Mid Oceanic Ridge is the longest and uninterrupted feature on Earth’s surface. But what is Mid Oceanic Ridge? How does it form? What are the forces responsible for its formation?
Brief about Plate boundaries
To understand Mid Oceanic Ridge, initially plate boundaries need to be understood. Plate boundaries are simply the boundaries between lithospheric plates. Broadly, there are three types of plate boundaries:
Divergent plate boundaries.
At Divergent Plate boundaries, the plate moves away from each other. It is also known as “Constructive plate boundary” because new lithosphere is developed.
Convergent plate boundaries.
At Convergent Plate boundaries, the plate moves towards each other. It is also known as the “Destructive Plate boundary” because here, the lithosphere is destroyed.
Transform faults are plate boundaries along which plates move laterally. Thus, it maintains the exposed area.
Concept of Isostasy
Nature has its own rules and such rules are very appropriate. Among them is the concept of isostasy. Basically, it is the balance maintained by nature.
The concept is linked with the response of the outer shell of Earth to the large stress generated on it. The stress may be due to the mountain. Thus, to compensate for the stress generated by mountains at one place, a basin is created by nature at another place. This is how nature maintains its balance.
What is Mid Oceanic Ridge?
As the name suggests, Mid Oceanic Ridges are the mountain ranges in the middle of the ocean basin. Mid Oceanic Ridges are linear, associated and the longest feature. It exceeds 70,000 km in length. It can be as wide as about 1000-4000 km and can be 3-4 km high from the ocean floor.
Mid oceanic ridges are associated with Divergent plate boundaries. At the Divergent plate boundary, formation of a new lithosphere takes place.
Where and how does the Mid Oceanic ridge forms?
Mid oceanic ridge forms at the boundaries of tectonic plates. Such ridges are formed due to the divergence of plates. It is also associated with the concept of seafloor spreading.
Seafloor spreading is the first concept to explain the kinematics of continental drift. According to this concept, the continents which were once connected to each other. The continents were separated from each other by ocean basin.
When lithospheric plates diverge, upwelling of mantle occurs. Such an upwelling is responsible for the formation of new lithosphere and ultimately seafloor spreading. It is also responsible for earthquakes. Jets of superheated seawater then solidifies creating a new lithosphere.
Some examples of Mid oceanic ridge
The most known example is mid atlantic ridge in the atlantic ocean. Mid atlantic ridge has rugged topography and has the presence of a rift valley. Rift valley gives the clue about spreading rates.
Another example is east pacific rise in the pacific ocean. East pacific rise has smoother topography and the rift valley is not present.
|The Reykjanes ridge, Iceland|
Some characteristic features of Mid oceanic ridge
Mid oceanic ridge is defined by volcanic eruptions. Such volcanic eruptions are also coupled by shallow focus earthquakes. Such shallow focus earthquakes are due to the layer known as Low velocity zone (which forms the base of lithosphere). The semi solid material from the low velocity zone rises up and causes earthquakes.
Tectonic extension and volcanic eruptions are the most common features of Mid oceanic ridge.
On the flanks of Mid oceanic ridges, certain fissures are also present.
Away from the axis of mid oceanic ridge, the oceanic crust cools down and becomes stable. It is known as a “plate boundary zone”.
Mid oceanic ridges are controlled by Normal faults.
Mid oceanic ridges are having the presence of transform faults. Such faults are responsible for the lateral movement of the plates. Transform faults are active between the offset ridge crest.
Rift valley and spreading rates
The presence of rift valley at the crest of Mid oceanic ridge gives idea about the spreading rates. Rift valley is a depression in the middle of Mid oceanic ridge. Also the morphology of Mid oceanic ridge gives ideas about the rift valley and spreading rates.
If the Mid oceanic ridge is having rugged topography as seen along mid atlantic ridge, there will be presence of rift valley. This rift valley will form when the activity of addition of magma is poor and so it indicates slow spreading. The spreading rate is less than 0.78 inches per year.
If the Mid oceanic ridge is having relatively smoother topography as seen along Galapogas spreading center, there will be presence of rift valley but it is not as prominent as in mid atlantic ridge. The spreading rate is in the range of 1.96 to 3.54 inches per year.
