Chapter 2: Plate Tectonics

Lesson 1

Objective #3

Ch.2, CRO #3

• Describe the process of sea-floor spreading.

• Describe where sea-floor spreading is occurring.

• Describe how sea-floor spreading was discovered and the evidence used to support it.

A huge discovery

• While mapping the sea floor using technology developed in WWII (SONAR), oceanographers found a huge chain of underwater mountain ranges and volcanoes.

• A huge rift (crack) ran down the center of these ranges, suggesting the crust was being pulled apart.

Mid-Ocean Ridges

The theory of sea-floor spreading

• In the 1950’s a theory known as “sea floor spreading” emerged. (Harry Hess)

• Sea-floor spreading – the process in which the ocean crust is pulled apart (spreading) and widening the ocean as new sea-floor is formed from volcanic eruptions of mantle lava along the ocean ridges.

Harry Hess

Sea-floor spreading

Sea-floor Spreading (Hess)

Bill Nye greatest discoveries clip

• Sea-floor spreading

Mid-ocean ridges

• The sea floor is spreading away from formations known as mid-ocean ridges.

• These are long mountain ranges on the sea floor divided by a rift valley and active volcanoes.

• The mid-Atlantic ridge runs right down the exact center of the Atlantic ocean.

• It is here that volcanoes create new oceanic crust to “grow the ocean”.

Evidence

• Sea floor spreading was discovered based on:– The observation that magnetic reversals were

found recorded in the ocean crust in a symmetrical pattern on both sides of the ridges.

– The observation that ocean crust gets older on each side of the ridges the farther away you get from them.

Magnetic Reversals

Connection to Continental Drift

• The ages of the rock on the Atlantic sea floor as well as the rate of spreading suggests that the Atlantic ocean began spreading about 180-200 million years ago.

• This matches the continental drift evidence that suggests that Pangaea broke up about this same time.

• Sea-floor spreading appears to be the force behind continental drift (causes continents to move); Alfred was right after all !!!

• an ode to alfred wegener