Editing Himalayas (section)

==Geology==
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{{Main|Geology of the Himalayas}}
[[File:Indian plate movement towards Eurasia 71 Mya onward.jpg|thumb|right|The movement of the [[Indian plate]] toward the [[Eurasian plate]] starting 71 million years ago at the average speed of {{convert|5–15|cm|in}} per year, which closed the [[Tethys Ocean|Neo-Tethys Ocean]] above and opened the [[Indian Ocean]] below.]]
[[File:230 Ma plate tectonic reconstruction.png|thumb|right|[[Cimmeria (continent)|Cimmeria]], having rifted from Gondwana shown drifting towards Eurasia, closing the Paleo-Tethys Ocean above, opening the Neo-Tethys Ocean below, and carrying parts of what is today the [[Tibetan Plateau]]]]
[[File:Karakoram Kohistan Ladakh Gangdese Belt.jpg|thumb|right|The accretions of the [[Karakoram]], the Kohistan-Ladakh [[island arc]], and the [[Gangdese batholith|Gangdese belt]] to Eurasia preceded the final India-Eurasia collision. The stars mark the [[Syntaxis (geology)|syntaxis]]-causing obtrustions.]]
[[File:Indus Nanga Parbat Himalayas from Khyber Pakhtunkhwa.webm|right|thumb|The [[Indus River]] in the foreground and the [[Nanga Parbat]] peak, the western [[syntaxis|anchor]] of the Himalayas, far in the background, a little faint but towering well above the cloud layer{{efn|as seen from a plane approximately above the historic [[Sawal Dher]] village, in [[Khyber Pakhtunkhwa]], Pakistan}}]]
[[File:2 2 himal tecto units.png|thumb|right|The [[Indus-Yarlung suture zone]], shown in green, separates the Himalayas from the [[Transhimalaya|Tibet transhimalaya]]]]
[[File:Folded Himalayan Rock Layers at Kali Gandaki in Nepal 2014.jpg|thumb|right|Folded layers of Himalayan rock, exposed in a cliff about {{convert|3|km|mi}} northeast of [[Jomsom]], in the [[Kali Gandaki Gorge]] in Nepal]]

[[Tectonics]], the recurring physical changes that affect the arrangement of the Earth's crust, and [[plate tectonics]], the movement of large regions of the Earth's crust in the manner of planar rigid bodies, are key to understanding the formation of the Himalayas.{{sfn|Molnar|2015|p=116}} The Earth's crust rests directly on its [[mantle (geology)|mantle]]. [[Tectonic plates]], comprising the crust and the upper portions of their underlying mantle, are moved around by convection in the [[asthenosphere]]. The [[oceanic crust]], found beneath oceans, is, on average, 7 km thick. It is created from [[upwelling]] magma at [[mid-ocean ridges]] and predominantly consists of [[basalt]], the principal igneous rock on Earth. In contrast, the [[continental crust]] underlying dry land has an average thickness of 35 km and is rich in [[silica]], which is less dense than basalt.{{sfn|Johnson|Harley|2012|p=2}} It makes the continental tectonic plates more buoyant than the oceanic.{{sfn|Molnar|2015|p=116}} 

India's defining geologic processes, which began 70 million years ago, had involved India [[rift (geology)|rifting]], or splitting away, from [[Gondwana]], and the [[Indian plate|Indian continental plate]] along with the [[Tethys Sea|Neo-Tethys oceanic plate]] above it jointly moving northward.{{sfn|Molnar|2015|p=116}} As these eventually reached the [[Eurasian plate]], the less buoyant oceanic plate [[subduction|subducted]], or slid under Eurasia and was carried into the deeper asthenosphere. In contrast, the [[Indian plate|Indian continental plate]] was obstructed because of its thickness and buoyancy. The lateral compression generated by the obstruction caused the plate to be sheared horizontally. Its lower crust and mantle slid under, but one layer of the upper crust piled up in sheets (called [[nappe]]s) ahead of the subduction zone.{{sfn|Molnar|2015|p=117}} Geophysicist [[Peter Molnar (geophysicist)|Peter Molnar]] noted that most of the Himalayas are "slices of rock that once were the top part of India's crust."{{sfn|Molnar|2015|p=118}} This is the process of mountain building, or [[orogeny]], in the Himalayas. 

