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Geology - The Ocean Floor

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    Study Guide Geology – The Ocean Floor 1. Con Ɵ nental Margins What Are Con Ɵ nental Margins? Continental margins are the areas where continents meet the oceans. Scientists classify them as active or passive based on whether plate tectonic activity is occurring. 1.1 Ac Ɵ ve Con Ɵ nental Margins An active continental margin is found where tectonic plates interact. Because of this movement, these margins are often linked to earthquakes and volcanoes . Active margins usually have: • A continental shelf close to land • A very steep continental slope • A deep ocean trench at the base of the slope • An uneven ocean floor , often with volcanic hills These features form because plates are colliding, sliding, or sinking beneath one another.

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    Study Guide 1.2 Passive Con Ɵ nental Margins A passive continental margin forms where there is no active plate movement . These margins are quieter and more stable. A passive margin includes: • A wide, shallow continental shelf • A gentle continental slope • A continental rise , made of accumulated sediments • A flat abyssal plain farther offshore Because tectonic activity is absent, these margins tend to be smoother and more predictable in shape.

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    Study Guide 1.3 Con Ɵ nental Shelves A continental shelf is a shallow, nearly flat area that extends outward from the edge of a continent beneath the ocean. Key features of continental shelves: • Near the coast, sediments are mostly sand • Farther offshore, sand gradually changes into fine - grained mud • Width varies from a few kilometers to more than 1,000 kilometers • Water depth increases from a few meters to about 200 meters Since shelves lie within the range of sea - level changes during ice ages, their sediments often record marine transgressions (sea level rising) and regressions (sea level falling). Continental shelves are made of sialic crust , which is rich in silicon and aluminum and is part of the continental crust. Even though they are important, continental shelves make up only about 8% of the ocean floor . 1.4 Con Ɵ nental Slopes Beyond the edge of the shelf lies the continental slope . This area marks the transition from shallow coastal waters to the deep ocean. Important characteristics: • Extends down to depths of about 15,000 feet • Has an average slope of 4 – 5 degrees • Covers a transition zone between continental and oceanic crust This transition zone can be complex and may include block faults and thrust faults , showing evidence of past tectonic movement.

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    Study Guide 1.5 Submarine Canyons Submarine canyons are deep, steep - sided valleys that cut through continental shelves and slopes. How they form: • Some canyon heads were carved by rivers when sea levels were lower during the Pleistocene ice ages • Others are shaped by ongoing sand movement from coastal currents Key features: • Often V - shaped • Can be more than 3 kilometers deep • Frequently end in abyssal fans , also called deep - sea fans Abyssal fans look like alluvial fans found on land. Strong bottom currents help shape and smooth these fans over time. Very rapid sand movement down steep slopes is called a sand fall . 1.6 Turbidity Currents Turbidity currents are powerful underwater flows of dense, sediment - filled water. They occur when:

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    Study Guide • Sand and mud on the continental slope are loosened by earthquakes or landslides • Sediments mix with water, creating a flow that is denser than seawater Important facts: • These currents can travel at speeds of up to 60 km per hour • They move downhill like underwater avalanches • The sediments they leave behind are called turbidites Although scientists have never directly observed large turbidity currents, strong evidence supports their existence. They are believed to be responsible for: • Breaking undersea telephone cables • Creating graded sediment layers • Transporting shallow - water fossils to deep ocean floors These clues show that turbidity currents play a major role in shaping the deep ocean environment. 2. Ocean Floor Sediments The ocean floor is covered by sediments , which are small particles that settle out of water over time. Scientists group sea floor sediments into three main types based on where they come from: • Terrigenous • Pelagic • Hydrogenous Each type forms in a different way and is found in different parts of the ocean. 2.1 Terrigenous Sediments: From the Land Terrigenous sediments come from the continents. They form when rocks on land break down through weathering and erosion. Key points: • Carried to the ocean by rivers, glaciers, wind, and gravity

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    Study Guide • Mostly deposited on the continental shelf , continental rise , and abyssal plain • Made up of sand, silt, and clay Once these sediments reach the ocean, strong currents , especially along the continental rise, help reshape and spread them out. This is why terrigenous sediments are most common near continents and less common far offshore. 2.2 Pelagic Sediments: From the Ocean Itself Pelagic sediments form far from land and settle slowly through the water column to the deep ocean floor. They are made of: • Tiny clay particles • Microscopic shells (skeletons) of marine organisms Some pelagic sediments are called oozes because they are thick and soft: • Calcareous ooze comes from organisms with calcium carbonate shells • Siliceous ooze comes from organisms with silica shells The clay particles (and sometimes volcanic ash) are often blown from land by wind and fall gently onto the ocean’s surface before sinking to the bottom. Pelagic sediments are least common on mid - ocean ridge crests because active volcanism there prevents thick sediment layers from building up. 2.3 Hydrogenous Sediments: From Seawater Hydrogenous sediments form directly from seawater , not from land or organisms. Important features: • Rich in minerals • Form when dissolved minerals precipitate out of seawater • Common examples include manganese nodules , which grow very slowly on the ocean floor

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    Study Guide These sediments usually form in deep ocean areas where sediment from land and organisms is scarce. 3. Ac Ɵ ve Con Ɵ nental Margins Active continental margins are places where tectonic plates are moving and interacting. A well - known example is along much of the Pacific coast . Because plates collide, slide past each other, or sink beneath one another in these regions, active margins are areas of intense geologic activity. Key Features of Ac Ɵ ve Margins Active continental margins are easy to recognize because they are associated with: • Frequent earthquakes • Volcanoes • Mountain belts Unlike passive margins, active margins do not have a continental rise or a broad abyssal plain. Instead, their structure is much steeper and more dramatic.

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