Sierra Madera
Key facts
- Country
- United States
- Diameter
- 13 km
- Age
- less than about 100 million years
- Location
- 30.60° N, 102.92° W
A cluster of rugged limestone hills rising from the desert plains of west Texas, south of Fort Stockton, surrounded by a broad ring of lower ground. It is on private ranch land.
Sierra Madera is a deeply eroded impact structure about 13 km across in the desert of west Texas, in the United States. At its heart a knot of hills rises from the plain, formed of rock that was pushed upwards by more than a kilometre during the impact.
What it hit. The impactor struck a thick sequence of Permian and Cretaceous sedimentary rocks, mainly limestones and dolomites. The rebound of the crater floor lifted Permian layers from deep below into a central uplift, where they are now steeply tilted, overturned and broken into blocks. The age is not known precisely; the youngest rocks affected are Cretaceous, so the impact happened less than about 100 million years ago.
Debate and proof. For years Sierra Madera was described as a 'cryptovolcanic' structure, a term used for round, disturbed areas thought to have been made by hidden volcanic explosions. In the 1960s, as interest in craters grew with the Apollo programme, geologists of the United States Geological Survey mapped the structure in detail. They found abundant shatter cones, cone-shaped fractures that form only under shock, and quartz grains with shock features. These showed that an impact, not a volcano, was responsible.
Why it matters. Sierra Madera was one of several American structures studied in the 1960s to understand how complex craters with central peaks form, knowledge that helped geologists interpret craters on the Moon. The impactor is unknown.
Visiting. The structure is on private ranch land and the central hills are not open to the public. Ask the landowner's permission before entering, and view it from the public road otherwise.
Did you know?
Rocks in the central hills were lifted more than a kilometre during the impact.
Sierra Madera was studied in the 1960s partly to help understand craters on the Moon.
It was once thought to be the result of a hidden volcanic explosion.