GC Leong Test : Volcanism And Earthquakes - 2

• Volcanoes are located in a fairly clearly-defined pattern worldwide, closely related to regions that have been intensely folded or faulted. 

• There are well over 500 active volcanoes and thousands of dormant and extinct ones. 

• They occur along with coastal mountain ranges, as off-shore islands and in the midst of oceans, but few in continents' interiors.

• The greatest concentration is probably that in the Circum-Pacific region, popularly termed the 'Pacific Ring of Fire', which has been estimated to include two-thirds of the world's volcanoes. 

• The chain of volcanoes extends for almost 2,000 miles from the Aleutian Islands into Kam Nchatka, Japan, the Philippines, and Indonesia (Java and Sumatra in particular), southwards into Solomon's Pacific islands, New Hebrides, Tonga and North Island, New Zealand. 

• On the other side of the Pacific, the chain continues from the Andes to Central America (particularly Guatemala, Costa Rica and Nicaragua), Mexico, and Alaska. 

• It is said that there are almost 100 active volcanoes in the Philippines, 40 in the Andes, 35 in Japan, and more than 70 in Indonesia.

• Volcanoes of the Mediterranean region are mainly associated with the Alpine folds, e.g. Vesuvius, Etna, Stromboli, Vulcano, and the Aegean islands.

• A few continue into Asia Minor (Mt. Ararat, Mt. Elbrus). The Himalayas have, surprisingly, no active volcano at all.

The phenomena are associated with the thermal or volcanic region in which the water below is being heated beyond the boiling point.

• Almost all the world's geysers are confined to three major areas: Iceland, the Rotorua district of North Island, New Zealand and Yellowstone Park of USA. 

• The world's best-known geyser is perhaps "Old Faithful" in Yellowstone National Park, Wyoming which erupts at regular intervals every 63 minutes on the average.

• Hot springs or thermal springs are more common and may be found in any part of the earth where water sinks deep enough beneath the surface to be heated by the interior forces. 

• The water rises to the surface without any explosion. 

• The water rises to the surface without any explosion. 

• Such springs contain dissolved minerals which may be of some medical value. 

• Iceland has thousands of hot springs. Some of them have been harnessed to heat houses, swimming pools and for other domestic purposes. Hot springs and geysers have become tourist attractions, e.g. in Japan and Hawaii.

The Distribution of Earthquakes: 

• The world's distribution of earthquakes coincides very closely with that of volcanoes. 

• Regions of greatest seismicity are Circum-Pacific areas, with the epicentres and the most frequent occurrences along the Pacific Ring of Fire'. 

• It is said that as many as 70 per cents of earthquakes occur in the Circum-Pacific belt. Another 20 per cent of earthquakes occur in the Mediterranean-Himalayan belt including Asia Minor, the Himalayas and parts of north-west China. 

• Elsewhere, the earth's crust is relatively stable and is less prone to earthquakes, though nowhere can be said to be immune to earth tremors.

• During an eruption material from the top of the mountain is blown off or collapses into the vent widening cone leading into a large crater. 

• Some volcanoes may have greatly enlarged depressions called calderas, which may be several miles across. 

• These result from violent eruptions accompanied by the subsidence of much of the volcano into the magma beneath. Water may collect in the crater of the caldera-forming crater or caldera lakes, e.g. Lake Toba in Sumatra.

Mt. Krakatau: The greatest volcanic explosion known to men is perhaps that of Mt. Krakatau in August 1883. Krakatau is a small volcanic island in the Sunda Straits, midway between Java and Sumatra.