Abyssal Hills : An Overview
There are thousands of small hills projecting above the abyssal plains which are supposed to be of volcanic origin. They are known by different names depending upon their slope and size. Some of the popular names are volcanic hills and islands, seamounts, sea knolls, table mounts or guyots. Such volcanic hills are circular or elliptical, dome-shaped or elongated with extensive base. Usually, they project to a height of 60 – 100 m. above the base and are generally 0.1 km to 100 km wide. When these hills rise above the sea level, they appear as volcanic islands or simply islands.
Image Credit:- utdallas.edu
A brief description of some of the abyssal hills is given as under:-
1. Abyssal Hill or Seaknoll is the name given to volcanic hills of lower heights.
2. Seamounts are isolated steep submarine volcanic hills, usually 2 – 100 km wide and less than 1,000 m high above the seafloor. The sides of the seamounts have steep slopes of 20 -25 Degree. Sometimes, the seamounts rise abruptly emerging above sea level. As such they appear as isolated islands. The Hawai Islands, the North Pacific and the Azores in the North Atlantic are outstanding examples of such type of islands.
3. Guyots are flat-topped hills, generally of the volcanic origin. Also known as tablemounts, these small hills are formed by truncation of seamounts by wave action later or sinking beneath the oceanic surface either by subsidence of the sea floor or by rise in sea level due to the melting of glaciers in the postglacial period. Their plateau like tops eventually sank too deep for wave erosion to continue wearing them down. Within the tropics, guyots are usually capped by coral reefs.
The abyssal hills may be found in groups or in isolation. When they are found in groups they give rise to typical morphology known as abyssal hill province. They are one of the most common features found on the ocean floor. They are numerous on the floor of the Pacific, Atlantic and the Indian Ocean. It is estimated that they are nearly 10,000 abyssal hills in the Pacific Ocean alone.
Image Credit:- oceansjsu.com
According to the latest findings, most of the volcanic hills on the abyssal plains are the result of divergent plates. When the Lithospheric plates diverge in the middle of the oceans, there is seafloor spreading which leads to volcanic activity and formation of a large number of volcanic hills. Thus most of the guyots were formed near the spreading centers and transported outward and downward by seafloor spreading.