The Seafloor

Alfred Wegener In 1912, a German scientist by the name of Alfred Wegener proposed the idea of moving continents. His hypothesis, called continental drift, was the first to propose the continents had once been one landmass, which he called Pangaea (pan-gee-ah). Flash Presentation

Evidence of continental drift 1) Rock similarities-similar type, age, and structure of rocks are found on the coasts of different continents.

Evidence of continental drift 2) Fossils-several fossils of plants and animals were found on widely separated continents.

Evidence of continental drift 3) Climatic evidence-glacial sediment was found in S. America, Australia, India. Not possible in current continental placement. Coal found in Antarctica suggests the continent was once closer to the equator.

Evidence of continental drift 4) Fit of continents-most notably, S. America and Africa.

Two reasons Wegener’s hypothesis rejected in early 1900s 1) No mechanism identified to move continents. 2) No process identified that would prevent continents from shattering if they moved. Later, the exploration of the oceanic crust, which the world assumed was flat, would provide Wegener with a mechanism for continental drift.

Types of Crust Two types of crust – oceanic and continental Continental crust has thickness of 40 km Oceanic crust has thickness of 6-7 km Deepest part of ocean is Marianas Trench in Pacific Ocean (just over 11 km deep!)

Continental Crust Elevation depends on how thick the crust is—the thicker the crust, the higher the elevation will be Continental crust is land that we live on and can see

Oceanic Crust Oceanic crust is covered by water Makes up about 60% of the Earth’s surface Composed mainly of basalt

Type of Crust Continental Oceanic Depth 40 km 6-7 km Found Land we see Covered by ocean Percantage coverage of Earth About 40% About 60% Composition Granitic Balsaltic (most dense)

New Advances in Technology… Until the 1940s and 1950s, we had misconceptions that oceanic crust was older than continental crust and remained unchanged. New advances in technology allowed our view of the seafloor to change: (1) Sonar (2) Magnetometer

Sonar Sonar uses sound waves to measure water depth.

Magnetometer A magnetometer is a device that can detect small changes in magnetic fields Towed by ships and records the magnetic field in rocks on seafloor Measurements used to construct topographic maps of the seafloor

Magnetic Reversals Technology in the mid 1900s allowed scientists to recognize a pattern in the basalt of ocean floors that indicated Earth’s magnetic field had changed periodically throughout time, referred to as a magnetic reversal. Remember the oceanic crust is composed of basalt. Basalt is rich in iron-bearing minerals, and is an accurate record of ancient magnetism As basalt cools, the iron minerals become oriented parallel to Earth’s magnetic field (similar to compass needles) When lava hardens, magnetic field is “locked” into place and gives record of Earth’s magnetic field at that time

Paleomagnetism Paleomagnetism is the study of rocks containing iron-bearing minerals that provides a record of Earth’s magnetic field. Oceanic crust (basalt) is rich in iron-bearing minerals As basalt cools, the iron minerals become oriented parallel to Earth’s magnetic field (similar to compass needles) When lava hardens, magnetic field is “locked” into place in that rock; tells the Earth’s magnetic field at that times

Magnetic Reversals A magnetic reversal is a change in Earth’s magnetic field. During normal polarity, the magnetic field has the same orientation as the Earth’s magnetic field. In reversed polarity, the magnetic field is opposite to Earth’s current magnetic field. These magnetic reversals create magnetic symmetry, where 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

Ocean Floor Topography Maps surprised scientists; revealed that underwater ridges had deep-sea trenches associated with them

Seafloor Spreading Henry Hess proposed explanation for data collected Seafloor spreading theory – new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches

Theory of Seafloor Spreading Provided explanation for magnetic reversals. This theory states that Earth’s crust is created at ridges on the ocean floor and destroyed at ocean trenches. Flash Animation

Seafloor Spreading Magma is less dense than surrounding rocks. It is forced upwards towards the crust along an ocean ridge and fills the gap that is created. When magma hardens, a small amount of new ocean floor is added to Earth’s surface. The sections of ocean floor slowly move away from the ridge.

Wegener’s Missing Link-Seafloor Spreading Continents are not pushing through ocean crust, as Wegener proposed; they ride with ocean crust as it slowly moves away from mid-ocean ridges.

Mid-Ocean Ridges Most prominent features of the ocean basins Run through all ocean basins and have a total length of over 65,000 km Average height of 1500 m Highest peaks are over 6 km tall and emerge from ocean as volcanic islands Mid-ocean ridges are the sites of frequent volcanic eruptions and earthquake activity

Mid-Ocean Ridges of the World

Ocean Rocks and Sediments Revealed: Ages of rock of the seafloor vary, and that the age variation changes in a predictable way Seafloor sediments are not as old as on the continental crust

Ocean Rocks and Sediments Rocks taken from areas near ocean ridges were younger than rocks taken from areas near deep-sea trenches The ocean crust is about 180 million years old, compared to continental crust that is 3.8 billion years old