Earth s Seafloors Ocean Basins and Continental Margins Introductory Oceanography Ray Rector - Instructor
OCEAN BASINS and CONTINENTAL PLATFORMS Key Concepts I. Earth s rocky surface covered by of two types of crust Dense, thin, low-standing oceanic crust Light, thick, high-standing continental crust II. Seafloor is divided into two topographic regions Shallow continental margins Deep-sea oceanic basins III. Continental margins and Deep Ocean basins are fundamentally different Composition Structure Age Tectonic origin IV. Ocean basins are rugged and have a wide variety of topographic features Mid-oceanic ridges and Transform fracture systems Abyssal Hills and Plains Oceanic islands, Seamounts, and Guyots Trenches and Island Arcs
Marine Provinces
Earth s Solid-Surface Topography
Topography of Earth s Ocean Basins
Topography of Pacific Ocean Basin
Elevation Relief Profile of Earth Surface 1. Sea level 2. Continental shelf 3. Continental slope 4. The deep ocean floor 5. Mean depth of ocean = 4 km below sea level 6. Mean altitude of land = 1 km above sea level 7. Mt. Everest = 8848m 8. Marianas Trench = 11022m
Seafloor-Shoreline Mapping Methods
Four Principle Methods of Mapping the Ocean Bottom 1. Ship-based Sonar 2. Satellite-based Radar 3. Ship-based Seismic Reflection 4. Submersible Survey
Means of Mapping the Ocean Bottom Sonar
Means of Mapping the Ocean Bottom Satellite Radar Use of space-based radar to reflect off ocean surface, giving a very precise sea surface profile; sea surface profile anomalies closely mimic the underlying seafloor profile
Means of Mapping the Ocean Bottom Seismic Reflection Use of underwater explosions to penetrate seafloor with seismic waves that reflect back, providing a subsurface image
Means of Mapping the Ocean Bottom Deep Sea Drilling Use of underwater drilling to penetrate seafloor and recover core samples of seafloor down to depths of over 3000 meters
Two Primary Types of Earth Crust Oceanic Continental
Earth s Continents and Seafloors 1) Two Different Types of Crust Continental Granitic rock Oceanic Basaltic rock 2) Continental Crust Lighter (2.7 g/ml) Thicker (30 km) High Standing (1 km elev.) 3) Oceanic Crust Denser (2.9 g/ml) Thinner (7 km) Low Standing (- 4 km elev.)
Cross Section: Oceanic Crust 1) Mafic-rich layers of rock made of olivine, pyroxene, calcium plagioclase and magnetite. 2) Formed at spreading centers 3) Ocean crust obducted onto continental edges called Ophiolite complexes 4) Worldwide, oceanic crust is fairly simple in structure and composition
Cross-Section Profile of an Ocean Basin Large-Scale Ocean Bottom Features Continental shelf, slope, and rise Abyssal plains and hills Mid-ocean ridge and rift valley Oceanic islands, seamounts, and guyots Ocean trench
Cross-Section of the North Atlantic Ocean Basin
AGE OF EARTH S OCEAN BASINS
AGE PROFILE OF NORTH AMERICA Tectonic Provinces of North America: (Ages are in Billions of Years)
Continental Margins of the World Submerged continental margins are shown in pale orange color
Two Types of Continental Margins Passive = Constructive = Atlantic Type Active = Destructive = Pacific Type Passive Margins Broad shelves Subdued coastline Little to no tectonic activity No Plate Boundary Active Margins Narrow shelves Rugged coastline Tectonically active Plate Boundary
Active versus Passive Margins Excellent Example: South America Active = West Coast Passive = East Coast
Continental Margin Features Major Features Continental Shelf Continental Slope Slope Break Continental Rise Submarine Canyons
Turbidity Sedimentation
Passive Continental Margin Example: Eastern Siberia -- Arctic Ocean Basin
Earth s Deep-Sea Basin Features Mid-Oceanic Ridge, Rises and Transform Fracture Systems Abyssal Hills and Plains Seamounts and Guyots Oceanic Islands and Plateaus Trenches and Island Arcs Profile: North Atlantic Ocean Basin
Deep Ridges and Rises
