evidence Fossils of the plant Glossopteris had been found on many parts of Earth, including South America, Antarctica, and India. Wegener reasoned that the area separating these fossils was too large to have had a single climate. Wegener argued that because Glossopteris grew in temperate climates, the places where the fossils had been found had been closer to the equator. This led him to conclude that the rocks containing these fossil ferns had once been joined.

Continental Drift – Climatic Evidence Coal forms from the compaction and decomposition of accumulations of ancient swamp plants. Wegener used the existence of coal beds in Antarctica to conclude that Antarctica must have been much closer to the equator sometime in the geologic past.

Continental Drift – Climatic Evidence Glacial deposits nearly 300 million years old on several continents led Wegener to propose that these landmasses might have once been joined and covered with ice. The extent of the ice is shown in white.

A Rejected Notion Two unanswered questions— Although Wegener had compiled an impressive collection of data, the hypothesis of continental drift was not accepted by the scientific community. Two unanswered questions— What forces could cause the movement? How could continents move through solids? Main reasons that continental drift was rejected.

A Rejected Notion It was not until the early 1960s, when new technology revealed more evidence about how continents move, that scientists began to reconsider Wegener’s ideas.

Ch. 17 Notes Day 2 Sea Floor Spreading

Objectives Vocabulary Summarize the evidence that led to the discovery of seafloor spreading. Explain the significance of magnetic patterns on the seafloor. Explain the process of seafloor spreading. Vocabulary magnetometer paleomagnetism magnetic reversal isochron seafloor spreading

17.2 Seafloor Spreading

Seafloor Spreading Many thought the ocean floor was flat until the mid-1900s. Many thought that oceanic crust was unchanging and was much older than continental crust. Technology in the 40’s 50’s,showed those ideas to be wrong.

SONAR- Sound Navigation and Ranging Sound waves could find the depth of the ocean floor. Sound waves travel through water at: R = 1482 m/s T= was measured D= was found R×T = D

Magnetometer A magnetometer is a device that can detect small changes in magnetic fields.

Ocean Floor Topography Maps made from sonar and magnetometers showed underwater mountain chains called ocean ridges. Deep-sea trenches were found. Geologists could not explain why there were trenches and mountains on the oceans floor.

Trenches

Mid-Atlantic Ridge mountains

Ocean Rocks and Sediments Analysis of deep-sea rocks and sediments found 1. Ages of the seafloor rocks differ. The farther from a ridge the older the rock. The oldest part of the seafloor is geologically young at about 180 million years old. 2. Ocean-floor sediment gets thicker farther from a ridge

Ocean Rocks and Sediments

Paleomagnetism is the study of Earth’s magnetic record. Rocks containing iron-bearing minerals provide a record of Earth’s magnetic field. Basalt, because it is rich in iron-bearing minerals, provides an accurate record of ancient magnetism.

Magnetism The Geomagnetic Time Scale Studies reveal a pattern of magnetic reversals over geologic time. A magnetic reversal is a change in Earth’s magnetic field. A magnetic field that is the same as the present has normal polarity. A magnetic field that is opposite to the present has reversed polarity.

The Geomagnetic Time Scale Towing magnetometers behind ships revealed an interesting magnetic pattern. In places where the magnetic readings of the ocean floor matched Earth’s present field, a stronger-than-normal reading (+) was recorded. In places where the magnetic data were reversed in relation to Earth’s present magnetic field, a lower-than-normal reading (–) was recorded.

The Geomagnetic Time Scale

Magnetic Symmetry The positive and negative areas of the seafloor form a series of stripes that were parallel to ocean ridges. The magnetic pattern on one side of the ridge is a mirror image of the pattern on the other side of the ridge.

Magnetic Symmetry The magnetic data matched the pattern that had been found in basalt flows on land. This allowed scientist to determine the age of the ocean floor. And make isochron maps. An isochron is a line on a map that connects points that have the same age.

Seafloor Spreading An American scientist named Harry Hess proposed the theory of seafloor spreading. Seafloor spreading states that new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches. Magma is forced toward the crust along an ocean ridge and fills the gap that is created.

Seafloor Spreading When the magma hardens, a small amount of new ocean floor is added to Earth’s surface. Each cycle of spreading and the intrusion of magma results in the formation of another small section of ocean floor, which slowly moves away from the ridge.

The Missing Link Seafloor spreading was the missing link to complete his model of continental drift. Continents are not pushing through ocean crust, as Wegener proposed; they ride with ocean crust as it slowly moves away from ocean ridges.

Section Assessment 1. Match the following terms with their definitions. ___ magnetometer ___ paleomagnetism ___ isochron ___ seafloor spreading A. a device that can detect small changes in magnetic fields B. a line on a map that connects points that have the same age C. the study of Earth’s magnetic record D. a theory that states that new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches A C B D

Section Assessment 2. How does the distribution of ocean-floor sediments support the theory of seafloor spreading? The thickness of ocean-floor sediments increases with distance from an ocean ridge which indicates that the seafloor is older with distance.