Welcome to Earth Science 2017-2018

Plate Tectonics Chapter 17

EES News and Notes Let’s start previewing with a self-guided reading of the opening part of the chapter. After that, we’ll move forward through the first section of notes. We’ll take our exam next week over minerals and all types of rocks (igneous, sedimentary and metamorphic); studying time to come!

How did we end up with the landmasses of today? Drifting Continents How did we end up with the landmasses of today?

Early Observations While unchanged, within a human lifetime, the Earth has changed drastically in the realm of geologic time. 1500s: Dutch cartographer Abraham Ortelius notices fit of continents based on coastlines.

Continental Drift German meteorologist, Alfred Wegener, developed hypothesis that Earth’s landmasses were at one time a single landmass. Pangea: A mega continent that is Greek for all the earth.

Continental Drift Wegener supposed that drift started about 200 million years ago. First to consider coastlines and rock, climate and fossil data to support his hypothesis.

Evidence from Rock Formations If Pangea broke apart, then we should be able to look at broken parts and compare. Similar types of rocks on both sides of Atlantic Ocean? Wegener observed rock layers in Appalachian Mountains were similar to those in Europe!

Evidence from Fossils Cynognathus Lystrosaurus Similar fossils of several different plants and animals were found on widely spread continents. Cynognathus and Lystrosaurus couldn’t swim between continents. Fossils pre-dated the time for the breakup of Pangea, supporting his hypothesis! Cynognathus Lystrosaurus

Climatic Evidence As a meteorologist, Wegener recognized clues from his studies. Glossopteris: seed fern found in SA, Ant and India. Because of the temperate climate needed, Glossopteris had to have been located closer to the equator originally.

Coal Deposits Sedimentary rocks provide clues to past environments and climates. Antarctica has coal beds. Coal beds only form in warm, wet regions. Therefore, Antarctica had to be closer to the equator previously.

Glacial Deposits Glacial deposits found in Africa, India, Australia and SA 290 million years old! These areas were once covered in an ice cap like that of Antarctica today. Therefore, these regions of these countries must have been located closer to the south pole previously.

A Rejected Notion Early 1900s, most scientists considered continents and ocean basins to be fixed features of the Earth. Wegener’s hypothesis was not accepted in his lifetime. Two major flaws existed in the hypothesis that others couldn’t look past.

A Rejected Notion First, it didn’t explain what could be strong enough to push continents (large masses) over such great distances. Second, it was ‘known’ that the mantle below the crust was solid, so how could things move through a solid?

A Rejected Notion Early 1960s, Wegener’s ideas are revisited with new information and new vigor. Advances in seafloor mapping and understanding Earth’s magnetic field provided evidence of continental movement and forces involved!

Convergence? Divergence? Seafloor Spreading Convergence? Divergence?

Mapping the Ocean Floor Prior to the mid 1900s, people thought the ocean floor was flat and old! Technology has changed this. Magnetometer: a device that detects small changes in magnetic fields. Used to study ocean floor rocks, in addition to sonar!

Ocean-Floor Topography Ocean floors flummoxed scientists! Underwater mountain chains ridges are the longest continuous chain on Earth! Can be 65,000 km long and 3km tall! Earthquakes/volcanism located along ridges.

Ocean-Floor Topography Underwater mountain chains also had deep sea trenches. Narrow, elongated depression on the seafloor. Mariana Trench: 11 km deep Why do these form around the Earth? What could create such deep trenches?

Ocean Rocks and Sediments Two crazy findings of ocean floor sediment: Ages of rocks vary across the ocean floor with variations that are predictable Near Ridges: Young crust Near Trenches: Old crust Age of crust increases with distance from ridges.

Ocean Rocks and Sediments Oldest parts of seafloor are ‘geologically’ young (180 myo) Oldest continental rock: 4 byo Why is oceanic crust so much younger?! Ocean-floor sediments are typically a few hundred meters thick By comparison, land sedimentary rocks can be 20 km thick!

Ocean Rocks and Sediments Why are ocean sediments not as thick as land sediments? Hypothesis: sediments are related to age of ocean crust. Observations: Thickness of sediments increases with distance from an ocean ridge! Pattern is also symmetrical from all ocean ridges.

Magnetism Earth’s magnetic field is based on flow of molten iron in outer core. This makes needles point north on a compass. Magnetic Reversal: the flow in the out core changes, changing the Earth’s magnetic field direction. This has happened many times in Earth’s history.

Magnetic Polarity Time Scale Paleomagnetism: The study of Earth’s magnetic field. Lava solidifies, and iron bearing minerals crystallize. As they crystallize, they behave like tiny compasses and align with the magnetic field. This is how we track changes between normal and reversed magnetic fields!

Magnetic Symmetry Oceanic crust is mostly basaltic rock (large amounts of iron- based minerals). Hypothesis: Rocks on the ocean floor might show magnetic reversals. Towed magnetometers and found that stripes emerged parallel with ocean ridges with matching polarity!

Magnetic Symmetry What does this mean? By matching patters on ocean floor with those on land, scientists were able to determine age of ocean floor from magnetic recording. Via isochron maps Isochron: Imaginary lines on a map that shows points that formed at the same time!

Seafloor Spreading Theory that explains how new oceanic crust is: Formed at ridges Slowly moved from ridges Destroyed at deep-sea trenches Based on topography, sedimentary and paleomagnetic data.

Seafloor Spreading - Process Magma (hot, low density) forced toward surface of crust along ocean ridge. As the ridge spreads apart, rising magma fills gap created. When solidified, new ocean floor forms. Spreading continues, more magma is forced up and solidifies.

Seafloor Spreading - Process The cycle continues the formation of the ocean floor in perpetuity. Mostly happens under the sea, but in Iceland, a portion of the Mid- Atlantic Ridge rises above sea level!

Seafloor Spreading Wegener collected data to support that continents drift across Earth’s surface, but… Couldn’t explain how they moved Couldn’t explain what caused the movement Continents DON’T push through ocean crust; ocean crust moves continents away from ridges.