Magnificent Himalayas, embracing the high clouds are great wonders of nature. Mountains, oceans, forests, rivers, etc., are marvelous gifts the nature has blessed us with. The incomparable beauty of these mountains attracts not only human beings, but also many other life forms. Indians consider Himalayas as an ultimate place of worship, meditation, and tranquility. Hence it is often referred as Heaven in Indian epics and mythology. Himalayas have been home to many Rishis and Saints since early time of the civilization. It has been a place of tourist attraction in the modern world. An Expedition to Himalayan Mountains has been the life ambitions of many adventurers of the world. The world's greatest rivers such as Ganga, Brahmaputra, Indus, Tsangpo originate in this gigantic mountain belt and have been the lifeline of millions. Himalayas have been an inseparable part of not only the modern civilized world but also the part of much earlier time animal kingdom. Therefore, Himalayas have been the subject extensive research.

Himalayan mountain belt acts as a barrier to the moisture laden southwesterly atmospheric circulation. The unique phenomenon of Indian monsoon system therefore exists because of Himalayas. Studies have suggested that the Himalayas play a major role in regulating global carbon dioxide through the process known as silicate weathering. The Ganga-Brahmaputra river system delivers thousands of tons of dissolved and suspended silicates to the Bay of Bengal in turn enriching the oceans with one of the nutrients supporting silicoflagellate production. These primary producers in-turn consume atmospheric carbon dioxide during photosynthesis, thus drawing down in large quantities one of the major greenhouse gases. Therefore, it is believed that the Himalayan weathering contributed significantly to the cyclic coldest climates in the geological past.

No wonder that the geologists get attracted to find the origin of this great mountain having magnificent morphological features. The Himalayan rocks are nothing but the product of sediment deposited in the ancient ‘Sea of Tethys' – which was closed when the Indian Plate collided with the Tibetan Plate some 50 millions of years ago. Huge quantity of compressional forces exerted along the plate-boundaries during collision caused this great mountain belts to rise. The rocks then underwent high-degree metamorphism to produce new generation of igneous and metamorphic rocks that have risen to remarkable heights resulting in the Himalayas that we see today. It has been a matter of debate among the geologists for several years about the episodes of this mountain building. The obvious first question that came to their mind was whether the Himalayan Mountain building activity was gradual or episodic? The geological evidences in the form of ‘Thrust Boundaries' along which the orogenic movements have taken place and cooling ages of various minerals forming the different rocks suggested this as episodic phenomena. If episodic, then in how many phases the orogeny has taken place? Twice.. thrice.. or more? Researchers of yesteryears have answered many of such questions.

Every action has a reaction. As one notices with the rising landmass, the increased erosion is also evident. This geological process is to maintain the balance between the land and ocean. The job of the surface erosion agents is to reduce the rising land to the mean sea level, i.e., rapid the rise, intense the erosion is the geological mantra of our mother nature! The Ganga-Brahmaputra river system draining the Himalayas alone discharges over 1200 km³ of freshwater every year to the Bay of Bengal. While it discharges the water, the run-off brings eroded sediment load in the form of suspended material and dissolved constituents. It is estimated that over 12 million km³ of Himalayan derived sediment has been discharged in to the Bay of Bengal since the first collision giving birth to the world's largest Bengal Fan. Therefore, the Bengal Fan sediment became the source of information to understand the tectonic processes responsible for building Himalayas. The physical erosion delivered various minerals in the form of silicate sediment, while the chemical weathering delivered several dissolved constituents to the Bay of Bengal. Increased erosion (physical or chemical or both) during a specific period, forms the basis to estimate the period of uplifting of Himalayas. The chemical weathering of the Himalayas alone has an impact on the global ocean Strontium (Sr) inventory, because the Himalayan Rivers deliver over 50,000 tonnes of dissolved Sr to the oceans every year with a ⁸⁷Sr/⁸⁶Sr ratio of >0.73 as against the global river ratio of <0.71. Thus the Sr-isotopic reconstructions for the oceans (anywhere on the Earth) in the geological past formed the basis for assigning age for Himalayan orogeny. In other words, the periods of rapid rise in the global ocean Sr-isotopic ratios mark the increased chemical weathering rates in the Himalayas as a result of rapid uplift.

