Underpinning Houston's land surface are unconsolidated clays, clay shales, and poorly-cemented sands up to several miles deep. The region's geology developed from river deposits formed from the erosion of the Rocky
Mountains. These sediments consist of a series of sands and clays deposited on decaying organic matter
that, over time, transformed into oil and natural gas. Beneath the layers of sediment is a water-deposited layer of halite, a rock salt. The porous layers were compressed over time and forced upward.
As it pushed upward, the salt dragged surrounding sediments into salt
dome formations, often trapping oil and gas that seeped from the
surrounding porous sands. The thick, rich, sometimes black, surface soil is suitable for rice farming in suburban outskirts
where the city continues to grow.
Despite over 150 active surface faults (estimated to be 300 active faults) with an aggregate length of up to 310 miles
(500 km) within the city of Houston alone, the region is generally earthquake-free. This is because clay below the surface
precludes the build-up of friction that would normally produce the ground shaking in earthquakes. These faults also tend to move at a smooth rate in what is termed "fault
creep," which further reduces the risk of an earthquake.