Development of Sea Floor Spreading

DSDP1950,s

SONAR

Sea- floor Spreading• The longest chain of mountains in the world is

the Mid-Ocean Ridge.• Scientists have mapped the ocean floor and mid-

ocean ridge using S.O.N.A.R. • The mid-ocean ridge extends into all of Earth’s

oceans. • The Island of Iceland is part of the ridge that

rises above the surface in the Atlantic Ocean. • Down the middle of the ridge is an area called the

rift valley – It is almost twice as deep as the Grand Canyon

Evidence for Sea-Floor Spreading:

Molten Material. The submersible ALVIN was sent down to study the rift valley.

Harry Hess• Harry Hess examined maps of the mid-ocean

ridge and proposed that the ocean floors move like conveyor belts, carrying the continents along with them.

• This movement begins at the Mid- Ocean Ridge.

• At the ridge, the molten material rises from the mantle and erupts (much like a volcano).

• The oldest rock is farthest from the center of the ridge.

• Harry Hess first proposed the idea of Sea-Floor Spreading.

Evidence• Rock Sampling. The Glomar Challenger was a drilling

ship the gathered samples from the ocean floor.

• The scientists determined the age- of the rocks in the samples and found that the farther-away from the ridge, the older the rocks were.

• The oldest rocks are about 3-4 hundred million years old.

• The newer rocks were always in the center of the ridges

EvidenceMagnetic Stripes. How

does the pattern of matching stripes show evidence of sea-floor spreading?

Sea floor spreading demonstrates that the sea floor moves apart at

the oceanic ridges and new oceanic crust is added to the

edges.

• Moving across the ocean floor perpendicularly to the oceanic ridges, magneometers alternately record stronger (positive) and weaker (negative) magnetic fields (called magnetic anomalies) in response to the influence of the sea floor rocks.

• Magnetic anomalies and the rocks causing them form parallel bands arranged symmetrically about the axis of the oceanic ridge.

Sea-Floor Spreading

• As basaltic rocks crystallize, some minerals align themselves with Earth’s magnetic field, as it exists at that time, imparting a permanent magnetic field, called paleomagnetism, to the rock.

• Periodically Earth’s magnetic field polarity (direction) reverses poles.

Sea-Floor Spreading

Because Earth’s size is constant, expansion of the crust in one area

requires destruction of the crust elsewhere.

• Currently, the Pacific Ocean basin is shrinking as other ocean basins expand.

• Destruction of sea floor occurs in subduction zones.• Seismicity is the frequency, magnitude and distribution

of earthquakes. Earthquakes are concentrated along oceanic ridges, transform faults, trenches and island arcs.

• Tectonism refers to the deformation of Earth’s crust.

• Development of the new Theory: • early 1950s – new evidence emerged to revive Wegener's

theory  – identification of the global mid-ocean ridge system – demonstration of the youth of the ocean floor  – magnetic 'stripes' found on the seafloor – pattern of earthquake hypocenters in well-defined belts

• emergence of the seafloor-spreading hypothesis by Dietz, Hess, and Vine

• suggested that mantle convection drives motion • demonstrated that oceanic crust formed at mid-ocean

ridges and spread laterally