Title: 17.2 Seafloor Spreading Page #: 88 Date: 3/18/2012 Table of Contents Title: 17.2 Seafloor Spreading Page #: 88 Date: 3/18/2012

Objective Students will be able to summarize evidence that led to the discovery of seafloor spreading. Students will be able to explain the significance of magnetic patterns on the seafloor. Students will be able to explain the process of seafloor spreading.

Word of the Day Basalt: A dark grey to black fine grained igneous rock.

Seafloor Spreading Main Idea: Pg. 473 Oceanic crust forms at mid-ocean ridges and becomes part of the seafloor.

Seafloor Spreading Mapping The ocean Floor: Pg. 473 Early 1900s: People believe that oceans are carved out by erosion. Believe that ocean floor is flat and old. 1940s: WWII submarine warfare results in the development of SONAR technology. Also results in MAGNOMETER technology.

Seafloor Spreading Mapping the Ocean Floor pg. 473 Sonar: Echolocation. Sub sends sound out, it bounces off an object and reveals its location. Magnometer: Device that can detect changes in magnetic fields (used to find enemy subs.)

Sonar Magnetometer

Seafloor Spreading Mapping the Ocean Floor pg. 473 Late 1940s-1950s: WWII ends. Scientists use new technology to study depth and topography of the ocean floor (sonar.) Scientists study magnetic fields of ocean floor rocks (magnometer.)

Seafloor Spreading Mapping the Ocean Floor pg. 474 1947: Bruce Heezen and Marie Tharp of Columbia University began to systematically map Earth’s seafloor features. Heezen went out on research cruises and gathered data from 1947 – 1965. Tharp: stayed on shore to construct maps. (Women were not allowed on research cruises.)

Heezen and Tharp

World Ocean Floor; Published 1974

Seafloor Spreading Mapping the Ocean Floor pg. 474 Surprise Findings! Ridges: huge underwater mountain ranges, 80,000 km long and up to 3 km high. Largest mountain range on Earth. Earthquakes and volcanoes are common along ridges.

Mid-Atlantic Ridge

Seafloor Spreading Mapping the Ocean Floor pg. 474 Surprise Findings! Deep-Sea Trenches: Narrow elongated depressions in sea floor. Can be 100s of km long and many km deep. Example: Marianas Trench; Mt. Everest (9 km above sea-level) can fit in Marianas Trench with 9 Empire State Buildings on it.

Seafloor Spreading Mapping the Ocean Floor pg. 474 Ridges and trenches lead to more questions: What formed mountains? What is source of valcanism on mountain range? What forces depressed Earth’s crust to create huge trenches?

Seafloor Spreading Ocean Rocks and Sediments pg. 475 2 Discoveries from samples of Deep Sea rocks and sediments: Age of rocks increases as you get farther away from a mid-ocean ridge – oldest rocks at trenches. Thickness of sediments increases as you get farther away from mid-ocean ridge (older crust has more time to collect sediments.)

Seafloor Spreading Oceanic Rocks only 180 million years old. Ocean Rocks and Sediments pg. 475 Oceanic Rocks only 180 million years old. Continental Rocks can be up to 3.8 billion years old. Layer of sediment on oceanic crust is only a few hundred meters thick. Layer of sediment on continental crust is 20 kilometers thick.

Thickness of sediments increases as you get farther from a mid-ocean ridge.

Seafloor Spreading Ocean Rocks and Sediments pg. 475 Oceanic rocks and sediments are symmetrical in age and depth of sediments (mirror images) on either side of a mid-ocean ridge.

Seafloor Spreading Earth has a magnetic field. Magnetic Reversal: Magnetism pg. 476 Earth has a magnetic field. Causes compasses to point north. A result of flow of molten iron in liquid outer core of the earth. Magnetic Reversal: When Earth’s magnetic field changes direction. Happens when there is a change in flow of the earth’s core.

Seafloor Spreading Magnetic Polarity Time Scale pg. 476 Paleomagnetism: Study of history of Earth’s magnetic field. Lava contains magnetite – when it solidifies crystals point toward the magnetic pole because of Earth’s magnetic field.

Magnetic Time Periods

Seafloor Spreading Magnetic Symmetry pg. 476 Magnetic Symmetry: Oceanic crust is mostly basaltic. Contains large amounts of iron bearing minerals. Magnetometers revealed a pattern in the ocean floor.

Seafloor Spreading Magnetic Pattern in Ocean Floor pg. 476 Regions with normal and regions with reverse polarity form a series of stripes across the floor parallel to mid-ocean ridges. Age and width of the stripes matched from one side of the ridge to the other.

Seafloor Striping

Seafloor Spreading Magnetic Pattern in Ocean Floor pg. 477 Isochron: A line on a map that shows points with the same age. Scientists were able to match patterns of reversals on land to the reversals on the sea floor to determine the age of the ocean floor.

Seafloor Spreading Seafloor Spreading pg. 479 Seafloor Spreading: Theory that explains how new crust is formed at ocean ridges and is destroyed at deep-sea trenches. Steps: Less dense magma rises at ridges and some solidifies. Solid magma makes new seafloor. Most magma gets turned horizontally away from ridge under the crust.

Seafloor Spreading Continental Drift questions answered: pg. 479 Continental Drift questions answered: Continents don’t move through oceanic crust. Continents are carried as passengers that ride along as ocean crust moves.