The chemical weathering studies based on the oceanic evolution of the Sr-isotopic ratios indicate that the Himalayan orogeny has taken place in three main phases: early Miocene, late Miocene and the Pleistocene. Whereas, the Himalayan derived sediment flux rates (physical weathering) calculated from the Ocean Drilling Program (ODP) sediment cores of the distal Bengal Fan (Leg 116) indicate that the orogeny took place in two episodes: middle to late Miocene and Pleistocene, thus giving rise to a controversy on the uplift history of the Himalayas, particularly during the early Miocene.

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The specimen contained two main components viz., the Fe-Mn hydroxide precipitated from the ambient seawater and silicate-detritus derived from the fine sediment suspension in the overlying water. The latter component is derived from the physical weathering (erosion) of land. The Fe-Mn crusts formed by hydrogenous process accrete at an extremely slow rate (few mm per million years). Further, along with theFe-Mn hydroxide accretion the silicate-detritus, which is present in the water column as suspended cloud also incorporates in to the specimen. That is, if the continental erosion was severe, then the silicate-detritus cloud would be denser. The time of densification of the suspended silicate cloud should result in increased flux of silicate-detritus in to the Fe-Mn crust accreted during such time. If the source of that silicate-detritus is identified, then its rate of accumulation (flux) should represent the rate of erosion of the source rocks. This innovative concept was adopted by the NIO-researchers to address the Himalayan uplift controversy. As the mountain uplifted, the surface erosion agents worked vigorously towards reducing the uplifted land to the mean sea level as a part of natural phenomenon. Additionally, the time-scale of changes in weathering regimes due to orogenic movements are in the order of millions of years closely comparable to the order of accretion rates of hydrogenous Fe-Mn crusts – the wonder proxy.

The scientists used isotopic methods to find out accretion rate of layers separated from the specimen (the bottom layer in contact with the substrate is the oldest and progressively becomes younger towards the surface of the specimen). The accretion rates obtained by cosmogenic ¹⁰Be and radiogenic ²³⁰Th dating-methods yielded uniform rate of 2.8 mm per million years. This accretion rate was used to translate the thickness of the Fe-Mn hydroxide layer of the specimen into time in the past (age). The silicate-detritus component from the Fe-Mn crust layers was extracted and quantified. The Strontium- and Neodymium isotopic characters of the silicate-detritus extracted from the crust layers indicated most of its origin from the Himalayas and Indonesia. Mixing models were prepared to unscramble the portions derived from each of those sources with time. After having completed this elaborate exercise of experiments, they found that the silicate-detritus in the specimen derived exclusively from the Himalayas has occurred in three distinct pulses: Early Miocene-, Late Miocene- and Pleistocene-pulse, in accordance with the uplift history based on the oceanic Sr-isotopic evolution, but in contrast with the distal Bengal Fan sediment flux based hypothesis of two-phase uplift.

The results were thus helped to couple both the physical erosion and chemical weathering during early Miocene, which was thought to have decoupled during that phase of uplift. Thus an important component that represents rapid-uplift (physical erosion), which was missing in the ODP-sediment records leading to the controversy was resolved. Now it is almost clear that both the physical erosion and the chemical weathering of the Himalayas were intense during the Early Miocene period, and hence the first major uplift of these mountains took place during that time.

What might have been the misguiding factor in the ODP sediment-flux results? The scientists feel that the location of the ODP Leg in the distal Bengal Fan is located in the zone of intense intraplate deformation. It is quite possible that the syn-deformation disturbances occurred during the major collision during the early Miocene might have obliterated the original flux-signals. Other evidences like, the cooling ages of several minerals in the High Himalayan granites and the Pb-isotopic signals in the Fe-Mn hydroxide fractions of the specimen also clearly indicate strong orogenic movements along the Main Central Thrust of the Himalayas during the early Miocene. Thus the first major phase of Himalayan Uplift during the early Miocene, which was not seen in the Bengal Fan sedimentary records was unraveled by this study.