Plate Tectonics and Seafloor Features

Isostasy: Lithosphere–Asthenosphere Equilibrium

Isostasy: The equilibrium between the lithosphere and the underlying asthenosphere.

Studying the Ocean Floor

Studying the ocean floor

Echo sounders produce sound waves that travel outward in all directions. The sound waves bounce off the nearest object and then return to the ship. Scientists know the speed of sound in seawater; they can then calculate the distance to the object that the sound wave hit.

Marine Sediment and Pillow Lavas

Marine sediment: very thin. Pillow lavas: volcanic rocks, very young compared with continental rocks.

Antarctic Ocean Floor Features

• Continental shelf: the edge of the continent that lies underwater.
• Continental slope: the area between the continental shelf and the abyssal plain.
• Ocean ridges: very long underwater mountain ranges.
• Transform faults: fractures in the oceanic crust that cross the rift of ocean ridges perpendicularly.
• Abyssal plain: a large flat area on the ocean floor.

Pacific Ocean Floor Features

Pacific Ocean floor: abyssal plain, mountain ranges often with volcanoes; ocean ridges, abyssal plain, trenches: narrow deep channels; island arcs: curved chains of volcanic islands next to ocean trenches.

Sea-Floor Spreading Process

Magma rises up from the mantle and flows out as lava through the rift of the Mid-Atlantic Ridge. As this new lava moves up, it pushes out the older lava deposits. As the lava cools it turns into rock, forming symmetrical bands on both sides of the ridge and creating new ocean floor. This process is known as sea-floor spreading.

Theory of Plate Tectonics

There are eight large plates and many smaller fragments called microplates. There are different types of plates:

• Oceanic plates: composed of oceanic lithosphere.
• Continental plates: composed of continental lithosphere.
• Mixed plates: composed of both oceanic and continental lithosphere.

1. A lithospheric plate is a fragment of the lithosphere. They are mobile!
2. Subduction: the process in which the oceanic lithosphere moves under the continental lithosphere or under another oceanic plate and sinks into the mantle.
3. Convection currents were proposed as the reason why plates moved. The idea is that there are areas where hot currents rise and separate, where ridges are formed, and areas where currents cool and descend. Today we think this explanation is not enough.
4. The lithospheric plates do not just float on the asthenosphere, but also actively contribute to their own movement in two ways: the gravity that pulls down (isostasy) on elevated ridges, and when a plate is being subducted its weight pulls it down lower.