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

  

• Silt stone

  

• Mudstone

  

• Calcium carbonate
• Sandstone

  

• 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

  

• Dolomite

  

• Conglomerate
• Limestone

  

• Diamictite

Metamorphic rocks

The Metamorphic rocks found in Lesser Himalaya are:

• Metasandstone

  

• Phyllite

  

• Banded gneiss
• Schist
• Slate

  

• Marble

  

• Banded Gneiss
• White quartzite

  

Igneous rocks

The igneous rocks found in Lesser Himalaya are:

• Trachyte

  

• 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

  

• Dolomite

  

• Greywacke
• Siltstone

  

• Shale

  

• limestone

  

Metamorphic

The Metamorphic rocks found in High Himalaya are:

• Granite gneiss
• Schists
• Gneiss
• Black schist

  

• Augen gneiss

  

• Mica schist

  

• Kyanite schist

  

• Actinolite in calcareous gneiss

  

• Kyanite gneiss

  

• Biotite schist

  

• Chlorite schist

  

Igneous

The igneous rocks found in High Himalaya are:

• leucogranites

  

• 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

  

• sandstones

  

• conglomerates
• greywackes

Metamorphic

The Metamorphic rocks found in Tethys Himalaya are:

• biotite quartzite
• marble