Chapter 3: Marine Provinces

Chapter Overview

• The study of bathymetry determines ocean depths and ocean floor topography.
• Echo sounding and satellites are efficient bathymetric tools.
• Most ocean floor features are generated by plate tectonic processes.
• Different sea floor features exist in different oceanographic locations.

Bathymetry

• Measures the vertical distance from the ocean surface to mountains, valleys, plains, and other sea floor features

Measuring Bathymetry

• Soundings
  – Poseidonus made first sounding in 85 B.C.
  – Line with heavy weight
  – Sounding lines used for 2000 years
• Fathom
  – Unit of measure
  – 1.8 meters (6 feet)
• HMS Challenger
  – Made first systematic measurements in 1872
• Deep ocean floor has relief
  – Variations in sea floor depth
• Echo Soundings
  – Echo sounder or fathometer
  – Reflection of sound signals
  – German ship Meteor identified mid-Atlantic
  ridge in 1925
• Lacks detail
• May provide inaccurate view of sea floor

• Precision Depth Recorder (PDR)
  – 1950s
  – Focused high-frequency sound beam
  – First reliable sea floor maps produced
  – Helped confirm sea floor spreading

Modern Bathymetry Measuring

• Multibeam Echo
  Sounders
  – Multiple simultaneous sound frequencies
• Seabeam
  – First multibeam echo sounder
  – Map sea floor strips up to 60 km (37 mi) wide
• Sonar
  – Sound navigation and ranging acronym
• Side scan sonar
  – GLORIA (Geological Long-range Inclined Acoustical instrument)
  – Sea MARC (Sea Mapping and Remote Characterization)
• Can be towed behind ship to provide very detailed bathymetric strip map

GLORIA Side Scanning Sonar

Sea Floor Mapping from Space

• Uses satellite measurements
• Measures sea floor features based on gravitational bulges in sea surface
• Indirectly reveals bathymetry

• Satellite-derived cean surface gravity
• Reveals bathymetry where ships have not conducted research

Seismic Reflection Profiles

• Air guns
• Strong, low-frequency sounds
• Details ocean structure beneath sea floor

Hypsographic Curve

• Shows relationship between height of land and depth of ocean
• 70.8% of Earth covered by oceans
• Average ocean depth is 3729 meters
• Average land elevation is 840 meters
• Uneven distribution of areas of different depths/elevations
• Variations suggest plate tectonics at work

Ocean Provinces

• Three Major Provinces

1. Continental margins
   –Shallow-water areas close to shore
2. Deep-ocean basins
   – Deep-water areas farther from land
3. Mid-ocean ridge
   –Submarine mountain range

Continental Margins

• Passive
  – Not close to any plate boundary
  – No major tectonic activity
  – East coast of United States
• Active
  – Associated with convergent or transform plate boundaries
  – Much tectonic activity

Active Continental Margins

• Convergent Active Margin
  – Oceanic-continent convergent plate boundaries
  – Active continental volcanoes
  – Narrow shelf
  – Offshore trench
  – Western South America

• Transform Continental Margin
  – Less common
  – Transform plate boundaries
  – Linear islands, banks, and deep basins close
  to shore
  – Coastal California along San Andreas Fault

Continental Margin Features

• Continental shelf
• Shelf break
• Continental slope
• Continental rise

Continental Shelf

• Flat zone from shore to shelf break
  – Shelf break is where marked increase in slope angle occurs.
• Geologically part of continent
• Average width is 70 km (43 miles) but can extend to 1500 km (930 miles)
• Average depth of shelf break is 135 meters (443 feet).
• Type of continental margin determines shelf features.
• Passive margins have wider shelves.
• California’s transform active margin has a continental borderland.

Continental Slope

• Where deep ocean basins begin
• Topography similar to land mountain ranges
• Greater slope than continental shelf
  – Averages 4° but varies from 1–25° gradient
• Marked by submarine canyons

Submarine Canyons

• Narrow, deep, V-shaped in profile
• Steep to overhanging walls
• Extend to base of continental slope, 3500 meters (11,500 feet) below sea level
• Carved by turbidity currents

Turbidity Currents

• Underwater avalanches mixed with rocks and other debris
• Sediment from continental shelf
• Moves under influence of gravity
• Sediments deposited at slope base

Continental Rise

• Transition between continental crust and oceanic crust
• Marked by turbidite deposits from turbidity currents
• Graded bedding in turbidite deposits
• Deposits generate deepsea fans, or submarine fans
• Distal ends of submarine fans become flat abyssal plains

Abyssal Plains

• Extend from base of continental rise
• Some of the deepest, flattest parts of Earth
• Suspension settling of very fine particles
• Sediments cover ocean crust irregularities
• Well-developed in Atlantic and Indian oceans

Abyssal Plain Volcanic Peaks

• Poke through sediment cover
• Below sea level:
  – Seamounts, tablemounts, or guyots at least
  1 km (0.6 mile) above sea floor
  – Abyssal hills or seaknolls are less
  than 1 km (0.6 mile) above sea floor
• Above sea level:
  – Volcanic islands

Ocean Trenches and Volcanic Arcs

• Convergent margins generate ocean
  trenches.
  – Deepest part of oceans
  – Most in Pacific Ocean
  – Deepest trench – Mariana Trench at
  11,022 meters (36,161 feet)

Island and Continental Arcs

• Volcanic arc on nonsubducted ocean plate
• Island arc
  – Islands in ocean
  – Japan
• Continental arc
  – Mountains on land
  – Andes Mountains

Pacific Ring of Fire

• Margins of Pacific Ocean
• Majority of world’s active volcanoes and
  earthquakes
• Marked by convergent boundaries

Mid-Ocean Ridge

• Longest mountain chain
• On average, 2.5 km (1.5 miles) above
  surrounding sea floor
• Volcanic
• Basaltic lava
• Divergent plate boundary

Mid-Ocean Ridge Features

• Rift Valley
  – Downdropped area on crest of ridge
  – Marked by fissures and faults
  – Small earthquakes
• Seamounts – tall volcanoes
• Pillow lava or pillow basalt – shapes formed when hot basaltic lava quickly cools
• Hydrothermal Vents
• Sea floor hot springs
• Foster unusual deep-ocean ecosystems able to survive without sunlight

Hydrothermal Vents

• Warm water vents – temperatures below 30°C (86°F)
• White smokers – temperatures from 30–350°C (86–662°F)
• Black smokers – temperatures above 350°C (662°F)

Fracture Zones and Transform Faults

• Transform faults along mid-ocean ridge offset spreading zones.
  – Linear ridge on spherical Earth
  – Seismically active
• Fracture zones along Pacific Ocean midocean rise
  – Seismically inactive
  – Occur beyond offset fragments of rise

Comparison Between Transform Faults and Fracture Zones

Fracture Zones and Transform Faults

Oceanic Islands

• Volcanic activity
• Hotspots
• Island arcs
• Islands that are part of continents