Plate Tectonics II: The Ocean Floor and the Seafloor Spreading Hypothesis

Which discoveries lead to understanding of Plate Tectonics?

(1) demonstration of the ruggedness and youth of the ocean floor; (2) confirmation of repeated reversals of the Earth magnetic field in the geologic past; (3) emergence of the seafloor-spreading hypothesis and associated recycling of oceanic crust; (4) precise documentation that the world's earthquake and volcanic activity is concentrated along oceanic trenches and submarine mountain ranges.

Sonar

- image the deep ocean using reflected sound. Developed before WWII.

Mapping Ocean Floor Debunked

-The deep ocean is a flat, featureless expanse. -The deepest regions of the ocean are found in the center of ocean basins. -Mountain ranges are only found on continents. -The ocean floor is covered in a deep pile of marine sediments, accumulated over billions of years.

Remnant Magnetism

1940's - geologists discover that the Earth's magnetic field is recorded by iron-bearing minerals during the formation of rocks. •This is trapped in the rock. • Magnetic minerals act like tiny compasses, pointing to the former position of the magnetic pole (declination). • Magnetic minerals also record the polarity ofthe Earth's magnetic field in the past.

Echo Sounding

A type of SONAR used to determine the depth of water by transmitting sound pulses into water. Reflection of sound signals.

Magnetometer

Detect faint magnetic fields. Developed before WWII.

War

Historically, what human activity has generated the most amount of funding for scientific research?

Wadati-Benioff Zone

Region of earthquake activity at a subduction zone.

Mantle Convection

The circulation of heat in the mantle that causes tectonic plates to move.

Sea-Floor Spreading

The process by which new oceanic crust forms as magma rises towards the surface and solidifies.

Plate Tectonic Theory

The theory that large segments of the outer part of Earth move relative to one another. Describes and provides evidence for the existence and movement of Earth's plates.

Seafloor Magnetic Polarity Stripes

This creates a symmetrical pattern of magnetic stripes of opposite polarity on either side of mid-ocean ridges. These patterns of stripes provide the history of seafloor spreading. Geophysicists can read these patterns from the magnetic anomalies they measure with a magnetometer.

The Cold War

Why was the World Wide Seismic Monitoring network (detection of atomic tests, monitoring of above ground testing ban)created?

Aseimic

a mountain ridge or chain of seamounts under the ocean. They do not produce seafloor spreading or seismic activity, except in the area of a "hotspot" at one end of the ridge.

Magnetic Field Reversion

• 1940's - geologists discover that the Earth's magnetic field randomly "flips" so that the N and S magnetic poles become reversed. • Reversals occur randomly and last for 100's of thousands to millions of years. • Pattern of reversals recorded in the rock through time is unique - like a bar code. • Reversals of the Earth's magnetic field are recorded in rocks as intervals of Normal(N) and Reversed (R) polarity.

Arthur Holmes

• Used radiometric dating to determine an accurate age for the Earth and to create a geologic timescale. •Proponent of the continental drift hypothesis. •Proposed that convection currents in the mantle pushed continents apart, creating new ocean basins in between. •Acknowledged that no hard evidence existed to support his hypothesis.

Harry Hess

•Suggested that the ocean floor itself might be moving. • Continents are carried along, embedded within the ocean floor. • Hypothesis is named "Sea Floor Spreading" by Robert Dietz at Scripps Institute.