Chapter 12: The Seafloor

Introduction The seafloor makes up the largest part of the Earth’s surface.

Introduction

Introduction Why is it important to know about the seafloor? ◦ The seafloor constitutes the largest part of Earth’s surface. ◦ Studies of the seafloor provide some of the evidence for plate tectonic theory, and various resources are found in seawater and on or in the seafloor.

Methods Used to Study the Seafloor Direct Sampling ◦ clamshell samplers ◦ piston corers.

Methods Used to Study the Seafloor

Oceanic Crust - Its Structure and Composition

The Continental Margins

Passive continental margins ◦ Passive continental margins lack volcanism and have little seismic activity. ◦ These margins usually have a wide continental shelf, and a slope which merges with a continental rise. Abyssal plains commonly extend seaward of the continental rise.

Features Found in the Deep-Ocean Basins Oceans evolve through a series of stages ◦ beginning with crustal thinning and rifting of a continent to create a rift valley ◦ followed by a long, narrow sea as a fully-developed spreading center evolves ◦ then eventually an ocean. As the cooling oceanic crust is subducted along ocean margins, the ocean begins to close on itself.

Submarine Hydrothermal Vents ◦ These vents on the seafloor at or very near spreading ridges are where circulating water is heated and discharged into seawater. ◦ As the hot water circulates through the oceanic crust it becomes a metal-rich brine from which deposits of various minerals and metals are deposited.

Submarine hydrothermal vents

Seamounts and guyots Seamounts and guyots are common, widespread features of the sea floor. ◦ Seamounts are remnants of extinct oceanic volcanoes. Many rise more than a kilometer from the ocean floor. They form as the oceanic crust moves over a hot mantle plume( hot spot). ◦ Guyots have the same origin but have flat tops created when the ocean volcano sinks to sea level. Once the top is near sea level, erosion by waves flattens the top. SEAMOUNTGUYOT

Seamounts and Guyots

Aseismic Ridges ◦ Aseismic ridges consisting of seamounts and guyots extend in a perpendicular orientation from many spreading ridges.

Sediments on the Deep Seafloor What are the two main types of sediment found on the seafloor? Sediments of the deep sea consist largely of the ◦ carbonate and siliceous skeletons of microscopic organisms (ooze) ◦ pelagic clays which are derived from continents and oceanic islands.

Sediments on the Deep Seafloor