Mid-Ocean Ridges Flashcards
What is the MAR?
Mid-Atlantic Ridge.
A submarine mid-ocean ridge.
One of the largest mountain ranges in the world.
How long is the Mid-Atlantic Ridge?
7000 km
What rock types are mid ocean ridges made up of?
Mafic and ultramafic igneous material, derived from the upper mantle.
Why is the Mid-Atlantic Ridge not entirely submarine?
Iceland is a hotspot
What type of mid-ocean ridge is the Mid-Atlantic Ridge?
Fast spreading ridge
How does magma form through the Mid-Atlantic Ridge?
When plates move apart, pressure is reduced, allowing the ultramafic peridotite to partially melt
How does magma move up through the Mid-Atlantic Ridge?
Magma accumulates in shallow magma chambers and move up through feeder sheeted dykes (dolerite) to the erupt on the sea floor as fissure lava flows, often forming basalt pillow lavas
How can we identify a pillow lava?
Usually basalt.
There will be a convex margin between the pillow lavas to show way-up.
Rounded usually (not circular)
What will ageing be like along mid-ocean ridges?
Along MORs, igneous rocks are new.
The age of the crust increases with distance from the MOR.
Why might we find an axial rift valley at a mid ocean ridge?
Forming at the centre of the MOR. This median rift is formed by the extension at the crust which is under tension
What are heat flow measurements like around mid ocean ridges? Why?
Heat flow measurements are high than average across MORs.
This is due to magma upwelling beneath, causing rocks to expand and may help increase the height of the ridge
What type of earthquakes typically happen at mid ocean ridges?
Shallow focus earthquakes.
Happens when magma is moving up (harmonic tremors)
Where do shallow focus earthquakes happen (not mid ocean ridges)?
Transform boundaries.
What is the significance of transform plate boundaries along mid ocean ridges?
Plates margins are straight, but the ridge isn’t. They move like a zigzag, with the vertical being divergent, and the horizontal being transform
What is an example of a transform fault?
The San Andreas fault
How much sediment can be seen around mid ocean ridges?
No sediment on the MOR.
Thickness increases with distance and deposition is symmetrical.
What organisms (dead) may be found in the sediments around mid ocean ridges?
Microscopic plankton, such as radiolaria or foraminifera
How can core samples taken along mid ocean ridges be dated?
Core samples of oozes and other fine grained sediments collected on ocean floor.
Can be dated using pelagic microfossils. The age is greatest, the further away
If you totalled the lengths of all mid ocean ridges, how long would it be?
60,000 km
What is the average depth of all mid ocean ridges?
2500 m
from 1800m in Iceland,
to -4000m in the Cayman trough
How often are transform faults found along mid ocean ridges?
every 50-500 km
Why are mid ocean ridges important?
They are the principle means of transferring of mass and energy from the upper mantle to the surface (by advection and partial melting)
Why can the interior of a mid ocean ridge not be directly sampled?
It is too hot and made of molten rock.
How can we take direct samples around the mid ocean ridges?
We can investigate the adjacent lithosphere by sampling the ocean floor and with Ocean Core Complexes (OCC).
Deep ocean submersibles take a range of measurements (temp, pressure, density, etc) and core samples
What two types of remote sampling can be used at mid ocean ridges?
Heat flow surveys, Gravity surveys
How can heat flow surveys be used at mid ocean ridges?
The heat is the highest directly above the axial rift (axial high).
Remote surveys can take temperatures from all around the rift
How can gravity surveys be used at mid ocean ridges?
Low density zone in the upper mantle below the MOR.
This shows a negative gravity anomaly (-ve Bouguer anomaly) due to expansion from more heating making rock less dense.
Decompression melting means that less dense material comes out.
How can we relate Bowen’s reaction series to mid ocean ridges?
The less dense material melts first from decompression melting.
Looks like a revere of Bowen’s reaction series
What are the conflicts with using survey data for mid ocean ridges?
Many remote surveys carried out, means that some give conflicting results due to the presence of water (sea).
Remote surveys includes electromagnetic, gravity, seismic, etc.
If we were to take a core sample around a mid ocean ridge, what would it look like?
Basalt pillow lavas,
Dolerite + dol sheet dykes
Gabbro
Peridotite
What is an ophiolite?
A piece of oceanic crust that have been obducted tectonically onto continental crust
What rock types make up ophiolite suites?
Basalt, dolerite, gabbro, peridotite (rare)
How do ophiolite suites from?
At a convergent boundary, when oceanic plate is subducting. Eventually two continental plates separated by a small amount of oceanic plate.
The convergence (pressure) of the continental plates causes the oceanic to be pushed upwards.
Becomes a collision boundary, oceanic plate in the middle of mountain ranges.
This is similar to the Wilson cycle
How can we determine the rate of sea floor spreading at mid ocean ridges?
Dating rocks, then measure their distance from the ridge. cm/y
Distance (cm) / Age of rocks
When calculating spreading rate, what must you remember to do?
Multiply by 2 as it is symmetrical
What determines the rate of spreading?
The amount of upwelling of partially melted material causing ridge push.
Subduction at other side - Slab pull.
Convection current may play a role
What does the rate of spreading determine?
How quickly continents move (continental drift).
How much energy is transferred from Earth’s interior and exterior.
How much new material (basalt) per year.
Ocean growth and reduction
What is an example of a slow spreading ridge?
The Mid atlantic ridge
How fast is the mid atlantic ridge spreading each year?
South atlantic = 4cm/yr
North atlantic = 2cm/yr
Slow spreading ridges have median rift valleys how big?
Up to 20km wide and 3km deep
If you were to draw a slow spreading ridge cross section, what would the overall shape be?
