A Dip in Earth's Gravity
In the flat, arid landscape of Uzbekistan's Xorazm Region, Earth's gravitational pull is measurably weaker than the global average. This is not a subtle scientific abstraction; it is a distinct negative gravity anomaly. The planet's gravity is not uniform. It varies based on the distribution of mass in the crust and mantle below. Where there is more mass, gravity is slightly stronger, and where there is less, it is weaker.
The Khorezm anomaly is detected by satellites like the twin spacecraft of the GRACE (Gravity Recovery and Climate Experiment) mission. These satellites orbit at an altitude of around 500 kilometers, precisely measuring the tiny changes in distance between them as they pass over different parts of the globe. These distance changes, accurate to within a few microns, reveal the gravitational tug of the features below. Over Khorezm, the satellites register a dip, indicating a significant mass deficit hidden deep underground. The cause is a colossal, buried dome of salt.
The Ancient Salt Dome
The feature causing this gravitational low point is a salt dome, or diapir, approximately 50 kilometers wide and 3 kilometers thick. This immense pillar of salt has pushed its way up from a deeper layer. Rock salt, primarily composed of the mineral halite, has a density of about 2,100 to 2,200 kilograms per cubic meter. The surrounding sedimentary rocks, such as sandstone and shale, are significantly denser, typically around 2,400 kg/m³ or more. This density difference, across such a vast volume of material, is what reduces the total mass in the region and weakens the local gravity field.
This salt dome is a product of geological processes that began over 150 million years ago. During the Late Jurassic period, this area was part of a shallow sea connected to the ancient Tethys Ocean. As the climate fluctuated, parts of this sea evaporated, depositing a thick layer of salt. Over millions of years, heavier sediments from rivers and marine environments buried this salt layer. Under immense pressure, the less-dense salt became buoyant and ductile, slowly flowing upwards through weaknesses in the overlying rock strata. This process, known as salt tectonics, formed the massive, hidden dome that exists today, its top still kilometers below the desert surface.