GEY 101 - Introductory Geology: Exploring Planet Earth

Mapping the ocean floor

Depth was originally measured by lowering weighted lines overboard
Echo sounder (also referred to as sonar)
- Invented in the 1920s
- Primary instrument for measuring depth
- Reflects sound from ocean floor
Multibeam sonar
Employs an array of sound sources and listening devices
Obtains a profile of a narrow strip of seafloor
Three major topographic units of the ocean floor
- Continental margins
- Deep-ocean basins
- Mid-ocean ridges

Continental margins

Passive continental margins
- Found along most coastal area that surround the Atlantic ocean
- Not associated with plate boundaries
- Experience little volcanism and few earthquakes
Features comprising a passive continental margin
- Continental shelf
  - Flooded extension of the continent
  - Varies greatly in width
  - Gently sloping
  - Contain important mineral deposits
  - Some areas are mantled by extensive glacial deposits
- Continental slope
  - Marks the seaward edge of the continental shelf
  - Relatively steep structure
  - Boundary between continental crust and oceanic crust
- Continental rise
  - Found in regions where trenches are absent
  - Continental slope merges into a more gradual incline – the continental rise
  - Thick accumulation of sediment
  - At the base of the continental slope turbidity currents deposit sediment that forms deep-sea fans
Active continental margins
- Continental slope descends abruptly into a deep-ocean trench
- Located primarily around the Pacific Ocean
- Accumulations of deformed sediment and scraps of ocean crust form accretionary wedges

Submarine canyons and turbidity currents

Submarine canyons
- Deep, steep-sided valleys cut into the continental slope
- Some are extensions of river valleys
- Most appear to have been eroded by turbidity currents
Turbidity currents
- Downslope movements of dense, sediment-laden water
- Deposits are called turbidites
- Turbidites are layered and exhibit graded bedding (decrease in sediment grain size from bottom to top)

Features of the deep-ocean basin

Deep-ocean trench
- Long, relatively narrow features
- Deepest parts of ocean
- Most are located in the Pacific Ocean
- Sites where moving lithospheric plates plunge into the mantle
- Associated with volcanic activity
Abyssal plains
- Likely the most level places on Earth
- Sites of thick accumulations of sediment
- Found in all oceans
Seamounts
- Isolated volcanic peaks
- Many form near oceanic ridges
- May emerge as an island
- May sink and form flat-topped seamounts called guyots

Coral reefs and atolls

Coral reefs
- Constructed primarily from skeletal remains and secretions of corals and certain algae
- Confined largely to the warm, clear waters of the Pacific and Indian Oceans
Atolls
- Coral islands – a continuous ring of coral reef surrounding a central lagoon
- Form on the flanks of a sinking volcanic island (hypothesis proposed by Charles Darwin)

Seafloor sediment

Ocean floor is mantled with sediment
Sources
- Turbidity currents
- Sediment that slowly settles to the bottom from above
Thickness varies
- Thickest in trenches – accumulations may exceed 9 kilometers
- Pacific Ocean – about 600 meters or less
- Atlantic Ocean – from 500 to 1000 meters thick
Mud is the most common sediment on the deep-ocean floor
Types of seafloor sediment
- Terrigenous sediment
  - Material weathered from continental rocks
  - Virtually every part of the ocean receives some
  - Fine particles remain suspended for a long time
  - Oxidation often produces red and brown colored sediments
- Biogenous sediment
  - Shells and skeletons of marine animals and plants
  - Most common are calcareous oozes produced from microscopic organisms that inhabit warm surface waters
  - Siliceous oozes composed of opaline skeletons of diatoms and radiolarians
- Hydrogenous sediment
  - Minerals that crystallize directly from seawater
  - Important deposits with economic potential include manganese nodules and sulfide deposits

Mid-ocean ridges

Characterized by
- An elevated position
- Extensive faulting
- Numerous volcanic structures that have developed on newly formed crust
Interconnected ridge system is the longest topographic feature on Earth’s surface
- Over 70,000 kilometers (43,000 miles) in length
- Twenty percent of Earth’s surface
- Winds through all major oceans
Along the axis of some segments are deep down faulted structures called rift valleys
Consist of layer upon layer of basaltic rocks that have been faulted and uplifted
Mid-Atlantic Ridge has been studied more thoroughly than any other ridge system

Seafloor spreading

Concept formulated in the early 1960s by Harry Hess
Seafloor spreading occurs along relatively narrow zones, called rift zones, located at the crests of ocean ridges
As plates move apart, magma wells up into the newly created fractures and generates new slivers of oceanic lithosphere
New lithosphere moves from the ridge crest in a conveyor-belt fashion
Newly created crust at the ridge is elevated because it is hot and therefore occupies more volume than the cooler rocks of the deep-ocean basin
Structure of the oceanic crust
- Three distinct layers
  - Upper layer – consisting of pillow lavas
  - Middle layer – numerous interconnected dikes called sheet dikes
  - Lower layer – gabbro, in a sequence of rocks called an ophiolite complex
- Magma that creates new ocean floor originates from partially melted peridotite in the asthenosphere