Formation of Himalaya
The process of formation of the Himalaya is explained by the Plate Tectonic Theory. According to this theory, there are different layers within the earth, the outer layer (lithosphere) is a rigid layer that is floating in the inner plastic layer (Asthenosphere). The lithosphere is divided into different blocks (Plates) which move relative past each other under the influence of the convection current due to temperature and density contrast within the asthenosphere. The Plates often collide with each other, move or break apart at different places on the earth. When they diverge from each other, they form depressions whereas when they collide with each other, they form mountains.
Nepal Himalaya
Located in the most central part of the Himalayas and covering about one-third of the specific land area, Nepal represents about one-third of the active Indo-Eurasia collision mountain belt. The Nepal Himalayas are the perfect place for those who want to study first-hand the active mountain-building processes on Earth. Numerous high snow ranges including the world's highest mount “Everest” located in Nepal make the Nepal Himalayas a paradise for trekkers and mountaineers that offer a unique topography with elevation ranging from 100 meters towards the south to 8848.86 meters towards the north.
The entire territory of Nepal is divided into five different geological regions from north to south: 1) Tethys Himalaya, 2) High Himalaya, 3) Lesser Himalaya, 4) Siwalik (Chure), and 5) Terai. These five geological regions of Nepal have different characteristics as they were formed by different processes at different times. These geological units have been piled atop one another by faults and thrusts at different periods, making the present geography in the Himalayas.
Tethys Himalaya
Tethys Himalaya consists of sedimentary rocks including Sandstone, Mudstone, Limestones. This unit houses Ammonites (around 200Ma), Crinoid (around 400 Ma) coral and gastropod shells that suggest the formation of these rocks at the bottom of sea. About 20Ma ago, Hot Higher Himalaya rocks started exhuming from beneath, pushing and folding rocks along the STDS and elevating the Tethys Himalaya to the present height.
Higher Himalaya
The Higher Himalayan rocks actually deposited as sediments of Precambrian earth over 1 billion years ago. Later during the formation of the Himalayas, the rocks were squeezed, melted and metamorphosed and became igneous and metamorphic rocks (granite, pegmatite, gneiss, schist, quartzite, etc.) due to high temperature and pressure. Minerals (biotite, garnet, kyanite, tourmaline, etc.) adjusted to high temperature and pressures are found in these rocks. About 20 million years ago, the MCT fault slid the High Himalayas over the Lower Himalayas. During the MCT activity, hot metamorphic and igneous rocks from about 30 km interior were brought up to the surface.
Lesser Himalaya
Within this geotectonic unit, metamorphic rocks (slate, phyllite, quartzite, marble, etc.) and sedimentary rocks (limestone, dolomite, sandstone, etc.) of moderate temperature and pressure are found. These rocks were formed from sediments accumulated in different places on land as well as sea during over 1.9 billion to about 50 million years ago. Due to heat and pressure of Himalaya Collision rocks were metamorphosed to moderate degree. Fossils such as Stromatolites (over 1 billion), Nummulite (about 45 million) found in this area confirm the depositional age range of the rocks. During, around 13 million years ago, the MBT thrusted the Lesser Himalaya over the Foreland Siwalik and the metamorphic and sedimentary rocks in the Lesser Himalaya from the north came to the surface carving the present topography.
Siwalik/Chure
From below the sea level (about 50 million years ago), to about 9 km above now, the Himalaya have been continuously rising. As a response to this rapid rise and climate interaction, high soil erosion, and numerous landslides occur in the Himalaya. As a result, the piles of sediments (sand, clay, gravel) accumulate in the Foreland Himalaya and formed the Siwalik rocks. Vertebrate Fossils such as early Ramapithecus, Stegodon, Crocodilia, etc., suggest the age of the Siwalik rocks from about 20 million years to 2.5 million years ago. The Main Frontal Fault (MFT), also known as the Himalayan Frontal Fault (HFT), defines the boundary between the Himalayan foothills and the Indo-Gangetic Plain. Along the MFT, all the Himalayan Mountains, including Siwalik/Chure, slid over the Terai plains towards the south. This process is still exhibited from time to time in earthquakes in the Himalayas.
Terai
About 10 million years ago, when the Himalayas were rising, the monsoons entered Asia and the process of soil erosion also accelerated. As a result, sediments (sand, gravel) started to pile up from high to low elevated regions. In this course, sediments accumulated in the Foreland and have been piling up along the banks of the Ganga and its tributaries and formed Terai.
Mineral Resources in different geological units
In Terai and Siwalik/Chure, there is the possibility of petroleum deposits and construction aggregates. Metallic minerals, industrial minerals, precious and semi-precious stones, and coals are expected in the Lesser Himalaya. Similarly, there is a possibility of precious and semi-precious stones, marble, and metallic minerals in the Higher Himalaya. The Tethys Himalaya has a wide potential of limestone, salt, gypsum, and natural gas.