Before the orogeny, the Eurasian coastline had been similar to today's Central Andes.{{sfn|Molnar|2015|p=128}} Along such coastlines, the adjoining oceanic plate subducts and erupts as volcanoes. [[Magma]], which eventually crystallizes into granite, rises into the Earth's crust below the active volcanoes but not to the surface.{{sfn|Molnar|2015|p=128}}   When India's continental plate pushed against Eurasia, not only did a part of the upper crust fold in nappes, but another stiffer part began to push against (or drag) Eurasia's ancient volcanic mountains farther north.{{sfn|Molnar|2015|p=128}} As a result, the crust of this formerly coastal region shortened under compression and thickened to become what is today the [[Tibetan Plateau]].{{sfn|Molnar|2015|p=128}} [[Isostatic equilibrium]], or the balance between the gravitational force pulling down on the crust and the force of buoyancy pushing up from the mantle, gives the Tibetan Plateau its notable thickness and altitude.{{sfn|Molnar|2015|p=128}}

The Indian plate was not the only landmass that had rifted from [[Gondwana]] and drifted northward toward Eurasia.{{sfn|Frisch|Meschede|Blakey|2011|p=174}} Before the India-Eurasia collision in [[Selandian|Middle Paleocene]] (60 [[Million years ago|Mya]]) and subsequent Himalayan orogeny, two other landmasses, the [[Qiangtang terrane]] and [[Lhasa terrane]],{{efn|Terrane: "A far traveled crustal block accreted to a continent.  Due to its remote origin, the terrane shows a different geological evolution compared to adjacent parts of the continent."{{sfn|Frisch|Meschede|Blakey|2011|p=197}}}} had drifted up from Gondwana.{{sfn|Frisch|Meschede|Blakey|2011|p=174}} Qiangtang, a geological region in what is today northern Tibet, had done so in [[Late Triassic]] (237&ndash;201 Mya).{{sfn|Frisch|Meschede|Blakey|2011|p=174}}   The Lhasa terrane collided with the southern boundary of the Qiangtang in the [[Early Cretaceous]] (145&ndash;100 Mya).{{sfn|Frisch|Meschede|Blakey|2011|p=174}} The collision caused the lithospheric mantle of the Lhasa terrane to thicken and shorten, forming a barrier that later prevented the Indian lithosphere from fully subducting under Tibet and leading to further thickening of the Tibetan plateau. The [[suture (geology)|suture zones]], or remains of the [[subduction|subduction zone]] and the [[terrane]]s that are joined, are found in the Tibetan plateau.{{sfn|Frisch|Meschede|Blakey|2011|p=174}} The Qiantang and Lhasa terranes were part of the string of microcontinents [[Cimmeria (continent)|Cimmeria]], today constituting parts of [[Turkey]], [[Iran]], [[Pakistan]], [[China]], [[Myanmar]], [[Thailand]] and [[Malaysia]], which had rifted from Gondwana earlier, closing the [[Paleo-Tethys Ocean]] above them and opening the Neo-Tethys Ocean between them and Gondwana, eventually colliding with Eurasia, and creating the [[Cimmerian Orogeny]].{{sfn|Frisch|Meschede|Blakey|2011|p=172}}  

After the Lhasa terrane had adjoined Eurasia, an active continental margin opened along its southern flank, below which the Neo-Tethys oceanic plate had begun to subduct. [[Magmatic activity]] along this flank produced the [[Gangdese batholith]] in what is today the [[Transhimalaya|Tibetan trans-Himalaya]]. Another subduction zone opened to the west, in the ocean basin above the Kohistan-Ladakh [[island arc]]. This island arc&mdash;formed by one oceanic plate subducting beneath another, its magma rising and creating continental crust&mdash;drifted north, closed its ocean basin and collided with Eurasia.{{sfn|Frisch|Meschede|Blakey|2011|p=173}}

The collision of India with Eurasia closed the Neo-Tethys Ocean.{{sfn|Frisch|Meschede|Blakey|2011|p=172}} The suture zone (in this instance, the remnants of the Neo-Tethys subduction zone pinched between the two continental crusts), which marks India's welding to Eurasia, is called the [[Indus-Yarlung suture zone]].{{sfn|Frisch|Meschede|Blakey|2011|p=172}} It lies north of the Himalayas. The headwaters of the [[Indus River]] and the [[Yarlung Tsangpo]] (later in its course, the [[Brahmaputra]]) flow along this suture zone.{{sfn|Frisch|Meschede|Blakey|2011|p=172}} These two Eurasian rivers, whose courses were continually diverted by the rising Himalayas, define the western and eastern limits, respectively, of the Himalayan mountain range.{{sfn|Frisch|Meschede|Blakey|2011|p=172}}

