12 2.6 Convergent Plate Boundaries
Convergent boundaries, where two plates are moving toward each other, are of three types, depending on the type of crust present on either side of the boundary — oceanic or continental. The types are ocean-ocean, ocean-continent, and continent-continent.
At an ocean-ocean convergent boundary, one of the plates (oceanic crust and lithospheric mantle) is pushed, or subducted, under the other (Figure 2.6.1). Often it is the older and colder plate that is denser and subducts beneath the younger and warmer plate. There is commonly an ocean trench along the boundary as the crust bends downwards. The subducted lithosphere descends into the hot mantle at a relatively shallow angle close to the subduction zone, but at steeper angles farther down (up to about 45°). The significant volume of water within the subducting material is released as the subducting crust is heated. It mixes with the overlying mantle, and the addition of water to the hot mantle lowers the crust’s melting point and leads to the formation of magma (flux melting). The magma, which is lighter than the surrounding mantle material, rises through the mantle and the overlying oceanic crust to the ocean floor where it creates a chain of volcanic islands known as an island arc. A mature island arc develops into a chain of relatively large islands (such as Japan or Indonesia) as more and more volcanic material is extruded and sedimentary rocks accumulate around the islands. Earthquakes occur relatively deep below the seafloor, where the subducting crust moves against the overriding crust.
Examples of ocean-ocean convergent zones are subduction of the Pacific Plate south of Alaska (creating the Aleutian Islands) and under the Philippine Plate, where it creates the Marianas Trench, the deepest part of the ocean.
At an ocean-continent convergent boundary, the denser oceanic plate is pushed under the less dense continental plate in the same manner as at an ocean-ocean boundary. Sediment that has accumulated on the seafloor is thrust up into an accretionary wedge, and compression leads to thrusting within the continental plate (Figure 2.6.2). The magma produced adjacent to the subduction zone rises to the base of the continental crust and leads to partial melting of the crustal rock. The resulting magma ascends through the crust, producing a mountain chain with many volcanoes. As with an ocean-ocean boundary, the subducting crust can produce a deep trench running parallel to the coastline.
Examples of ocean-continent convergent boundaries are subduction of the Nazca Plate under South America (which has created the Andes Mountains and the Peru Trench) and subduction of the Juan de Fuca Plate under North America (creating the Cascade Range).
A continent-continent collision occurs when a continent or large island that has been moved along with subducting oceanic crust collides with another continent (Figure 2.6.3). The colliding continental material will not be subducted because it is too light (i.e., because it is composed largely of light continental rocks), but the root of the oceanic plate will eventually break off and sink into the mantle. There is tremendous deformation of the pre-existing continental rocks, forcing the material upwards and creating mountains.
Examples of continent-continent convergent boundaries are the collision of the India Plate with the Eurasian Plate, creating the Himalaya Mountains, and the collision of the African Plate with the Eurasian Plate, creating the series of ranges extending from the Alps in Europe to the Zagros Mountains in Iran.
In addition to fish, the ocean is also home to a variety of birds. These birds are referred to as seabirds because they spend a considerable amount of time around the sea. Many seabirds spend their entire lives in the ocean, only coming to land to reproduce. Some seabirds dive and swim below the surface, such as penguins, whose wings are modified into flippers, or cormorants, whose webbed feet act as paddles. Others stay at the surface, only hunting in the uppermost meter of the water (Keddy). Please go to this website to read more about seabirds: https://ca.audubon.org/what-s-seabird
Birds evolved from reptilian ancestors
Birds split from the main reptile branch about 150 million years ago with an intermediate form known as Archaeopteryx, which was about the size of a crow. They have reptilian characteristics, such as their feet and lower legs are covered with scales and terminate in claws, their reproductive physiology is basically reptilian (laying eggs), their feathers are believed to have been derived from scales, and have many other developmental and structural similarities with reptiles. Yet, unlike reptiles, birds are warm-blooded (endothermic) and maintain a constant internal body temperature.
Marine birds
Marine birds tend to be larger and stronger than their land counterparts. They have a lightweight, low-density body structure with hollow bones. Their wings are long, pointed and cupped underneath to support a large and muscular organism aloft for long periods of time.
They do not drink freshwater. Instead, they have salt glands over their eyes that remove the salt, thus permitting them to drink seawater, freeing them from dependence on freshwater from the land. The salty fluid is expelled through their bills.
The albatross
- This is one of the largest of the oceanic birds, with a wingspan of up to 12 feet
- They are the best gliders in the world – may remain aloft for months at a time, taking advantage of the west wind drift (winds that blow completely around Antarctica)
Pelicans
- This prehistoric-looking bird feeds by diving on its prey and entrapping it in its large gular pouch
- It has air sacs in its shoulders that absorb the impact with the water
Arctic Terns
- This bird feeds on small fish swimming near the surface that it catches by diving. It may even swim below the surface after them
- It lives at high latitudes and breeds on all far north rocky coasts
- However, it dislikes cold weather, so makes the longest migration of any animal, 15,000 miles each way
Penguins
- Penguins are found only in the southern hemisphere
- They are flightless birds that "fly" through the water after fish, squid and krill
- They slow their heart rate
- Have subcutaneous fat layers for insulation
- Have heavy plumage for insulation
- Their bodies are streamlined
- They have heavy feet for paddling and kicking against the water
- Their blood is shunted to the heart and brain and the extremities are deprived of blood, in order to slow heat loss
- These apply to most diving birds
Emperor Penguin: These are the largest diving birds in the world, up to 4 ft in height
- They're the best bird divers in the world, can routinely dive for 5-10 minutes (most bird dives last 30 seconds to a minute) and at best can stay down for 15-18 minutes, diving to over 1000 feet
- They eat small fish and krill – penguin populations have increased as a result of the killing off of the great whales – more krill is available as food for penguins
Penguin Adaptations
- They fluff their feathers to create trapped air in a dead space
- They have long feathers
- They rock back and forth on their heels on the ice to limit the area in contact with the ice and slow heat loss
- They can reduce blood flow to and heat loss from their wings and feet
- They exhibit "huddle" behavior – gathering in large groups and constantly changing position so that each gets a turn in the center, where it's warmest
The first paragraph, by Keddy (University of California, Davis), is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts. Download this book for free at https://geo.libretexts.org/Courses/Diablo_Valley_College/OCEAN-101%3A_Fundamentals_of_Oceanography_(Keddy)
The rest was written by Dr. Cristina Cardona.