Mid-Ocean Ridge, Rises & Fracture Systems Key Points Mid-ocean ridge systems represent the most extensive chain of active mountains on Earth Active faulting and volcanism Sea bottom is covered by rugged bedrock of young pillow basalt Little to no pelagic sediment Site of active hydrothermal vents
Mid-Ocean Ridge and Fracture Systems Key Points Mid-ocean ridge systems represent the most extensive chain of active mountains on Earth Active faulting and volcanism Sea bottom is covered by rugged bedrock of young pillow basalt Little to no pelagic sediment Site of active hydrothermal vents
Mid-Ocean Ridge Hydrothermal Vents
Seamounts, Islands, and Tablemounts Excessive basaltic volcanism at spreading centers create seamounts and islands along mid-ocean ridge. Over time, seamounts and islands move off the elevated ridges and into deeper and deeper water as oceanic crust becomes older, cooler, denser Eroded flat-topped islands eventually sink below sea level to become tablemounts or guyots. Tops of guyots deeper with distance from ridge Seamounts, and guyots eventually become buried beneath thickening pelagic sediment pile to become part of the abyssal plains and hills
Abyssal Plains and Hills Key Points Thick pelagic sediment covers a rugged subsurface bedrock of basalt Abyssal plains are the flattest, most featureless provinces on Earth Abyssal hills are tops of seamounts sticking out Abyssal plains and hills cover the most extensive tracts of ocean seafloor Subsurface imaging of abyssal plains and hills from seismic reflection studies and deep sea drilling
Abyssal Plains and Hills Key Points Abyssal sediments are predominately clays and oozes Soft pelagic sediment is loaded with benthic organisms Benthic organisms crawl over and and burrow through the sediment = Bioturbation
PLATE TECTONICS and the SEAFLOOR Seafloor Spreading Subduction Transform Faulting Continental Rifting and Collision Hot Spots
Seafloor Ridge and Trench Map
Earth Processes That Create Seafloor Features Continental Margins 1) Features resulting from Continental Rifting Continental Shelf and Slope Deep-Sea Oceanic Basins 1) Features generated by Seafloor Spreading Mid-Ocean Ridges and Fracture Systems Oceanic Islands, Seamounts and Plateaus 2) Features generated by Subduction Trenches and Island Arcs Forearc Islands 3) Features resulting from Sedimentary processes Abyssal Plains and Hills Continental Rises Submarine Canyons
OCEAN BASINS and CONTINENTAL PLATFORMS Summary of Concepts I. Earth s rocky surface covered by of two types of crust Dense, thin, low-standing oceanic crust Light, thick, high-standing continental crust II. Seafloor is divided into two topographic regions Shallow continental margins Deep-sea oceanic basins III. Continental margins and Deep Ocean basins are fundamentally different Composition Structure Age Tectonic origin IV. Ocean basins are rugged and have a wide variety of topographic features Mid-oceanic ridges and Transform fracture systems Abyssal Hills and Plains Oceanic islands, Seamounts, and Guyots Trenches and Island Arcs
OCEAN BASINS and SEAFLOORS Discussion
Seafloor Sediments Shallow Underwater Carbonate Sand Dunes - Bahamas
Ocean Basins are Vast Sinks for Sediment
Two Major Types of Marine Sediment Depositional Environments Shallow Margin = Littoral and Neritic Deep Sea = Pelagic Littoral Pelagic Neritic
Seafloor Sediment Provinces Cross-Section Profile of an Ocean Basin Continental shoreline = Littoral Province Continental shelf = Neritic Province Continental slope to rise = Transitional All deep sea regions = Oceanic or Pelagic
Four Compositional Types of Seafloor Sediments 1. Terrigenous Sources: Erosion of land; volcanic eruptions; wind-blown dust Material: Gravels, Sands, Silts, and Clays 2. Biogenous Sources: Organic; accumulation of plant and animal hard parts Material: Calcareous and Siliceous Oozes 3. Hydrogenous Sources: Precipitation of minerals from solution Material: Carbonates, Metal Oxides and Sulfides 4. Cosmogenous Sources: Extraterrestrial dust and meteorites Material: Tektite particles, Glassy spheres, Silicate dust
