Table of Contents
In this post we will study about Continent – Ocean Convergence. Understanding Continent – Ocean Convergence is important to understand the Formation of The Rockies, the Formation of the Andes and other similar fold mountain systems.
We have studied in See Floor Spreading how convectional currents in the mantle drive the lithospheric plates. Rising vertical limbs of the convection currents in the mantle create a divergent plate boundary and falling limbs create a convergent plate boundary.
In convergence there are sub-types namely:
- Collision of oceanic plates or ocean – ocean convergence. [Explained in the previous post]
- Collision of continental and oceanic plates or ocean – continent convergence [This post].
- Collision of continental plates or continent – continent convergence [Next Post].
- Collision of continent and arc or continent – arc convergence [Next Post].
In all types of convergence, denser plate subducts and the less denser plate is either up thrust or folded or both [up thrust and folded].
Continent – Ocean Convergence Or The Cordilleran Convergence
- Continent – Ocean Convergence is also called Cordilleran Convergence because this kind of convergence gives rise to extensive mountain systems. A cordillera is an extensive chain of mountains or mountain ranges. Some mountain chains in North America and South America are called cordilleras.
- Continent – Ocean Convergence is similar to ocean – ocean convergence. One important difference is that in continent – ocean convergence mountains are formed instead of islands.
- When oceanic and continental plates collide or converge, the oceanic plate (denser plate) subducts or plunges below the continental plate (less denser plate) forming a trench along the boundary. The trenches formed here are not as deep as those formed in ocean – ocean convergence.
- As the ocean floor crust (oceanic plate) loaded with sediments subducts into the softer asthenosphere, the rocks on the continental side in the subduction zone become metamorphosed under high pressure and temperature.
- After reaching a certain depth, plates melt. Magma (metamorphosed sediments and the melted part of the subducting plate) has lower density and is at high pressure. It rises upwards due to the buoyant force offered by surrounding denser medium. The magma flows out, sometimes violently to the surface.
- A continuous upward movement of magma creates constant volcanic eruptions at the surface of the continental plate along the margin.
- Such volcanic eruptions all along the boundary form a chain of volcanic mountains which are collectively called as continental arc.
[Arc: narrow chain of volcanic islands or mountains.
Island arc: A narrow chain of volcanic islands. Island arc is usually curved. The convex side will have a trench if it’s an oceanic arc. Japan, Philippines, Hawaii (hotspot island arc) etc. are oceanic arcs. They are formed due to ocean – ocean convergence.
Continental arc: A narrow chain of volcanic mountains on continents. Cascade range (parallel to Rockies), Western Chile range (parallel to Andes) etc. are examples of continental arcs. They are formed due to continent – ocean convergence]
- Continental margins are filled with thick geoclinal sediments brought by the rivers. As a result of convergence, the buoyant granite [geoclinal sediments] of the continental crust overrides (is placed above) the oceanic crust [continental crust in up thrust by the oceanic crust]. As a result the edge of the deformed continental margin is thrust above sea level.
- The advancing oceanic plate adds more compressive stress on the up thrust continental margin and leads to its folding creating a fold mountain system.
- In some cases, the advancing oceanic plate compresses the continental arc (orogenic belt) leading to its folding (Rockies and Andes).
[As the oceanic plate subducts, the sediments brought by it accumulates in the trench region. These accumulated sediments are called as accretionary wedge. The accretionary wedge is compressed into the continental margin leading to crustal shortening.
Convergence == Crustal Shortening
Divergence == Crustal Widening
Crustal Shortening at one place is compensated by Crustal Widening in some other place]
- With the formation of the orogenic belt (fold mountain belt), resistance builds up which effectively stops convergence. Thus, the subduction zone progresses seaward.
- With the culmination of compression, erosion continues to denude mountains. This results in isostatic adjustment which causes ultimate exposure of the roots of mountains.
- Examples are found in the Rockies, deformed in late Mesozoic and early Tertiary period, and the Andes, where the deformation begun in the Tertiary Period is still going on.
Formation of the Andes – Continent – Ocean Convergence
- The Andes are formed due to convergence between Nazca plate (oceanic plate) and the South American plate (continental plate). Peru – Chile trench is formed due to subduction of Nazca plate.
- Andes are a continental arc (narrow, continental volcanic chain) formed due to the volcanism above the subduction zone. The pressure offered by the accretionary wedge folded the volcanic mountain, raising the mountains significantly.
- The folding process in Andes is still continuing and the mountains are constantly rising.
- Volcanism is still active. Ojos del Salado active volcano on the Argentina – Chile border is the highest active volcano on earth at 6,893 m. (Olympus Mons on Mars is the highest volcano in the solar system. It is 26 – 27 km high)
- Mount Aconcagua (6,960 m, Argentina), the highest peak outside Himalayas and the highest peak in the western hemisphere is an extinct volcano.
Formation of the Rockies – Continent – Ocean Convergence
- The North American plate (continental plate) moved west wards while the Juan de Fuca
plate (minor oceanic plate) and the Pacific plate (major oceanic plate) moved eastwards. The convergence gave rise to a series of parallel mountain ranges.
- Unlike the Andes, the Rockies are formed at a distance from the continental margin due to the less steep subduction by the oceanic plates.
- Trenching is less conspicuous as the boundary is filled with accretionary wedge and there are a series of fault zones that makes the landforms a bit different from Andes.
Wadati – Benioff zone: Earthquakes along Convergent boundary
- A Wadati–Benioff zone is a zone of seismicity corresponding with the down-going slab in a subduction zone (the intensity of
earthquakes increases with depth of subduction).
- Differential motion along the zone produces numerous earthquakes, the foci of which may be as deep as about 670 kilometres.
- Wadati–Benioff zone earthquakes develop beneath volcanic island arcs and continental margins above active subduction zones.
- They can be produced by slip along the subduction thrust faultor slip on faults within the down going plate.
- Most disastrous earthquakes are deep seated ones or deep focus earthquakes. Such earthquakes are common around the subduction zone.
Chile, Japan, Himalayan belt see high intensity disastrous earthquakes due to the subduction process.
We will study about the formation of Himalayas in the next post.