Seafloor Spreading and Mid-Ocean Ridges

• In the late 1940s, scientists explored the seafloor in detail using echo sounders to determine ocean depth.
• These depth data were used to create topographic maps of the seafloor, revealing vast mountain ranges called mid-ocean ridges.
• Mid-ocean ridges are longer than any mountain range found on land.

Seafloor Spreading

• Seafloor spreading explains continental drift and involves new oceanic crust forming at mid-ocean ridges while older crust moves away.
• As the seafloor spreads, the mantle melts and forms magma, which rises through cracks in the crust along the mid-ocean ridge.
• When magma erupts, it is called lava, which cools and crystallizes into basalt, forming rounded pillow lavas.
• The closer the oceanic crust is to a mid-ocean ridge, the younger it is, and as the seafloor continues to spread apart, the older oceanic crust moves progressively away from the ridge.
• Seafloor spreading provided the mechanism to explain continental drift.
• Mid-ocean ridges form either through lava eruptions that cool and build up around the ridge or through cracking and movement of rocks along the seafloor.
• Sediment accumulates on top of the oceanic crust, with almost no sediment near the mid-ocean ridge.
• Far from the ridge, sediment becomes thick enough to create the abyssal plain.
• Continents move as the seafloor spreads along mid-ocean ridges.

Magnetic Reversals

• Earth's magnetic field, created by the iron-rich liquid outer core, experiences changes in direction.
• Normal polarity is when magnetized objects point north, while reversed polarity is when they point south.
• Magnetic reversals occur every few hundred thousand to few million years.
• Basalt on the seafloor contains iron-rich magnetic minerals that align with Earth's magnetic field when lava cools and crystallizes, recording the field's direction at the time.
• Parallel patterns in the magnetic signature of rocks on either side of mid-ocean ridges have been discovered.
• Scientists measured the magnetic signature of rocks using a magnetometer, revealing parallel magnetic stripes, with each pair having similar composition, age, and magnetic character.
• Each stripe represents crust formed and magnetized at a mid-ocean ridge during normal or reversed polarity.
• Magnetic stripes confirm that ocean crust forms at mid-ocean ridges and is carried away in opposite directions.

Other Evidence

• Measurements of thermal energy leaving Earth show more energy released near mid-ocean ridges than from abyssal plains.
• Sediment dating reveals that sediment closest to the mid-ocean ridge is younger, and sediment thickness increases with distance from the ridge.

Learn about seafloor spreading and mid-ocean ridges. The process involves new oceanic crust forming at mid-ocean ridges. Older crust moves away as the mantle melts and forms magma, which rises and erupts as lava.