Mud volcanoes are formed as a result of the increase in tectonic activity, or pressure build-up under the Earth's surface. These elevations closely resemble their magma-erupting counterparts; the only difference being that the former erupt muck (ton and tons of it), instead of lava.
At first, they form bulges, which later on develop into cones. They are characterized by different kinds of features, such as domes, griffins, scoria cone, salse, and muck-shield.
Mud pool and vents, are their geothermal features. These pools (hot springs) appear, when the surrounding terrain is dry and there is not much water available. Apparently, the Yellow Stone National Park is a mud pool.
Mud volcanoes are mostly found in areas of high volcanic activity, or places with oil and natural gas depositions. Due to reaction with the emitted gases, the soil in the surrounding area becomes salty. This is the reason why you might find most of them in arid regions (without any vegetation).
The eruption was of such high intensity that the sludge covered an entire village including 18 schools, 20 factories,15 mosques, and a cemetery. It is still active and emits 125,000 cubic meters of sludge daily.
These volcanoes are common, in fact, a large number of them located on land and sea; but not much information is available about them. Research on the presence of them in the sea has just started.
Pakistan: In Baluchistan, an area of Pakistan, there are about 18 active volcanoes of this kind. They are mostly found in clusters. Even the world's highest and largest one is found here. Its altitude is 300 ft. There are two known groups of them in this country; one is Chandragup, while the other is Jabl-ul-Ghurab.
The Chandragup is also called Chandrakup, which means 'volcanoes of the moon'. These are mostly situated in the high terrain areas. They can also be seen as small islands in the Arabian Sea.
Romania: The Berca volcanoes are located in the Bazau county, in Romania. One special feature about them is that, the muck which erupts is cold and it comes from the Earth's continental crust layers. Unlike the other ones, these are easily accessible. The soil being salty, there is hardly any vegetation here.
Azerbaijan: Most of these volcanoes in the world are located in Azerbaijan, near the Caspian Sea. They are mostly situated around Alat, Gobustan, and Salyan. Here, there are a lot of small ones. The small cones out here emit cold muck, water, and gases, continually.
They symbolize the presence of oil and gas reservoirs, under the land or sea. On 25th October, 2005, a mud volcano erupted with great force;15 kilometers away from the capital city Baku. Flames shot high up in the sky and muck was deposited in the surrounding area. This is a very rare calamity and happens only once in a blue moon.
China: There are several volcanoes of this kind in the Xinjiang region of China. Scientists have recently discovered China's largest one in this region. There are two active ones and several inactive ones, in South Taiwan. They are typically characterized by flowing, muddy waters.
There is a small inactive one in the Luoshan village of Taiwan. People of this village have invented a new tofu, made from the soybeans grown in the volcanic region. This unique tofu has become very famous with the tourists.
Many volcanoes are found in other areas, such as Taman Peninsula and Krasnodar Krai district, in Russia; Alaska and Columbia, in the U.S; Apennines and Sicily, in Italy; and the Andaman Islands in India.