2-1 the seafloor and ocean marginsquestions - Tagged

Chapter Overview

Topic: The Seafloor and Continental Margins

Focus: Mapping the ocean floor, features, characteristics of continental margins, and deep-ocean basins.

Mapping the Ocean Floor

Bathymetry

• Originally, ocean depth was measured through weighted lines lowered overboard, representing one of the earliest methods.

• Echo Sounder (Sonar)

  • Invented in the 1920s, it revolutionized underwater exploration.

  • It serves as the primary instrument for measuring ocean depths, functioning by emitting sound waves that travel through water. Once they strike the ocean floor, the reflected sound waves return to the surface, allowing scientists to gauge depth accurately.

• Multibeam Sonar

  • Utilizes an array of sound sources and listening devices and thus improves the accuracy of seafloor mapping.

  • It captures a profile of a broad and narrow strip of seafloor, providing detailed information about underwater terrain in real time.

• Satellite Measurements

  • Modern technology employs radar altimeters onboard satellites.

  • These satellites detect subtle variations in the ocean's surface caused by gravitational pull from underlying seafloor features, allowing for mapping of underwater structures from space.

Major Topographic Divisions of the North Atlantic Ocean

Features:

• Major continental masses such as North America and Africa significantly influence oceanic structures due to their geological makeup and tectonic activity.

Continental Margins

• Passive Continental Margins

  • These margins are predominantly found along the Atlantic Ocean coast, where the continental shelf extends without significant tectonic activity.

  • They are not associated with tectonic plate boundaries and are characterized by minimal volcanism and few earthquakes, making them relatively stable regions.

• Features of Continental Margins

  • Continental Shelf

    • Represent flooded extensions of continents that vary widely in width.

    • They host important mineral deposits, including oil and gas reserves, making them economically significant.

  • Continental Slope

    • This is the steeply inclined section that marks the seaward edge of the continental shelf, transitioning to the deep ocean.

    • It signifies the boundary between continental crust (composed mainly of granite) and oceanic crust (primarily composed of gabbro).

  • Continental Rise

    • A gradual incline that merges from the continental slope and is characterized by sediment accumulation.

    • Forms due to turbidity currents that deposit sediments, resulting in structures known as deep-sea fans that spread out across the ocean floor.

• Active Continental Margins

  • Found primarily around the Pacific Ocean, these margins exhibit geological instability.

  • They show features like the abrupt descent of the continental slope into deep-ocean trenches, significantly impacting marine ecosystems.

  • Accretionary wedges form in these areas from deformed sediments and remnants of oceanic crust, contributing to complex geological structures.

Features of Deep-Ocean Basins

• Deep-Ocean Trench

  • These trenches are the longest, narrowest, and deepest ocean features, primarily concentrated in the Pacific Ocean.

  • They are significant sites of lithospheric plate subduction where ocean floors are pushed back into the mantle, often associated with volcanic activity on land.

• Abyssal Plains

  • These are extensive, level areas of the ocean floor, found far from land and often characterized by thick sediment layers accumulating from marine snow and biological detritus over millions of years.

• Seamounts

  • Isolated volcanic peaks under the ocean that can either rise high enough to become islands or exist beneath the surface as flat-topped guyots.

  • Formed from basaltic lavas, they create extensive underwater structures, contributing to marine biodiversity and geological complexity.

Oceanic Ridge Anatomy

• Oceanic Ridges

  • Represent broad, linear swells forming along divergent plate boundaries, often characterized by high heat flow from geothermal activity.

  • Consist of numerous volcanic structures, forming the longest continuous topographical feature on Earth, which stretches over 70,000 kilometers in various oceanic environments.

Origin of Oceanic Lithosphere

• Seafloor Spreading

  • Formulated by Harry Hess in the early 1960s, this theory explains the development of new lithosphere along mid-ocean ridges.

  • As new material rises and solidifies, it occurs primarily at the rift zones, and this material moves outward, much like a conveyor belt.

Structure of the Oceanic Crust

• Composed of four distinct layers:

  1. Layer 1: Sedimentary layers, primarily consisting of marine sediments accumulated over time.

  2. Layer 2: Pillow lavas formed during volcanic activity, indicating the flow of lava under water.

  3. Layer 3: Interconnected dikes composed of basalt, providing insights into the volcanic history of the ocean floor.

  4. Layer 4: Gabbro, part of the ophiolite complex, representing the crystallized remains of magma beneath the ocean crust and contributing to our understanding of geological processes.

Hydrothermal Vents

• Often referred to as black smokers, these extraordinary ecosystems are located along ocean ridges.

• They exist in total darkness, relying on chemosynthesis rather than photosynthesis; seawater interacts with oceanic crust to dissolve and precipitate minerals, ultimately forming metal sulfide deposits which host unique biological communities.

Destruction of Oceanic Lithosphere

• Subducting plates play an essential role in the lifecycle of oceanic basins, indicating where destruction occurs along subduction zones as crust is returned to the mantle, leading to the formation of new geological features and rejuvenation of the surface.

Atoll Formation

• An atoll is defined as a circular coral reef that encloses a shallow lagoon and low-lying islands, often formed over volcanic seamounts as coral thrives around the rim, exemplifying the complex interaction between biological and geological processes in marine ecosystems.