Lecture 4: The Ocean Basins Flashcards
How much of the Earth’s surface is covered by Ocean Basins
Three-fifths
When did we start exploring Ocean basins and what boosted our exploration?
Explored since 1870s, boosted by massive increase in funding from 1950s onwards (triggered by the cold war)
What did people previously believe ocean basins were?
‘Sunken Continents’
How do we use echo sounding to survey ocean basins?
Use a sound source to fire seismic waves down to the sub-bottom sedimentary layers
Measure time since deployed and picked up again
Use known travel time in water to calculate depth
How do we use depth sounding to survey ocean basins?
Lower a weight over the side of a ship, attached to piano wire.
They’d then measure how much wire had been unreeled by the time the weight hit the sea bed
What ways do we survey the ocean basins?
Depth sounding Echo sounding Seismic reflection profiling Dredging and dating of samples Drilling Submersibles Magnetic field measurements Gravity field measurements
What are the max depths in the ocean?
Ocean trenches - up to 11km deep (Mariana Trench)
How deep are large flat ocean basins?
4km deep
How long and high are mid-ocean ridges?
65km long and approx. 2km high (ocean only 2km deep)
What is the seafloor covered with, and what is underneath?
Sediment, but underneath is volcanic rocks
Where does current/recent underwater volcanic activity occur?
MORs and at the end of aseismic ridges
What are aseismic ridges?
chains of volcanoes on the sea floor (can be big enough to be above sea level)
What is bathymetry?
Underwater topography
Are mid-ocean ridges continuous?
No
What are mid-oceans ridges broken up with?
Fracture zones
What do fracture zones do?
Break up mid-ocean ridges and offset them
What angle are offsets always to mid-ocean ridge segments?
Perpendicular
What is the part of fracture zones between offset ridges?
a transform fault (plate boundary)
How thick is layer 1 of the oceanic crusts?
0-10km thick (average approx 0.4km)
What are the three main sources for layer 1 of oceanic crusts?
Terrigenous sediment
Pelagic clays
Pelagic oozes
What is terrigenous sediment?
Sediment transported from land to ocean by rivers
Where is terrigenous sediment thickest?
Near continents
What is pelagic clays?
Fine particles, carried in suspension
Where do pelagic clays settle?
only settle out in deep calm water
What are pelagic oozes?
Remains of microscopic organisms (plankton)
How thick can layer 1 of the oceanic crust be?
On old oceanic crust, sediment can be up to 10 kilometres thick (but squashed down by the pressure of its own weight)
How thick is layer 2 of the oceanic crusts?
1-2.5km thick
What is the structure of layer 2 of the oceanic crust like?
Fairly homogenous
Pillow lavas definition
blobs of lava that cooled rapidly upon contact with cold seawater
Sheeted dykes definition
vertical volcanic layers
What is layer 2 of the oceanic crust mostly made of?
Basalt
What is layer 3 of the oceanic crust mostly made of
Gabbro
How thick is layer 3 of the oceanic crust?
Approx 5km
What is the different between gabbro and basalt?
Same chemical composition, but gabbro cools slower and therefore has larger crystals
What is below layer 3 of the oceanic crust and what is it made of?
The upper mantle, made of peridotite
What is the total thickness of the oceanic crust?
5-8km
What depths can earthquakes occur at?
At a range of focal depths
What depth range is a shallow earthquake?
0-50km
What % of Earthquakes are considered shallow?
75%
What depth range is a intermediate earthquake?
50-300km
What % of Earthquakes are considered intermediate?
20%
What depth range is a deep earthquake?
300-700km
What % of Earthquakes are considered deep?
5%
Where can intermediate and deep earthquakes be found?
Most are around the edge of the Pacific
Where do shallow earthquakes occur?
basically happen all over the world (mostly on plate boundaries)
Where do all earthquakes occur (within the earth)
in the rigid outer layer of the Earth – called the lithosphere
Why do all earthquakes occur in the lithosphere?
it is the only layer rigid enough to undergo brittle fracturing
Why do earthquakes not occur in the asthenosphere
It is close to melting point and so behaves plastically
What types of earthquakes occur in mid-ocean ridges?
Earthquakes here are normally shallow
What are mid-ocean ridge earthquakes normally formed by?
Extensional stress (normal faulting)
What is normal faulting
Where plates are being pulled apart
What types of earthquakes occur in transform faults?
Mostly shallow
What are transform fault earthquakes normally formed by?
Strike-slip faulting
What is strike-slip faulting
Blocks of lithosphere are sliding past each other horizontally, creating friction and earthquakes
What % of earthquakes occur in oceanic trenches?
95%
How do focal depths range in oceanic trenches?
From shallow to deep
What stresses can be in oceanic trenches?
can be extensional (normal faulting) or compressional (thrust faulting)
What is the Benioff zone?
the area where an oceanic lithosphere subducts into the mantle
What does a plot of earthquake foci around a subduction zone reveal?
reveals a dipping plane called a Benioff zone
How can the dip of a Benioff zone vary (and average)
Dip can vary between 15 and 75 degrees (average is 45)
What does the Benioff zone show?
the lithosphere remains rigid as it subducts, but only down to a depth of 700km
What happens to the lithosphere beneath 700km?
it is probably to hot and plastic to fracture
Why can’t we date dredged basalts easily?
They’ve been altered by water
Why can’t we use deep sea drilling for large scale detailed maps?
It is expensive and difficult
What magnetic material does basalts and gabbro’s contain?
Magnetite
What happens to basalt and gabbro when molten?
It is ferrimagnetic (above the Curie Point, 500 degrees), therefore groups of atoms can align themselves with any external magnetic field
When the rock is molten, the atoms align themselves with the Earth’s magnetic field
When it cools down the atoms are locked in place
What is the alignment of basalt and gabbro’s atoms with the earth’s magnetic field called?
Thermoremanent magnetization (TRM)
What did sampling of Basalts and Gabbro’s show?
different directions of atoms
This is due to reversals in the earths field
What is the reversals in the direction of atoms in Basalts and Gabbro’s called?
Magnetic straigraphy
What can we use magnetic stratigraphy
Divide the earth’s history into ‘normal’ and reverse periods
What is one period of time between magnetic reversals called?
A chron
How many chrons are there?
4
What are Oceanic Magnetic Anomalies parallel to and symmetrical with?
MORs
What do the pattern of reversals match?
Patterns already established on the continents
The fact that the pattern of reversals matches the pattern on land allows us to do what?
Calculate the time period of oceanic rocks
How do we calculate the date of underground rocks using magnetic alignment on land?
The symmetrical pattern of stripes is time-calibrated so ocean floor anywhere can be dated simply by counting up the stripes
Ocean basin definition
places where there is oceanic crust beneath the sea
How did we originally measure sea depth?
We lowered a weight over the side of a ship, attached to piano wire. They’d then measure how much wire had been unreeled by the time the weight hit the sea bed
What was the majority of the rock dredged up off the sea bed?
Volcanic rock - basalt
What did drilling of the seabed reveal?
Volcanic rock is everywhere on the ocean bed, but is just overlaid by lots of sediment in some places
What is the section where the seabed starts rising on a passive continental margin called?
the continental rise
What is the steepest part of a passive continental shelf called?
the continental slope
What is the shallow section of a passive continental boundary closest to land called?
The continental shelf
Does a passive continental margin have a trench?
No
What is a typical gradient for a continental slope?
1m up for every 14m across
Why do Mid-Ocean ridges break up?
because the plates are moving across the surface of a sphere
What is the absolute limit of earthquake depth?
700km
What is the chemical composition of magnetite?
Fe3O4