During the India-Eurasia collision, two elongated protrusions located on either side of the northern border of the Indian continent generated areas of extreme deformation. A point where mountain ranges with different directions of extension, and thus formed by tectonic forces at varying angles, converge is called a [[syntaxis]] (''Greek'': convergence).{{sfn|Frisch|Meschede|Blakey|2011|p=174}} The two syntaxes, [[Nanga Parbat]] and [[Namche Barwa]], on the northwestern and northeastern corners of the Indian continent, respectively, are characterized by the quick upward movement of land or rocks that were once deeply buried and significantly altered by extreme heat and pressure.{{sfn|Frisch|Meschede|Blakey|2011|p=174}} Geologists have estimated the rate of uplift of these rocks to be {{convert|7|mm|in}} per year, or {{convert|7|km|mi}} per million years.{{sfn|Frisch|Meschede|Blakey|2011|p=174}} The protruding regions have some of the highest mountain peaks at {{convert|8125|m|ft}} and {{convert|7756|m|ft}}, respectively.{{sfn|Frisch|Meschede|Blakey|2011|p=174}} The regions also have the greatest [[terrain|topographical relief]] in the interior of a continent, approximately {{convert|7000|m|ft}} over a horizontal distance of {{convert|20–30|km|mi}}.{{sfn|Frisch|Meschede|Blakey|2011|p=174}} Nanga Parbat has a narrow, [[anticline]], or arch-shaped fold whose crest dips sharply to the north, perpendicular to the general direction along which the Himalayas extend.{{sfn|Frisch|Meschede|Blakey|2011|p=174}}   The Indus and Yarlung Tsangpo, which originally emptied into the New-Tethys, now bend around the Nanga Parbat and Namche Barwa, respectively, to eventually empty into the Indian Ocean.

Geologists Wolfgang Frisch, Martin Meschede, and Ronand Blakey write, "India rapidly marched northward towards Asia with a velocity of ca. 20 cm/yr, a plate velocity that exceeds any modern example. This velocity considerably slowed to ca. 5 cm/yr following the collision, yet India continued to protrude into Asia for more than 2000 km. ... The irregular northern margin of the Indian continental crust first came into contact with Eurasia along its northwestern corner, approximately 55 Ma. As a consequence, India underwent a counter-clockwise rotation to close the remaining part of the Neotethys in scissor-like fashion from west to east. The closure of the Neotethys was completed approximately 40 Ma."{{sfn|Frisch|Meschede|Blakey|2011|p=174}}

Today, the Indian plate continues to be driven horizontally at the Tibetan Plateau, which forces the plateau to continue to move upwards.<ref>{{cite web |url=https://www.geolsoc.org.uk/Plate-Tectonics/Chap3-Plate-Margins/Convergent/Continental-Collision|title=Plate Tectonics -The Himalayas |publisher=The Geological Society |access-date=13 September 2016}}</ref> The Indian plate is moving at {{convert|67|mm|abbr=on}} per year, and over the next 10 million years, it will travel {{cvt|1500|km|||}} into Asia. Approximately 20&nbsp;mm per year of the India-Asia convergence is absorbed by [[Thrust fault|thrusting]] along the [[Main Frontal Thrust|Himalaya southern front]]. This leads to the Himalayas rising by about 5&nbsp;mm annually, making them geologically active. The movement of the Indian plate into the Asian plate also makes this region [[seismic]]ally active, leading to earthquakes from time to time.<ref>{{Cite web |date=2019-01-17 |title=Devastating earthquakes are priming the Himalaya for a mega-disaster |url=https://www.nationalgeographic.com/science/article/earthquakes-priming-himalaya-disaster |access-date=2024-03-28 |website=Science |language=en}}</ref>

The Himalayan mountain range consists of three sub-ranges: (1) the Higher- or "Tethys" Himalayas, (2) the Lesser Himalayas, and (3) the Siwaliks.   The nappes&mdash;large, stacked sheets of rock&mdash;found in the Tethys Himalayan mountain range, are primarily composed of sedimentary rocks, such as [[limestone]] formed from the accumulation and compression of sediments like sand, mud, and shells deposited in the Neo-Tethys seabed during the [[Paleogene]]" (66 Mya&ndash;23 Mya).{{sfn|Frisch|Meschede|Blakey|2011|p=174}} Below the sedimentary rocks in the Higher and Lesser Himalayas is a bottom layer, or [[basement (geology)|basement]], composed of [[metamorphic rock]] formed much earlier during the [[Pan-African orogeny|Pan-African]]-[[Cadomian orogeny]] between 650 Mya and 550 Mya.{{sfn|Frisch|Meschede|Blakey|2011|p=174}} The lowest subrange, the Siwaliks, represents the sedimentary rock deposits washed off the rising Himalayas in a [[foreland basin]], a low-lying crustal region, at their foot.{{sfn|Frisch|Meschede|Blakey|2011|p=174}} It primarily consists of sandstones, shales, and conglomerates formed during the [[Neogene]] period (23 Mya to 2.6 Mya).  

Geologists Wolfgang Frisch, Martin Meschede, and Ronand Blakey further write, "The Siwaliks are both underlain and overlain by thrusts; they have been overridden by the nappe stack of the Higher and Lesser Himalayas and, in turn, are thrust over more interior parts of the Indian continent. Each of the three mega-units is internally imbricated into several individual nappes. Fensters (windows) and klippen provide important structural information regarding the thrust belts and help document the existence of broad thrust sheets, some of which record thrust distances in excess of 100 km. A fenster or window is an erosional hole through a thrust sheet that exposes a tectonically lower unit framed by a higher unit; a klippe is detached by erosion and forms a remnant of a nappe or higher thrust sheet
that rests on top of a lower unit."{{sfn|Frisch|Meschede|Blakey|2011|p=173}}