Type and Locality of Marine Sediments
Marine Sediment Sampling Methods Bucket-Scooping Piston Coring Submersible Drilling
Marine Sediment Sampling Tools Camel Grab Drill Core Box Core Piston Core
Core Sample Processing and Analysis
Marine Sediment Sampling Locations
Terrigenous Sediments Sources: Erosion of land; volcanic eruptions; wind-blown dust; icebergs Material Comp: Quartz, Feldspar, Mica, Amphibole, Fe Oxide, & Clays Material Size: Cobbles, Gravels, Sands, Silts, and Clays Ash from volcanic eruptions Sediment from rivers Wind-blown material Sediment-filled glacial ice flows
Biogenous Sediments Sources: Carbonate reefs; Benthic shelled-animals; Plankton Material Comp: Calcium carbonate and Silica Material Size: Gravel, Sand, Silt, and Clay Coral and Sponge Reefs Benthic shelled-animals Microscopic Plankton
Type and Locality of Marine Sediments
Continental Margins of the World Seafloor that includes shoreline, continental shelf and slope Submerged continental margins are shown in pale orange color
Continental Margin Settings Seafloor that includes shoreline, continental shelf and slope
Shallow Marine Sediments Key Points 1) Shallow marine sediments that deposit along shorelines and offshore shelf are termed littoral and neritic 2) Coast and shelf sediments are of two types: Land-derived inorganic rock and mineral fragments of gravel, sand, silt, and clay Organic carbonate and silica materials of marine life skeletons from reefs and sea bottom habitats Carbonate, silica and phosphate seawater precipitates 3) Shelf sediments mostly arrive via rivers 4) Coastal sediments may reach deep waters via turbidity currents moving down submarine canyons
Deep Ocean Basins of the World Deep seafloor excluding the continental shelf and slope From continental rise to mid-ocean ridge
Key Points Deep Marine Sediments 1) Deep ocean sediments are termed pelagic 2) Pelagic sediments are predominately very fine-grained 3) Two types of pelagic sediments Inorganic clays from land Biogenic oozes from plankton 4) Two types of biogenic oozes Calcareous Siliceous 5) Abundant benthic organisms crawl over and burrow through the sediment = Bioturbation
Calcareous Ooze Sediments Accumulation of calcium carbonate hard parts from dead microscopic plankton Mainly consists of cocolithophores and foraminifera tests Calcite-shelled plankton abundant in warmer surface waters Accumulate above the Carbonate Compensation Depth (CCD) foraminifera cocolithophores
Distribution of Calcareous Ooze Sediments 1) Calcareous oozes principally deposit in relatively shallow, low- to mid-latitude regions of deep ocean 2) Concentrated on tops and flanks of mid ocean ridges
Silica Ooze Sediments Accumulation of silica hard parts from dead plankton Mainly consists of diatoms and radiolarian tests Abundant in deeper, cooler surface waters high latitude
Silica Ooze Sediments Accumulation of silica hard parts from dead plankton Mainly consists of diatoms and radiolarian tests Abundant in cooler surface waters high latitude
Comparing Silica and Carbonate Oozes
Types of Hydrogenous Sediment Sources: Precipitation of minerals from solution Material: Carbonates, Metal Oxides and Sulfides Black Smoker Chimneys Manganese Nodules
Types of Cosmogenous Sediment Sources: Extraterrestrial rock, dust,and debris Material: Silicates, glass, and metals Microtektites Tektite Strewn Fields
Type and Distribution of Marine Sediments
Percentage Distribution of Pelagic Sediments 1) Calcareous Oozes = covers 48% of deep seafloor 2) Abyssal Clays = covers 38% of deep seafloor 3) Siliceous Oozes = covers 15% of deep seafloor
Rates of Deposition of Marine Sediments
Total Sediment Thickness in Ocean
MARINE SEDIMENTS Discussion