Chapter 2: Plate Tectonics Lesson 1 Objective #3
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