Volcanoes

Both A and R are true and R is the correct explanation of A.

Explanation — A major volcanic belt is found along the Mid-Oceanic Ridge in the Atlantic Ocean. This is because volcanic eruption is closely related to sea floor spreading, where molten magma rises from beneath the Earth's crust, forming new ocean floor. This rising magma causes frequent volcanic activity along the ridge.

Both A and R are true and R is the correct explanation of A.

Explanation — Most of the times volcanic eruptions are disastrous causing complete destruction. The destruction caused by lava flow makes life difficult near volcanoes. It leads to loss of shelter, economic hardship, and damage to the environment. This destruction makes lava activity mostly unprofitable to man as people are forced to abandon their land and houses.

A volcano is a vent or opening in the crust of the Earth, connected by a conduit to an underlying magma chamber from which molten lava, volcanic gases and steam are ejected. It is usually in the form of cone or dome according to the material ejected.

The main causes of volcanism can be summarised as follows:

1. The temperature inside the earth's crust increases with increase in depth at the rate of 1°C per 32 metres. The high temperature and pressure inside the earth crust, which increases with increase in depth is responsible for instability. Molten magma under great pressure has the capacity to liquefy great volume of gases.
2. The surface water that percolates through the cracks and crevices, turns into steam due to hot rocks. The steam being very powerful, forces itself through the weak points of the earth crust along with molten material.
3. The plate tectonics or the movement and splitting of the major and minor plates of the earth cause cracks, crevices and fissures in the earth's crust through which the magma is poured out.

On the basis of their activity, volcanoes can be classified into three types:

1. Active volcanoes — They constantly eject lava, gases and ash. For example- Sabancaya in Peru.
2. Dormant volcanoes — They remain dormant but can suddenly erupt and again go dormant. For example- The Vesuvius volcano in Italy.
3. Extinct volcanoes — They are volcanoes where there has been no volcanic activity since long. Their crater usually developes into a lake. For example- For example- Arthur's seat at Edinburgh in Scotland.

The four major types of volcanic eruptions are:

1. Ash and cinder cones
2. Lava Dome
3. Lava Shield
4. Composite or Strato cone

Volcanicity is a process which involves the intrusion of magma in the Earth's crust or the extrusion of such molten material on the Earth's surface. It gives rise to volcanic eruptions.

The volcanic eruption ejects molten lava, volcanic gases(mostly sulphur dioxide), ashes, cinder, pumice, steam etc.

An abrupt circular depression formed by extrusion of volcanic material, by collapse or by impact of a meteorite is called crater. For example, sunset crater, Arizona.

The four major types of landforms associated with volcanoes are:

1. Volcanic Plateaus — The erupted lava spreads to form plateau. For example- Columbian plateau.
2. Volcanic Mountains — These mountains are formed from material ejected from the fissures of the Earth's crust. For Example- Mauna Loa in Hawaii.
3. Volcanic Plains — Sometimes lava forms a very fertile plain. For example- Plain in Java, Malwa region in Deccan plateau.
4. Caldera Lake — The crater forms large lakes that may serve as great source of perennial rivers. For example- Lake Nayos in South Africa.

Three beneficial effects of volcanoes are:

1. They lead to formation of geysers and springs. These are important from health and medical point of view.
2. The crater lakes serve as great source of perennial rivers. For example, Lake Nyos in South Africa.
3. Precious stones are found in volcanoes. For example, diamonds in South Africa.

Three adverse effects of volcanoes are:

1. They cause extreme destruction of forests, cities and lives in large area.
2. Volcanoes eject poisonous gases that cause pollution and health problems.
3. The volcanic area becomes inhabitable by man. The rain water is immediately percolated due to high porosity.

Volcanoes generally occur in the weaker zone of the earth crust and are characterized by young fold mountains. The following are the three belts of volcanic eruptions:

1. The Circum-Pacific Belt (Ring of Fire) — It extends through the Andes of South America, Central America, Mexico, the mountains of western U.S.A, the Aleutian Islands, Kamchatka, the Kurile Islands, Sakhalin, Japan, Philippines, Celebes, New guinea, the Solomon Islands, New Caladonia and New Zealand.
2. Mid-Atlantic— This belt includes the volcanoes of Mid-Atlantic ridge. It also includes the volcanoes of Lesser Antilles, Southern Antilles, Azores, St. Helena, etc.
3. Mid-Continental— This belt includes the Volcanoes of Alps Mountains, Mediterranean Sea (Stromboli, Vesuvius, Etna, etc.) and volcanoes of Aegean Sea, Mt. Ararat (Turkey), Elbrus, Hindukush and the Himalayas.

