Magdalena Fault EarthCache

The earth's crust, a portion of the lithosphere, undergoes large-scale motions and deformations. The lithosphere is broken into tectonic plates which move in relation to each other based on three major boundary types. Plate movement is based on the idea of mantle convection, where heat rises to the surface of the earth, cools, and falls back to the center, creating convection circles. The origin of heat generation within the earth is explained well here: Interior heating

Above is an excellent overview of possible plate boundaries. Image source: UW-Stevens Point

1. Convergent: two plates collide. When a dense plate (oceanic plates are always the densest) collides with a less dense plate (continental or oceanic), the denser plate gets subducted, or pushed under, the less dense plate. This combination produces mountain ranges, earthquakes, and volcanoes: the Andes mountains on the west coast of South America are a volcanic arc formed by the subduction of the Nazca oceanic plate beneath the South American continental plate. This boundary is associated with powerful earthquakes such as the one that struck Sumatra, Indonesia, in 2004. Continental plates also may collide with each other, where neither will be subducted but instead will form mountain ranges like the Himalaya where the Indian plate crashed into the Eurasian plate. Faults composing convergent boundaries are generally reverse faults (and low-angle thrust faults).

2. Divergent: two plates move away from each other. New crust is formed with this boundary: as plates pull away to be subducted elsewhere, "new" material must be created to compensate. Magma wells up at this boundary to form volcanoes or long mountain ranges (depending on the rate at which the plates diverge), such as the Mid-Atlantic Ridge (oceanic) and the volcanoes Erta Ale, Kilamanjaro, and Ol Doinyo Lengai of the East African Rift (continental). Earthquakes are usually shallow at this boundary, and faults associated with divergence are generally normal faults.

3. Transform: two plates slide past each other. Crust is neither destroyed nor created; rather, plates slide and grind past each other. The San Andreas fault is a classic example where the Pacific plate is sliding north past the North American plate, effectively slicing off a good chunk of California. Earthquakes are shallow, but may be immensely powerful. Bends in the fault cause a buildup of pressure (in actuality, the bend is a convergent boundary of the transform fault) which sometimes leads to a sudden release of energy--an earthquake. Faults are strike-slip, or 'transform'.

The Rio Grande Rift

Land located approximately on either side of the Rio Grande River is pulled in opposite directions (diverging), causing enormous blocks of crust to slump down towards the rift:

The Rio Grande Rift isn't exactly successful in cutting the North American plate in half. Movement is slow, and earthquake activity is relatively rare (check out the USGS's earthquake site to see real-time locations of quakes around the world).

Another example within the U.S. of a 'failed' rift system is located in the midwest: the Reelfoot Rift of New Madrid, MO. There is no obvious surficial expression of this rift, though in 1811-12, a couple of major earthquakes powerful enough to ring church bells in Boston and make the Mississippi River run backwards for a day ripped through the midwest. Thankfully populations were low in that day, but if an earthquake of that size were to occur again, it would cause significant loss of life and billions of dollars of damage. It's hardly a question of "if," but "when." Small earthquakes occur along the fault zone every day (see the USGS earthquake site again). The rift is likely associated with the splitting (divergence) of the North American plate to create the Atlantic Ocean, ~200 million years ago.

Though the exact causes of plate motion and specific movement directions are still investigated to this day (do subduction zones pull plates apart, or do divergent zones push plates apart? Or both?), all rifts are associated with an upwelling of magma which either aids or causes the plates to split.

19 km below Socorro, NM lies an inflating magma body responsible for two earthquakes in July and November 1906 (magnitude approximately 5.8). The sill-like body is estimated to rise about 2mm/year, and is associated with the Rio Grande rift. Check out New Mexico Tech's page on recent seismic activity in the area., which includes information about the latest earthquake 'swarm' around August-September 2009.

The Magdalena Mountains themselves are associated with volcanism: "Magdalena Peak and Elephant Mountain are more recent volcanic eruptions, approximately 13 million years ago" (Dave Love, quoted in 'The Mountains of New Mexico' by Robert Julyan). There's even a beautiful ignimbrite location within the Maggies...but that's for another Earthcache! They belong to a series of calderas that trend northeast/southwest along the Mogollon-Datil volcanic field which remarkably get younger the closer to Socorro you investigate.

To capture this Earthcache, you must:

1. Email me the elevation of the Rio Grande River just east of Socorro, NM and the elevation of the point where you stand.

2. Tell me in the same email what type of fault (reverse, thrust, normal, or transform) you see when looking at the Magdalena Mountains from this location.

3. Take and post a photo of the fault scarp (the mountains!) when you find the cache. Please try to include at least one person in your party in the photo. Your GPSr does not need to be visible since this cache is findable without one.