Like stepping stones, with the lowest being at the rift valley
The upwards ‘steps’ are normal faults
What do sonar images show us along the mid atlantic ridge?
Vast areas of ocean floors with rough abyssal hills on either side of the ridge
What is the magma chamber at slow spreading mid ocean ridges like?
Insufficient partial melting to maintain magma chamber, though there may be mush zones where small volumes of melt exist within softened magma
How common are eruptions along slow spreading mid ocean ridges?
Magma is short lived and discontinuous. Each eruption is a distinct event
What are fault scarps?
Steep slopes or cliffs.
What causes the magma chamber to reach the surface?
Feeder dykes
What is an example of a fast spreading mid ocean ridge?
The East Pacific rise between the Nazca and Pacific plate
How fast do fast spreading mid ocean ridges spread?
up to 16 cm per year
If you were to draw a fast spreading ridge cross section, what would the overall shape be?
Rounded (ish) with no particular pattern besides the symmetry
What does the axial trough of fast spreading mid ocean ridges look like?
High elevation, dome like topography resulting from many layers of lava
What is the rift valley like at fast spreading mid ocean ridges?
There isn’t one.
There is just a crack along the smooth crest
How do eruptions happen at fast spreading ridges?
Deep hot rock rises quickly and heat passes onto the plate.
Rock is poor thermal conductor so heat cannot escape quickly and lithosphere becomes hooter, weaker, and more ductile
What is the magma chamber of a fast spreading mid ocean ridges like?
Continuous
How can spreading at mid ocean ridges affect global sea levels?
Oceanic crust moves away from ridge, cools, sinks, becomes more dense. This increases the volume of ocean basin and decreases sea level.
However, fast ridges = large amounts of new basalt pillows - decrease volume - push sea levels up. This is because it causes displacement if water onto the continents. (this is more important)
Why is peridotite solid in the mantle?
Pressure
Why might some magma be cooled near the surface?
Water infiltration
How does decompression melting contribute to the formation of magmas at mid ocean ridges?
As pressure drops (spreading), molecules in rocks move more, so decrease the melting point.
What happens to mantle material as pressure drops? (not melting)
Adiabatic cooling.
experiences cooling effect due to volume increase
Once melting starts at mid ocean ridges, how much rock melts while rising?
About 3% per 10km
At mid ocean ridges, what does seismic tomography show about magma chambers?
Magma chambers are made of molten rock lying on a partly molten mush zone
Melted zone is only a few 10s metres thick and about a km wide
What is a liquidus?
Conditions at which rock completely melts
What is geotherm?
The temperature as a function of depth.
The geothermal gradient. Rate in change of temp with depth
What is solidus?
Conditions at which rock starts to melt
How do submarine fields of hot springs form?
Along fast spreading mid ocean ridges, hot rock rises from mantle. Few hundred metres wide
What happens to the water at the hot springs at mid ocean ridges?
Water becomes super heated. >400C. This is now called hydrothermal fluid. But the water does not boil due to hydrostatic pressure
What is basalt made up of?
Feldspar, augite, some olivine. Contains small amounts of accessory minerals (metallic elements).
What does the hydrothermal fluid do with the basalt?
The high temperatures in the water dissolve the metallic elements
What happens to the jets of hot solution at hydrothermal vents?
They react with the cold water
What type of mineral is found at hydrothermal vents?
Chalcophile minerals.
Sulfides of zinc, copper, and iron precipitate out. These make it look like smoke (hence black smoker)
Pyrite is one of the main minerals produced
What do the precipitated minerals accumulate to form?
Build chimneys and mounds of sulfide minerals (a few metres tall) around the vent. This happens with the help of chemosynthetic bacteria
Where are black smokers found?
In areas of active volcanism on the ocean floor.
Some vents are well away from activity but instead along fault lines on a slow spreading ridge
What metal traces can be found in black smokers, but not economically viable?
Platinum, gold, and silver
Why would we mine black smoker deposits?
Such large quantities of minerals are ejected onto the seafloor that hydrothermal vents have formed rich ore deposits.
What is an example of when hydrothermal minerals were mined?
Ophiolite suites in Cyprus.
How could black smokers become an economically viable source?
It blocks the vent.
Sea water percolates through the mound, precipitating sulfide and seawater minerals (e.g. anhydrite).
Increased temps dissolve anhydrite, leaving gaps in which more sulphides can precipitate, new chimney collapses. Over time, an economic deposit accumulates
What is metasomatism?
The change of rock due to addition or subtraction of chemical constitution.
Hydrothermal alteration of rocks.
What is serpentine?
It has snake like bands.
It comes from olivine
How is serpentine formed?
Water is incorporated into atomic structure of olivine.
Increase volume and reduce density
What is the process of metasomatism?
Chemicals of original minerals react with hot water and its dissolved chemicals to form new minerals which replace the original crystals
What is heat flow vs heat flux?
Heatflow - Total amount of energy transfer (watts)
Heat flux - Rate of energy transferred (watts/metre^2)
How does hot water from hydrothermal vents move upwards?
Spiral upwards by convection
How is the whole ocean floor involved in heat exchange?
Cool sea water percolates into crust along fractures and faults, cooling the crust as the water gets warmer
What temperatures do white smokers form?
Lower temps than black smokers/
<300C
What deposits are white smokers formed from?
barium, calcium, silicon
(white)
Where are white smokers found?
Fault lines on slow spreading MORs and OCCs
What is the zone of metasomatism?
Metal sulfides are precipitated below sea bed in this zone
Other than the deposits, what makes white smokers white?
Precipitation of silica, anhydrite, and calcite
