Unit 6: The Sea Floor · Mid-Oceanic ridges & Hydrothermal Vents Warm-water vents are below...

Unit 6: The Sea Floor

Turn to Page 62 in Introduction to the World’s Oceans

• Study the bathymetric chart

• What features do you see?

• How do the Atlantic and Pacific Oceans differ?

• What do you think accounts for those differences?

Ocean Bathymetry: What do you see?

What is Bathymetry?

• Bathymetry is from the Greek: bathos = deep, and metry = measure

• is the study of ocean contours

• A bathymetric chart shows what the ocean floor would look like if all the water drained out

Bathymetry continued

• More is known about the surface of Venus, Mars, and the dark side of the moon than the ocean floor

• Only 5% of the oceans have been mapped with the same precision as the moon

• there are areas the size of Kansas where no ship soundings have been made and even well surveyed areas are based on widely separated survey tracks.

Early bathymetric studies were performed with a sounding line or lead line to study the depth of the ocean floor

• Sailors measured depth in fathoms (6 ft) by stretching the rope between two outstretched arms

Precision Depth Recorder (PDR) developed in the 1950s used a focused beam to

measure depth to 1 m in accuracy

Multi-beam echo sounders and side-scan sonar allows mapping of swaths up to 60 km

wide

Bottom Coverage & Data Density by Survey Method Leadline Single Beam Multibeam

1-2 K soundings per survey

500 - 750 K soundings per survey

400,000 1,000,000 K soundings per survey

Image courtesy of NOAA & UNH

National Geographic Video

http://video.nationalgeographic.com/video/player/news/environment-news/us-ocean-floor-mapping-vin.html

Satellite Altimetry measures differences in sea surface elevation A sea surface anomaly is caused by the different gravitational pull of ocean mounts and trenches

Satellite Bathymetry continued

Sea Floor Bathymetry as Measured by Satellite over 4.5 year period

NOAA Satellite Bathymetry animation

• http://www.youtube.com/watch?v=w1TZObXGUo4

• http://www.youtube.com/watch?v=JkeglSqQMyQ&feature=related

USS San Francisco

• In 2005 the nuclear attack sub USS San Francisco collided with an uncharted sea mount at flank (maximum) speed

• The sub was sailing at a depth of 500 ft. in the South Pacific about 350 south of Guam

• The sub was almost lost

• They repaired it by cutting the bow off the retiring USS Honolulu and installing it on the San Francisco.

Why Study Bathymetry?

Ocean-Floor Topography Varies with Location Hypsographic Curve

Continental Margins – the submerged outer edge of a continent Deep Ocean Basin – the deep seafloor beyond the continental margin Mid-Ocean Ridges – The submarine mountain range in the middle

What are the three classifications of ocean floor?

Deepest not in the middle, but off to either side of the

shallower mid-ocean ridge

Mid-Atlantic Ridge

Continental Margins

Passive margins Active margins

passive margin active margin

Continental Margins

Continental Shelf - shallow, submerged edge of the continent. Shelf break - abrupt transition from continental shelf to the continental slope. Continental Slope - transition between the continental shelf & the deep-ocean floor. Continental Rise - accumulated sediment found at the base of the continental slope.

Continental Shelves Are Seaward

Extensions of the Continents

The features of a passive continental margin:

(a) Vertical

exaggeration 50:1

(b) No vertical

exaggeration.

Continental Margins – Submarine

Canyons

California’s canyons

turbidity currents

Submarine Canyons Form at the Junction between Continental Shelf & Continental Slope

How do canyons form? river erosion during previous glaciation periods turbidity currents: rapid movement of a mass of sediment down a

slope ~ avalanche often caused by earthquakes

A mid-ocean ridge is a mountainous chain of young, basaltic rock at an active spreading center of an ocean.

Topology of Deep-Ocean Basins Differs from That of the Continental Margin

Hydrothermal Vents hydrothermal vents: spring of hot, mineral- and gas-rich seawater

near the spreading centers of mid-ocean ridges

In 1977, Robert Ballard & J. Grassle discovered new life forms in Alvin, at

3000 m near the Galápagos Islands

Hydrothermal vents are sites where superheated water containing dissolved minerals & gases escapes through fissures, or vents.

