Assam extends from 89° 42' E to 96° E longitude and 24° 8' N to 28° 2' N latitude, it has an area of 78,438 km , similar to that
of Ireland or Austria.

Geologic origin of Assam

• According to the plate tectonics, Assam is in the eastern most projection of the Indian Plate, where it thrusts underneath the Eurasian Plate creating a subduction zone.
• It is postulated that due to the northeasterly movement of the Indian plate, the sedimentary layers of an ancient geocyncline called the Tethys (in between Indian and Eurasian Plates) were pushed upward to form the Himalayas.
• It is estimated that the height of the Himalayas is increasing by 4 cm each year.
• Therefore, Assam possesses a special geomorphic environment, with large plains and dissected hills of the South Indian Plateau system abutting the Himalayas to the north, north-east and east.
• Studies also conclude that the Brahmaputra is a paleo-river, older than the Himalayas, which often crosses higher altitudes in the Himalayas eroding at a greater pace than the increase in the height of the mountain range to sustain its flow.
• The height of the surrounding regions still increasing forming steep gorges in Arunachal Pradesh.

Physiography

• The Brahmaputra becomes a braided river in Assam, and, along with its tributaries, creates the flood plain of the Brahmaputra Valley.
• The Brahmaputra Valley in Assam is approximately 80 to 100 km wide and almost 1000 km long.
• The width of the river itself is as much as 16 km at some places in the state.
• The hills of Karbi Anglong and Dima Hasao district and those in and around Guwahati and North Guwahati (along with the Khasi and Garo Hills) are originally parts of the South Indian Plateau system.
• These are eroded and dissected by the numerous rivers in the region.
• Average height of these hills in Assam varies from 300 to 400m.
• The southern Barak Valley is separated by the Karbi Anglong and North Cachar Hills from the Brahmaputra Valley in Assam.
• The Barak originates from the Barail Range in the border areas of Assam, Nagaland and Manipur and flowing through the district of Cachar, it confluences with the Brahmaputra in Bangladesh.
• Barak Valley in Assam is a small valley with an average width and length of approximately 40 to 50 km.

Mineral resources

• Assam is endowed with petroleum, natural gas, coal, limestone and many other minor minerals such as magnetic quartzite, kaolin, sillimanites, clay and feldspar.
• A small quantity of iron ore is also available in western parts of Assam.
• The Upper Assam districts are major reserves of oil and gas. Petroleum was discovered in Assam in 1889.
• It is estimated that Assam and surrounding region possess around 150 million tonnes of petroleum reserves.
• Presently, Assam is the 3rd largest producer of petroleum (crude) and natural gas in the country accounting for 16% and 8% respectively of the total production of this
  mineral in the country.
• A Tertiary coal belt is located in Tinsukia, Dibrugarh, Sivasagar, Karbi Anglong and Dima Hasao districts with an estimated reserve of 370 million tonnes.
• Assam coal is friable in nature and has a high sulphur content. It is mainly utilised by local railways, steamers, and hydro power stations.
• Low moisture, low volatile cooking coal has been discovered in the Hallidayganj Singrimari area.
• Assam has rich limestone reserves. The major reserves of limestone are in Dima Hasao and Karbi Anglong districts.
• A total reserve of 97 million tonnes of limestone has been found in these two districts and almost half of the reserve is of cement grade.
• Moreover, there is an estimated reserve of 365 million tonnes of limestone near Umrangshu in Dima Hasao district.
• Among the minor minerals, there are quartzite reserves in the Nagaon district, kaolin reserves in Karbi Anglong and the Lakhimpur districts and sillimanite-bearing rocks in Karbi Anglong district.

Climatic condition

• With the ‘Tropical Monsoon Rainforest Climate‘, Assam is a temperate region and experiences heavy rainfall and humidity.
• Winter lasts from late October to late February. The minimum temperature is 6 to 8 degrees Celsius.
• Nights and early mornings are foggy, and rain is scanty.
• Summer starts in mid May, accompanied by high humidity and rainfall.
• The maximum temperature is 35 to 38 degrees Celsius, but the frequent rain reduces this. The peak of the monsoons is during June.
• Thunderstorms known as Bordoicila are frequent during the afternoons.

Biodiversity

• There are a number of tropical rainforests in Assam, including the Dehing Patkai rainforest.
• Moreover, there are riverine grass lands, bamboo orchards and numerous wetland ecosystems. Many of these areas have been protected by developing national parks and reserved forests.
• The Kaziranga National Park and Manas National Park are the two World Heritage Sites in the region. The Kaziranga is the home for the rare Indian rhinocero s, while Manas is a tiger sanctuary .
• Apart from the rhinoceros and the tiger, the spotted deer or chital / futukihorina (Axis axis), the swamp deer or dolharina (Cervus duvauceli duvauceli), the clouded leopard (Neofelis nebulosa), the hoolock gibbon or holoubandor, pygmy hog or nol-gahori (Sus salvanis), the wild buffalo, the hispid hare, the golden langur (Chloropsis cochinchinensis), the golden cat, the giant civet, the binturong, the hog badger, the civet cat and the porcupine are all found in the state.
• Moreover, there are abundant numbers of Gangetic dolphins, mongooses, giant squirrels and pythons.
• Major birds in Assam are: the blue-throated barbet or hetuluka (Megalaima asiatica), the white-winged wood duck or deuhnah (Cairina scultulata), the Pallas’s fish eagle or kuruwa (Haliaeetus leucoryphus), the great pied hornbill or rajdhonesh (Buceros bicornis homrai), the Himalayan golden-backed three-toed woodpecker or barhoituka (Dinopium shorii shorii), and the migratory pelican.
• Assam is also known for orchids and also for valuable plant species and forest products.
• White-winged wood duck is the state bird of Assam.