Seafloor Spreading and Tectonic Activity

Seafloor Spreading

• Seafloor spreading is a geological process where new oceanic crust forms at mid-ocean ridges, then slowly moves away from the ridges.
• This process occurs at divergent boundaries. Tectonic plates move apart, forming new seafloor.
• The movement of tectonic plates is driven by convection currents in the Earth's mantle.
• Seafloor spreading is supported by evidence from studying molten materials, magnetic stripes in the seafloor, and drilling samples from the ocean floor.
• Molten materials erupt from cracks in mid-ocean ridges, forming unique "pillow" structures.
• Magnetic stripes on the seafloor exhibit a symmetrical pattern, reflecting reversals in Earth's magnetic field.
• Older rocks are found farther from the mid-ocean ridge, while younger rocks are closer to it. This age pattern supports the idea of seafloor spreading.
• The Earth doesn't get bigger as the seafloor spreads. Subduction occurs where the older oceanic crust slides beneath the continental crust, into the mantle.

Learning Competency

• Learning Competency: Explain how seafloor spreads.

Learning Objectives

• Discuss the process of seafloor spreading.
• Explain the role of convection currents in seafloor spreading.
• Describe three pieces of evidence supporting seafloor spreading.

Echo Sounders/Sonar

• Echo sounders produce sound waves that travel outward in all directions and bounce off the nearest object, returning to the ship.

Features of the Seafloor

• Modern maps of the Atlantic and Pacific Oceans show darker blue areas corresponding to deeper seas.
• Deep sea trenches are found along the west coast of Central and South America and in the mid-Atlantic, east of the southern tip of South America.
• Isolated mountains, large mountain ranges, and stretches of flat, featureless regions are also common.

Seafloor Magnetism

• Scientists used magnetometers to discover a pattern of normal and reversed magnetic stripes in ocean floor basalts. This pattern helps date the rocks.
• The ocean floor's features include the continental shelf, continental slope, abyssal plain, mid-ocean ridge, seamounts, and trenches.
• Reversals in Earth's magnetic field are recorded in the rocks created at mid-ocean ridges.

Polar Reversals

• Earth's magnetic field sometimes reverses, alternating between periods of normal and reversed polarity.
• When the north and south magnetic poles are aligned, it's called normal polarity. When they are reversed, the magnetic field is reversed polarity.

Magnetic Stripes

• Scientists discovered a symmetric pattern of normal and reversed magnetic stripes on the seafloor. These stripes mirror the reversals in Earth's magnetic field, confirming seafloor spreading.

Seafloor Ages

• Seafloor is youngest near mid-ocean ridges, progressively older the farther one travels away from the ridge.
• Orange areas on seafloor age maps correspond to the youngest seafloor. The oldest parts of the seafloor are located near continents or deep-sea trenches.

Hess's Theory of Seafloor Spreading

• Harry Hess proposed the theory of seafloor spreading to explain continental movement.
• Hess suggested that continents do not move across oceanic crust, but that continents and ocean floor move together, with new crust formed at mid-ocean ridges through upwelling magma.

Convection Currents

• Convection currents within the Earth's mantle drive the movement of tectonic plates.
• Heat from the Earth's core causes hot mantle material to rise, cool, and sink, creating a cycle that moves the plates.
• Ridge push and slab pull contribute to plate movement.

Divergent Boundary-Fast-spreading Ridge

• Hot mantle rock rises at mid-ocean ridges.
• Melt forms beneath the lithosphere.
• Magma rises, becomes injected into the crust, and forms new crust at the mid-ocean ridge.
• Lava erupts onto the ocean floor, adding new crust.
• Plates move apart, cool, and thicken.

Processes of Seafloor Spreading

• The movement of oceanic plates at divergent boundaries causes fractures (due to tensile stress) in the lithosphere.
• Hot basaltic magma rises from the mantle and fills these fractures.
• The magma cools and solidifies, forming new seafloor.
• The new seafloor moves away from the ridge, and the cycle repeats.
• Older rocks move farther from the spreading centers than younger ones, near the spreading center.

Efficiencies that the seafloor spreads

• Evidence from molten materials (pillow rocks).
• Evidence from magnetic stripes (reversals recorded in rocks).
• Evidence from drilling samples (older rocks farther from mid-ocean ridges, youngest rocks at the ridge).

Why Doesn't the Earth Get Bigger?

• Seafloor spreading is balanced by subduction. Older oceanic crust slides beneath continental crust at subduction zones, returning to the mantle.