The
Zagros Mountains in southwestern Iran present an impressive landscape of long
linear ridges and valleys. Formed by collision of the Eurasian and Arabian
tectonic plates, the ridges and valleys extend hundreds of kilometers. Stresses
induced in the Earth’s crust by the collision caused extensive folding of the
preexisting layered sedimentary rocks. Subsequent erosion removed softer rocks,
such as mudstone (rock formed by consolidated mud) and siltstone (a slightly
coarser-grained mudstone) while leaving harder rocks, such as limestone
(calcium-rich rock consisting of the remains of marine organisms) and dolomite
(rocks similar to limestone containing calcium and magnesium). This
differential erosion formed the linear ridges of the Zagros Mountains. The
depositional environment and tectonic history of the rocks were conducive to
the formation and trapping of petroleum, and the Zagros region is an important
part of Persian Gulf production.
This
astronaut photograph of the southwestern edge of the Zagros mountain belt
includes another common feature of the region—a salt dome (Kuh-e-Namak or
“mountain of salt” in Farsi). Thick layers of minerals such as halite (common
table salt) typically accumulate in closed basins during alternating wet and
dry climatic conditions. Over geologic time, these layers of salt are buried
under younger layers of rock. The pressure from overlying rock layers causes
the lower-density salt to flow upwards, bending the overlying rock layers and
creating a dome-like structure. Erosion has spectacularly revealed the uplifted
tan and brown rock layers surrounding the white Kuh-e-Namak to the northwest
and southeast (center of image). Radial drainage patterns indicate another salt
dome is located to the southwest (image left center). If the rising plug of
salt (called a salt diapir) breaches the surface, it can become a flowing salt
glacier. Salt domes are an important target for oil exploration, as the
impermeable salt frequently traps petroleum beneath other rock layers
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