Radiating dykes When molten rock intrudes into the continental crust and slowly freezes there into g

Radiating dykesWhen molten rock intrudes into the continental crust and slowly freezes there into granitoids (a word that denotes a variety of compositions, but the same larger crystal sizes than volcanic rocks due to slow cooling in the Earth), and is then exhumed for us to enjoy by erosion, the feeder system that brought the magma from below is sometimes visible in the landscape. West and East Spanish Peaks are found in the Rockies of Colorado, standing proud on the edge of the Eastern Plains, and froze in the surrounding sedimentary rocks (deposited in a Mesozoic inland sea that waxed and waned across the centre of North America) some 28 million years ago. Rising steeply some 1800 metres from the Colorado Plateau that they rose into, West Peak (from which the photo was taken) maxes out at 4,155 metres, making it the easternmost peak over 4,000 in the USA.They were designated a Wilderness area in 2001, and were originally recognised as a National Natural Landmark in 1976 due to their interesting geology. The two mountains are a feature known to geologists as stocks, large masses of intruding granite that solidified underground, with the East Peak being a near circular blob some 8x5km, and West Peak around 4x3. There is a mixture of rocks in these plutons, though they tend towards the silica rich end of the range, granites and granodiorites.The dykes (aka Dakota Walls) seen radiating like spokes from the base of the mountain are made of the same rock, and extend up to 23km away from the pluton they fed, being up to 30 metres wide and high. Over 400 have been quantified, with 2 systems of radiating dykes and an older system of parallel ones. They originally pushed through weak points or remobilised pre existing faults in the sedimentary rocks, manifesting as the forefront of the pressing magma rising from below, and then fed the growing mountain to be until the supply from the depths ran out. The surrounding sedimentary rocks are softer, so they have been preferentially eroded (along with much of the original granite), revealing the structures hidden within.It was a landmark for the indigenous people since time immemorial, along with the Spanish (who knew the peaks as los Dos Hermanos, or the two brothers). and French trappers, followed by later settlers. The Plains tribes knew them as Wahatoya, the Breasts of the Earth, and they are visible from as far as 160km away. During the settling of the West, they were a prominent landmark on the Santa Fe trail. Care should be taken exploring these peaks, since 19th century gold mining has left them riddled with unsealed shafts.LozImage credit: G. Thomashttp://sangres.com/mountains/spanpks/index.htm#.VyaFPORrM8UUSGS report on the fascinating geology of the region, free access: http://pubs.usgs.gov/pp/0594g/report.pdf -- source link

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