The temperature inside the Earth increases at the rate of 1° per 32 metres of descent. It results in very high temperature as well as extreme pressure deep inside the Earth. The molten lava inside the Earth, turns surface water (which percolates through the cracks and crevices to the interior) into steam. This steam and other compressed gases put pressure on the Earth's crust. The movement of Tectonic plates cause cracks, crevices and fissures in the Earth's crust through which the magma is poured out.

On the basis of intensity of eruption, volcanoes are of three types:

1. Active volcanoes — They constantly eject lava, gases and ash. For example- Sabancaya in Peru.
2. Dormant volcanoes — They remain dormant but can suddenly erupt and again go dormant. For example- The Vesuvius volcano in Italy.
3. Extinct volcanoes — They are volcanoes where there has been no volcanic activity since long. Their crater usually developes into a lake. For example- For example- Arthur's seat at Edinburgh in Scotland.

Volcanic eruptions generally occur in the weaker zone of the earth crust and are characterized by young fold mountains. For example - the Andes mountains are young mountains which are part of 'Ring of Fire'.

Sabancaya in Peru in South America is an example of an active volcano.

Vent & caldera

Lava Shields

Arthur’s Seat

Circum-Pacific belt

Sills

1. Iceland lies on the Mid-Oceanic Ridge, a region with high volcanic activity. Due to this, the heat beneath the Earth’s crust is intense. When groundwater seeps deep into the Earth, it gets heated by this underground heat and rises back to the surface as a hot spring.

2. People bathe in hot springs because the warm water is rich in minerals like sulphur, which are believed to have healing properties, especially for skin conditions and other health benefits.

1. Indonesia experiences frequent volcanic eruptions because it lies along the Pacific "Ring of Fire"—a zone of intense seismic and volcanic activity surrounding the Pacific Ocean. This region is formed by multiple tectonic plate boundaries, making it one of the most volcanically active areas in the world, with over 120 active volcanoes across the Indonesian archipelago.

2. When a volcanic eruption occurs under the sea, it can cause a sudden displacement of a large volume of water. This disturbance creates powerful sea waves that travel rapidly across the ocean, known as tsunamis. Thus, underwater volcanic activity can directly trigger a tsunami.

Labelled self-explanatory diagram of a volcano is given below:

It is a large, more or less circular depression or basin associated with a volcanic vent. Its diameter is many times greater than that of the included vents. Calderas are believed to result from subsidence or collapse and may or may not be related to explosive eruptions.

The Circum-Pacific belt of volcanoes is called as Ring of Fire. It is called so because the belt has numerous active volcanoes. It is the chain of volcanoes making up mountain belts and island arcs surrounding the pacific ocean basin.

A mobile silicate melt coming out at the time of volcanic eruption, which can contain suspended crystals and dissolved gases as well as liquid is known as magma. It is formed due to great pressure and temperature inside the Earth's crust. It contains a mixture of minerals, water vapour, sulphur and dissolved gases.

The magma that reaches the Earth's surface due to volcanic eruption is called lava. The molten lava solidifies as it reaches the surface of earth forming different structures.

Earthquakes are closely associated with volcanic belts because volcanic belts are usually situated around tectonic plates. Movement of tectonic plates results in earthquakes and also causes cracks, crevices and fissures in the Earth's crust through which the magma is poured out. Thus, an earthquake may trigger a volcano and vice versa.

Lava activity is profitable to man as:

1. Lava forms plains, which are very fertile. For example- plains in Java, Malwa region in Deccan plateau and parts of Brazil.
2. Lava activity also provides us precious stones, for example, diamonds in South Africa and nickel deposits of Sudbury in Canada.
3. Geysers and springs are formed due volcanic activity and are very useful from health point of view as they contain sulphur and other useful minerals.
4. The crater lakes are formed as result of volcanic eruption. They may serve as source of perennial rivers.
5. Volcanic spots become tourist spots.
6. Sometimes remnants of volcanoes after erosion help in preserving traces of old civilization.

The Circum-Pacific Belt is also called the 'Ring of Fire' because this belt consist of numerous active volcanoes which keep erupting.

Volcanos have constructive effects in favour of man because:

1. Lava from plains are very fertile. For example- plains in Java, Malwa region in Deccan plateau and parts of Brazil.
2. Lava activity also provides us precious stones.
3. Geyser and springs are formed that provide water with health benefits.
4. Crater lakes are formed which serve as source of water to perennial rivers. For example-Lake Nyos.
5. Sometimes remnants of volcanoes after erosion helps in preserving traces of old civilization.

Mt. Vesuvius in Italy is a dormant volcano because it first erupted in 79 AD. After that it remained dormant for over 1500 years and then suddenly erupted with great force in 1631 AD. Since then, it has erupted about 7 times with the last eruption seen in 1944.