Rocks of the Himalayan Mountains in Nepal
All three classes of rocks (igneous, metamorphic, sedimentary) are found in the Himalayan mountains. Although the Himalayas are only 50-60 million years old, most of the rocks in it are much older. The oldest known rock in Nepal is about 2 billion years old, while a great many of them are over 570 million years old. This post identifies the 3 different classes of rocks in 5 different geologic units of Nepal and Tibet.
Rock classification
Igneous rocks
Igneous rocks are formed when magma cools. Their classification is dependent on the how they formed, their texture, mineral composition, chemical composition, and their shape. It’s a bit much even for geology majors. The point I am trying to make is that there are different classes/types of igneous rocks, but only a few are found in the Himalayas.
Metamorphic rocks
Metamorphic rocks are formed when intense temperature and pressure alter a pre-existing rock physical and or chemical structure. This alteration occurs deep within the earth, by tectonic process like continental drift, and by its proximity to magma.
Sedimentary rocks
Sedimentary rocks are formed from the collection of mineral and or organic particles and or weathered and eroded rocks. The sedimentary rocks form when an abundant pressure (lithostatic) squeezes the sediments together. Under the right amount of pressure, chemical, and physical changes occur in the sediment causing the sediments to bind together.
Geologic units in Nepal
Nepal is separated into units by fault lines. The fault lines run east and west. Their names are Main Frontal Thrust Fault (MFT), Main Boundary Thrust Fault (MBT), and Main Central Thrust Fault (MCT), and South Tibetan Detachment Fault (STDF). These faults separate geologic divisions, which are Indo-Gangetic Plain, Siwaliks, Lesser Himalaya, Higher Himalaya, and Tethys Himalaya.
Indo-Gangetic Plain
There isn’t too much going on in the Indo-Gangetic Plain. It is primarily alluvial deposits, weathered and eroded rocks from the Himalayan mountains. Only a small portion of Nepal is in the Indo-Gangetic plain. I couldn’t find any specimens that were collected in this area.
From the Indo-Gangetic Plain, heading north, we cross the Main Frontal Thrust Fault into Siwaliks.
Siwaliks
The Siwalik region occupies a space in southern Nepal just north of the Terai and south of the middle hill mountains. This area is one of the most geologically active areas of Nepal. It is increasing in altitude of 3 to 4 mm per year. The area is known for its calcium carbonate and mineral deposits, and its sandstone. The rocks found in this area tend to be 2 to 18 million years old.
Sedimentary rocks
The sedimentary rocks found in Siwaliks are:
- Marl
Merl - Silt stone
Siltstone - Mudstone
Mudstone - Calcium carbonate
- Sandstone
Sandstone - Shale
Shale
On the northern edge of Siwaliks is the MBT, which separates the Siwaliks from Lesser Himalaya.
Lesser Himalaya
Lesser Himalaya is known for its metamorphic and sedimentary rocks, however there are also some igneous rocks in this region. The rocks in this area generally date to over 570 million years old.
Sedimentary rocks
The sedimentary rocks found in Lesser Himalaya are:
- Chert
Chert - Dolomite
Dolomite - Conglomerate
- Limestone
Limestone - Diamictite
Metamorphic rocks
The Metamorphic rocks found in Lesser Himalaya are:
- Metasandstone
Metasandstone - Phyllite
Phyllite - Banded gneiss
- Schist
- Slate
Slate - Marble
Marble - Banded Gneiss
- White quartzite
White Quartzite
Igneous rocks
The igneous rocks found in Lesser Himalaya are:
- Trachyte
Trachyte - Pegmatite
pegmatite
On the northern edge of Lesser Himalaya is the MCT, which separates Lesser Himalaya from High Himalaya.
High Himalaya
High Himalaya is the area between Lesser Himalaya and Tethys Himalaya. Some rocks in High Himalaya are dated to 500-2,000 million years ago. However, there are other rocks much younger, 60 million years old, due to metamorphic processes. Metamorphic rocks are found at the base and sedimentary rocks are found at the top.
Sedimentary rocks
The sedimentary rocks found in High Himalaya are:
- Merl
Merl - Dolomite
Dolomite - Greywacke
- Siltstone
Siltstone - Shale
Shale - limestone
Limestone
Metamorphic
The Metamorphic rocks found in High Himalaya are:
- Granite gneiss
- Schists
- Gneiss
- Black schist
Black Schist - Augen gneiss
Augen Gneiss - Mica schist
Mica Schist - Kyanite schist
Kyanite Schist - Actinolite in calcareous gneiss
Actinolite in Calcareous Gneiss - Kyanite gneiss
Kyanite Gneiss - Biotite schist
Biotite Schist - Chlorite schist
Chlorite Schist
Igneous
The igneous rocks found in High Himalaya are:
- leucogranites
Leucogranite - granite
Granite - diorite
- gabbro
- tonalite
the last tectonic zone is the Tethys Himalaya. It is north of the South Tibetan detachment Fault, which separates it from Higher Himalaya zone.
Tethys Himalaya
This area of the Himalayas is generally considered the Tibetan plateau. it is approximately 500 million years old and is comprised of sedimentary and metamorphic rocks.
Sedimentary
The sedimentary rocks found in Tethys Himalaya are:
- dolomite
Dolomite - sandstones
Sandstone - conglomerates
- greywackes
Metamorphic
The Metamorphic rocks found in Tethys Himalaya are:
- biotite quartzite
- marble
Marble
