Download marine provinces, part a

A.3-09/06
MARINE PROVINCES, PART A
I. BATHYMETRY TECHNIQUES
(the measurement of ocean depths and the charting of the shape or topography of the ocean floor)
A. Soundings - Posidonius
1. Posidonius wished to answer the question “how deep was the ocean?”
2. He used a sounding line to create depth measurements
a.) fathom – standard unit of depth of ocean (6 feet)
b.) fathme = outstretched arms
B. Soundings - HMS Challenger
1. In 1872, the first systematic bathymetric measurements of the ocean made by
many different soundings at different points. (3.5 year mission)
2. Measurements found that the ocean floor had relief and wasn’t flat.
C. Echo Sounders (Early 1900’s)
1. It sends a sound (a ping) and echoes when it hits something. The time
measured is divided by two, and then time x velocity = distance is used to
compute depth.
a.) speed of sound in water – 1507 m/s
2. Meteor had used echo soundings to gather information about the ocean floor
(in 1925).
3. DISADVANTAGE: Echo sounders have a beam that stretches out as it goes
deep. And high points send ping back within the range (~4600 m).
D. Precision Depth Recorder (PDR)
1. It uses a high-frequency sound beam with a 1 m resolution, much more detail
than the echo sounder.
2. Maps were created from PDR’s and research vessel tracks, which confirmed
sea floor spreading.
E. Multibeam Echo Sounders
1. These use multiple sound beams at once to map features
a.) SeaBeam – the first multibeam echo sounder
2. Sound emitters are directed away from ship on both sides, with receivers on
the hull.
3. DISADVANTAGE: A research vessel must “mow the lawn.”
F. Side-Scan Sonar (SOund Navigation And Ranging)
1. These are towed behind a survey ship on a cable so that it “flies” just above the
ocean floor to create a “strip map”
a.) Sea MARC (SEA Mapping And Remote Characterization)
b.) GLORIA (Geological Long-Range Inclined Acoustical instrument)
2. DISADVANTAGE: A research vessel must “mow the lawn.”
A.3-09/06
G. Satellites
1. Satellites make judgments about the ocean floor by looking at the ocean
surface.
2. Gravitational pulls exist from different parts of the bottom of the ocean. High
seamounts create bulges, while deep trenches cause indents.
3. Once waves, tides, currents and atmospheric effects are taken out of the
equation, the only thing remaining reveals the bathymetry of the ocean floor.
H. Air Gun Explosions
1. The air-gun explodes underwater and emits low frequency sounds.
2. Some of these sounds can reach further into the ground than others.
3. When they reach as far as they can, they bounce back to a receiver that is
floating away from the boat. And then, (velocity x time = distance) is used to
gather depth measurements, but now with rock layers.
II. HYPSOGRAPHIC CURVE
A. PURPOSE
1. The purpose of the curve is to show the relationship between the height of the
and the depth of the oceans.
2. hypsos = height
B. FINDINGS
1. 70.8 % of the earth’s surface is covered by oceans.
2. 29.2 % of the earth’s surface is covered by land.
3. The average depth of the ocean is 3729 meters.
4. The average elevation of land is 840 meters.
5. Continental shelf and slope make up 21% of oceanic area.
6. Ocean basins - continental rise, abyssal plain and hills, mid-ocean ridge, and
trenches make up 79% of oceanic area.
7. Areas of the ocean at relatively great depths are much greater than the areas of
land at great elevations.
8. Because the HC plots area (Km^2) vs elevation or depth (Km)- volumes
(Km^3) are also revealed.
a.) The Mean Earth Sphere Depth depicts the level where the volume of
crust and ocean are equal (2440m below present sea level).
b) In effect, this process can be thought of as filling the ocean basins with
land and making the earth a sphere- the Earth’s solid surface.
c) Displacement of water leads to another reference point “Mean Ocean
Sphere Depth” – 2686m (2440m + 286m), which is depth of a layer of
water on an entirely spherical earth.