Mid-Oceanic ridges & Hydrothermal Vents

Warm-water vents are below 30⁰ C and emit clear water. White smokers emit water between 30 ⁰ and 350 ⁰. Water is white because of light-colored dissolved minerals. Black smokers are above 350⁰ C (662 ⁰F) and emit black water because of presence of dark metal sulfides. Question: Why don’t they emit steam instead of water?

Seamounts are volcanic projections from the ocean floor that do not rise above sea level. Flat-topped seamounts eroded by wave action are called guyots. Abyssal hills are small, extinct volcanoes or rock intrusions near the oceanic ridges.

Volcanic Seamounts & Guyots

Project Above the Seabed

Trenches are depressions in the ocean floor caused by the subduction of a converging ocean plate.

Deep-Ocean Basins

Island Arcs, chains of volcanic islands & seamounts, are usually found parallel to the edges of ocean trenches.

Deep-Ocean Basins

(left) As two oceanic plates converge, an island arc is formed by volcanic activity.

Trenches & Island Arcs Form

in Subduction Zones

Video Marianas Trench

http://www.youtube.com/watch?v=BYjyGfRp3F4

Coral atolls form when volcanic islands sink leaving behind the coral reef surrounding a lagoon where the volcano used to be.

Atolls

Fanning Island

The Abyss • abyssal plains: flat, featureless expanses of seafloor

• covered w/ sediment between continental margins &

oceanic ridges

• 3700-5500 m

• Why is the abyssal plain so flat? It is flatter than any

similar plain on land

Sea Floor Spreading Evidence

• In the 1950s,

scientists discovered

parallel patterns of

magnetic stripes on

the sea floor.

• This provides

evidence of seafloor

spreading and a

record of magnetic

pole reversals.

Global Distribution of Earthquakes

Global distribution of earthquakes also provides evidence for theory of plate tectonics and seafloor movement.

Age of the Seafloor

• Late 1960s deep-sea drilling began on

the Glomar Explorer

• Radiometric dating of ocean rocks to

determine seafloor age

• Oldest seafloor is about 200 million

years old compared to 3.5 billion years

old for oldest rocks on land.

Age of Seafloor

Symmetric pattern of age distribution - youngest at the ridges and oldest at the subduction zones

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Education, Inc.

Plate Tectonics Theory

• Lithosphere – tectonic plates that

float on ductile asthenosphere

• Large scale geologic features occur

at plate boundaries

Plate Tectonic Processes

Global Plate Boundaries

• Slab Pull Theory: Older dense plates at the subduction zones sink into the underlying asthenosphere providing most of the driving force for plate motion.

Driving Forces of Plate Motion

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Education, Inc.

Types of Plate Boundaries

Divergent Boundary Features

• Plates move apart at

the mid-ocean ridge

forming a rift valley

• New ocean floor is

created

• Shallow focus

earthquakes happen

here

Generation of a Divergent Boundary

Convergent Boundary Features • Plates move toward each other

• Oceanic crust destroyed

• Trenches and volcanic arcs are found here

• Deep focus earthquakes

Three Types of Convergent Boundaries

Oceanic – Continental Convergence

• Ocean plate is

subducted

• Continental arcs

generated

• Explosive andesitic

volcanic eruptions

Oceanic – Oceanic Convergence

• Oceanic crust is destroyed

• Friction of plates melts sediments and crust

• Molten magma erupts in form of island arcs

Continental –Continental Convergence

• No subduction

• Tall mountains

uplifted –

Himalayans

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Transform Boundary Features

• Oceanic Transform Fault – ocean floor only

• Continental Transform Fault – cuts across

continent

• All transform faults

occur between

mid-ocean ridge

segments.

Global Hot Spots • Scattered around the earth are

approximately 40 fixed areas of volcanic activity known as hot spots

• Hot spots remain stationary as the plates move and are useful for tracing plate movements

• As ocean plate moves over a hot spot, a chain of volcanos is formed.

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Education, Inc.

Global Hotspot Locations

Hawaiian Islands Hot Spot