How the Aravalli Hills formed and why they look the way they do

The Aravalli Hills look modest today but they sit on some of the oldest and most studied rocks in India. Geologists care about them because they preserve a long record of how a piece of the earth’s crust in northwestern India was built, deformed, heated, and intruded by magma, then worn down.

The Hills’ features are better understood as part of a belt of ancient rocks that many scholars have called the Aravalli-Delhi orogenic belt. Within this belt, geologists have distinguished two broad successions of rocks, called the older Aravalli Supergroup and the younger Delhi Supergroup.

Making the Aravalli

Before the Aravalli sediments were laid down, an older crustal foundation already existed in the region. Research has described this basement and the overlying Aravalli succession as a connected system, with sediments accumulating on older crust. Many researchers have interpreted the earliest phase as one in which the crust stretched and thinned, creating basins. Rivers and shallow seas then filled these basins with sand, mud, and carbonate sediments and in some places with volcanic material.

This means the ‘raw material’ for the Aravalli Hills first appeared as sedimentary layers in the basins rather than as a preexisting ridge.

According to geologists, these basins later closed when tectonic forces pushed the crustal blocks together. The layered rocks folded, broke along faults, and were carried over one another along thrusts. As the rocks were buried deeper and were heated, many of them changed (metamorphism), with sandstone turning into quartzite, mudstone into phyllite or schist, and limestone into marble. Studies have also confirmed the presence of large crustal structures consistent with this kind of compression and thrusting at depth.

There were also episodes when magma rose into the crust and crystallised as granitoid bodies. Using zircon dating, geologists have constrained when such intrusions formed. One 2003 study reported a zircon age of about 967.8 million years, give or take 1.2 million years, for the Chang pluton granitoid gneiss in central Rajasthan. This kind of dated intrusion is a mark that tectonic and thermal activity continued long after the earlier sedimentation.

Even after compression and intrusion, the present landscape didn’t appear overnight. Over a very long span of time, while the more resistant rocks, especially quartzite, tended to remain as ridges, wind and water wore the weaker rocks down faster. A 2022 review of the Delhi Supergroup and Delhi Basin emphasised that the belt’s history spans long intervals and multiple tectonic and thermal episodes, which geologists have said could explain why the present hills are really remnants rather than a young, steep mountain range.

Geology to environment

The geology of the Aravalli Hills influences the local soils and movement of water by the kinds of rocks present, the way those rocks break and weather, and the rainfall patterns.

Quartzite is common in many Aravalli ridges. This type of rock is tough because it forms when sandstone is buried and heated until the grains recrystallise and lock together. When quartzite reaches the surface, it often forms rocky ridges with thin soils. Thin soils hold less water and fewer nutrients, which affects what plants can grow and how quickly the slopes erode after being disturbed. In practical terms, the ridge can support vegetation but it’s also easier to damage and slower to recover if the soil is stripped away.

Hard crystalline and metamorphic rocks usually don’t store much water in pores the way sand does. Instead groundwater often lies in fractures and joints and in weathered zones, which means it’s unevenly distributed. Some locations yield water because the rock fractures connect well while nearby locations are dry because the rock is less broken or the weathered layer is thin.

In many parts of the Aravalli Hills, most of the monsoon rains arrive in a few months and there are long dry periods in between. One July 2025 study of the Northern Ridge in Delhi described a climate with hot summers and annual rainfall averaging 710 mm — the kind of seasonality known to favour plants that can tolerate drought.

Because the Hills’ soils are typically thin and there’s little water outside the southwest monsoon, even small disturbances can have lasting effects. Quarrying and mining both remove soil and alter the local drainage. A field study in the Asola Bhatti area in 2018 reported patterns of soil erosion consistent with such disturbances and linked them to lower soil richness and diversity. The researchers also noted that the soil could regenerate in some sites but only if conditions allowed.