Lecture 7 Mapping the Ocean Floor Earth’s Internal Structure Plate Tectonics and the Ocean Floor
Mapping the Seafloor Bathymetry: measurement of ocean depths and charting of the ocean floor topography. First measurements made aboard the HMS Challenger during its 3.5-year voyage (December 1872 – May 1876).
127,500 km voyage of the HMS Challenger
Mapping the Seafloor Sonar: Sound Navigation and Ranging Sound energy used to measure water depths. Echo Sounders: Transmit sound waves into ocean water. Produces an echo when it bounces off the seafloor.
Distance = Velocity x Time Mapping the Seafloor A clock measures the travel time for sound waves (2–way). Speed of sound in water is 1,500 m/s (4,900 ft/s). Travel time and speed used to determine distance traveled. Distance = Velocity x Time
Mapping the Seafloor After World War II, sidescan sonar is developed. Used to create first photograph-like images of seafloor. Does not provide actual water depth data.
Mapping the Seafloor High-Resolution Multibeam Sonar: Developed during the early 1990s. Provides bathymetric data for wide strips of seafloor. Very high resolution (distinguishes depth differences of less than 1 m). Only about 5% of the seafloor has been mapped in detail with this method.
Mapping the Seafloor from space: Massive seafloor features exert stronger gravity causing elevated areas of ocean surface. Satellites use microwaves to map ocean surface variations within a few cm.
Map of gravitationally-produced irregularities of the ocean surface.
Mapping the Seafloor Seismic Reflection Profiles: views of the rock structures beneath the seafloor. Images of the subsurface acquired from strong, low-frequency sounds produced by explosions or air guns.
Earth’s Internal Structure
The Crust: thin outer, rocky layer of Earth. Thicker crust is more elevated than thinner crust.
Continental Oceanic Crust Type Average Thickness 35-40 km 7 km (> 70 km beneath mountainous regions) 7 km Average Density 2.7 g/cm3 3.0 g/cm3 Average Composition Granitic Rock Basalt and Gabbro (Greater amounts of Mg and Fe) Age of Oldest Rocks Approximately 4 billion Years 180 million years
Gradual increase in temperature, pressure, and density with depth causes changes in the physical properties of rocks.
Lithosphere: rigid, relatively cool outermost shell that consists of the crust and the uppermost mantle. Average thickness of 100 km. More than 250 km thick below oldest portions of continents.
Asthenosphere: soft, relatively weak layer of the mantle. Temperature/pressure conditions are such that rock is near its melting point and is easily deformed.
The Outer Core: liquid shell of iron-nickel alloy that is 2,260 km thick. The flow of metallic iron in this zone generates the Earth’s magnetic field.
Seismic waves used to determine the physical properties of Earth’s interior.
The Mobile Geosphere The Theory of Plate Tectonics: The Earth’s lithosphere is broken into numerous slabs called tectonic plates. The plates are in continual motion - average rate of 5 cm (2 inches) per year. Rigid lithosphere plates move about on the asthenosphere which is softer and capable of gradual flow.
Moving plates interact with one another along their margins forming 3 types of plate boundaries.
Individual plates can be capped with both continental crust and oceanic rust.
Greatest concentrations of earthquakes reveal the locations of plate boundaries.
The Driving Force of Plate Motion Heat transferred from planet’s interior by convection within the mantle.