If the mid oceanic ridge is having smooth topography as seen along east pacific rise, there will be absence of rift valley. The absence of rift valley indicates faster spreading.
Forces responsible for formation of mid oceanic ridge
Two types of forces are responsible for the formation of Mid oceanic ridge. They are ridge push and slab pull. Ridge push is the force occurring at the divergent plate boundary and slab pull is the force occurring at the convergent plate boundary.
Features along mid oceanic ridge
The most common feature is the median rift valley. It is formed due to the phenomena of seafloor spreading.
Cracks like fissures and faults.
Certain volcanic features like seamounts and underwater lava flows are also present. Seamounts are tall , cone shaped volcanic features.
When hot basaltic lava spills on cold seawater, the margins of lava get chilled. This creates pillow lavas. Such pillow lava is solid from the upper side and is liquid from inside.
Hydrothermal Vents are the next important feature along the mid oceanic ridge. Such hydrothermal vents are like hot springs. The cold sea water seeps into the oceanic crusts through cracks and fissures.
Then such cold water approaches the hot underground magma chamber. The cold water then picks up the heat and dissolved solids from the magma and is reverted back to the ocean surface. The temperature of the water that comes out from the hydrothermal vents determines its appearance.
Warm water vents
Warm water vents have temperature below 30℃ and the water emitted from it is clear in colour.
White smokers have temperatures between 30℃ to 350℃. The colour of the emitted water is white as it contains white compounds such as barium sulphide.
Black smokers have temperatures above 350℃. The colour of emitted water is black due to the presence of metal sulphides.
Hydrothermal vents are also home for certain microorganisms such as some bacterias. These are the organisms that are capable of surviving in extreme conditions of temperature and without sunlight. Such bacteria are referred to as “Archean bacteria”. Such organisms are dependent for their survival on hydrogen sulphide released from hydrothermal vents.
Another important feature along the mid oceanic ridge are transform faults. Transform faults are the main contributors for giving a zigzag appearance to the mid oceanic ridge. Also fracture zones are significant contributors.
Oceanic islands are also an important feature of the mid oceanic ridge. For example Ascension island along mid atlantic ridge.
Difference between oceanic ridge and oceanic rise
The spreading rate highly influences the appearance of the mid oceanic ridge. Fast spreading generates broad and smoother morphology along the mid oceanic ridge. Fast spreading segments of the mid oceanic ridge generates large amounts of rocks.
During seafloor spreading, rocks move away from the spreading centers at a swift rate. Rocks are not capable of thermal contraction and subsidence. These gently sloping and fast spreading segments of mid oceanic ridge are known as “oceanic rise”. The most common example is east pacific rise.
Slow spreading generates rugged topography along the mid oceanic ridge. The central rift valley tends to be broader and more developed. The rocks are allowed to undergo thermal contraction and subsidence phenomena.
These steep sloping and slow spreading areas along the mid oceanic ridge are considered as “oceanic rise”. The best known illustration is mid atlantic ridge.
A current set of ultraslow spreading centers has been perceived. These were uncovered along southwest Indian and Arctic segments of the mid oceanic ridge. The spreading rate is about 0.8 inches per year.
It is characterised by a fully matured rift valley(which is broad and deep) and volcanoes occur at considerable interim. It might expose the mantle for a million years.
Records of magnetic fluctuations along mid oceanic ridge
As explained above, the mid oceanic ridge is associated with the concept of seafloor spreading. Mid oceanic ridge is also characterised by volcanic eruptions and magma spills. When magma spills out from the mid oceanic ridge and gets consolidated to form rock, the magnetic minerals like magnetite present in the magma also get consolidated.
However, the magnetic minerals are capable of orienting themselves along the existing magnetic field directions. Thus, magnetic minerals serves as a record for understanding the past magnetic field direction.
On both sides of the mid oceanic ridge, at equal distance rocks having the same orientation of magnetic minerals are found. Thus away from the axis of mid oceanic ridge, rocks with records of older magnetic field directions are obtained.
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