Exploring the Ocean Floor

• Topography

• The arrangement of physical features in an area

• Bathymetry

• the study of underwater ground characteristics
• measure ocean depth
• chart topography

• Bathymetric Techniques

• Sounding [Posidonius (85 BC)]

• Let out 2 km (1.25 mi) line with attached weight until touched

• Echo sounders [1900s]

• sound signal (ping) bounces off bottom, calculate dist

• Precision depth recorder (PDR) [WWII]

• high frequency sound beam

• Multibeam echo sounders [today]

• SeaBeam - able to scan area 60 km (37 mi) wide

• Satellite mapping from space [most recent]

• microwave beams, measure gravitation irregularities

• Early Measurements "Sounding"

• Involve a line with a weight on the end
• Knots are tied at different (premeasured) points
• Throw overboard and see at what knot the line stops
• Knotted line is called PLUMB LINE
• Posidonius (85 BC) used 2 km lines

• Sound waves (SONAR)

• SOund Navigation And Ranging
• Single Beam Sonar "Echo sounders"

• Uses echoes to determine ocean floor features
• Sound ‘pings’ off floor. The return time is recorded
• Must know how fast sound travels in water
• Use started in early 1900s

• Sound in Water

• Sound in water travels much faster than sound travels in air
• Avg speed in Air = 340 m/s
• Avg speed in Water = 1500 m/s (3355 miles/hr)
• Depth can be measured by using the following equation:

• d = (½) t * v

• d is sea depth
• t is time recorded
• v is avg sound speed (velocity) in water

• What's the deal with that ½?

• For sounding to work, the beam must travel down to the bottom, then return.
• That counts as 2 trips, down and up.
• Therefore, you need to divide the distance (depth) in half to compensate. You just want the "down" measurement.

• Practice Problem

• It took 10 seconds for a sound pulse to travel through the water and return to the sonar instrument.  Calculate the sea depth.
• d = ½ t * v
• d = ½ (10 seconds) (1500 meters/second)
• depth = 7,500 meters

• Increased precision and accuracy

• Precision Depth Recorder (PDR)

• high frequency sound beam
• provides better recording information
• can plot over a range of 400 fathoms (730 m) on a page about 50 cm wide
• use started in World War II

• Today's technology

• Multibeam Sonar
• Uses many ‘pings’ to create 3D images
• Can locate shipwrecks, downed planes, schools of fish

• Satellite mapping

• Use of microwave beams
• Seabed causes distortions to gravitational forces
• Differences measured against norms to determine topography

• Underwater landscapes

• Mirrored and magnified when compared to the continents.
• Largest underwater mountains are TALLER than on land
• Underwater plains are larger and flatter than those on land

• Height Comparison

• Mount Everest

• Highest altitude
• Altitude: 8,850 m (29,035 feet) tall

• Mauna Kea

• Tallest mountain
• Altitude: 4,205 meters (13,796 feet)
• But if measured from underwater, it's over 10,000 meters high (32,000+ feet)

• Challenger Deep

• Deepest place on Earth
• Depth: 10,994 meters (36,070 feet)

• Common Seafloor Features

• Continental Margin

• Region closest to land
• Has 3 parts
• Continental Shelf

• Underwater extension of the coastal plain
• Extends from shoreline to slope

• Continental Slope

• Steep slope leading from the edge of a continent down to the seafloor

• Continental Rise

• Hill of sediment at the bottom of the steep slope near the edges of continents

• Seamounts and Island Chains

• Cone Shaped undersea mountains of volcanic origin
• May occur in chains (ex: Hawaii)
• Often, active volcanic island at one end
• Guyot: seamount with a flat top

• Island Arcs

• Chains of volcanic active islands next to deep sea trenches
• Happens due to subduction

• subduction: one tectonic plate shifts beneath another

• How are island chains formed?

• “Hot spot” or “Volcanic Plume” stays in place while tectonic plate slides past it.

• Ring of Fire

• The Ring of Fire is a ring of volcanoes around the Pacific Ocean that result from subduction of oceanic plates beneath lighter continental plates.
• Most of the Earth's volcanoes are located around the Pacific Ring of Fire because that's the location of most of the Earth's subduction zones.

• Abyssal Plains

• Flat, featureless area making up a large part of the seafloor
• Made up of eroding earth, sediments, and benthic ooze

• Mid-Ocean Ridge

• Underwater mountain range where new seafloor is created
• Creates New Oceanic Crust (seafloor spreading)
• **largest is the Atlantic Mid-Ocean Ridge

• Sea Floor Spreading

• Magma rises, cools, and turns into rock.
• The new rock is pulled away by the two tectonic plates diverging (pulling apart)
• Newer rocks will be found close to the spreading zone. Older rocks will be further away.

• Trenches

• Deepest parts of the ocean
• Narrow, canyon like
• Deepest: Mariana’s Trench in Pacific Ocean: "Challenger Deep"
• **Earthquakes and volcanic activity associated with trenches**

1. Continental slope
2. Seamounts
3. Abyssal plain
4. Mid-Ocean ridge
5. Island arcs
6. Continental shelf
7. Trench
8. Continental rise