earthstory reblogged
Offroading in Anza Borrego State Park, east of San Diego, California
Orbicular Gabbro
This polished slab comes from the Peninsular Ranges plutons, found to the east of San Diego California. This is a rare, and fairly photogenic, chunk of orbicular gabbro. The smallest of the spheres are about 1 cm in diameter, the largest in this picture are about 5 cm or so.
Spherical textures do show up in some igneous rocks, although rarely. When crystals are growing from a liquid they tend to form shapes defined by the arrangement of atoms and no mineral easily produces a structure that is spherical. To form spheres like these, growth of the minerals must happen in stages, with successive layers added around an original or rounded core, such that the newly grown mineral is just following an already-round shape.
The core of the spheres is made of gabbronorite, an igneous rock with grains large enough to be seen by the human eye made of plagioclase and pyroxenes. Those cores are surrounded by rims of olivine and pyroxene. Geologists who examined this rock suggest that the original rock, the gabbronorite, was flooded by fluids coming off of another nearby magma body. Those fluids caused some of the gabbronorite to erode, rounding the inner clasts, and then they precipitated the olivine and pyroxene layers as the conditions changed.
-JBB
Image credit: http://bit.ly/1XxZyca
References: http://www.bgs.ac.uk/bgsrcs/rcs_details.cfm?code=GBNO http://gsabulletin.gsapubs.org/content/84/1/1.short http://sdsu-dspace.calstate.edu/handle/10211.10/2145
Source: facebook.com
The Gulf of California rift This photo was taken in Sierra de San Pedro Mártir national park, Baja California Nord, Mexico, looking east. The park sits in high ground in the northern part of the Baja peninsula. The surrounding rocks are part of the Peninsular Ranges batholith – grown from magmas intruding the crust as the Farallon plate subducted offshore. In the foreground you can make out some of these granitic rocks. When North America moved so far to the west that it completely subducted the Farallon plate and met the Pacific plate, it broke the Farallon plate into small pieces, some of which survive today. The Farallon plate used to subduct off the western coast of Baja, but today subduction has migrated far to the south. Instead, a different type of fault system has grown. To the north, this fault system becomes the San Andreas Fault system; to the south, it has rifted the continent apart. Normal faults have opened a basin that today is filled by the San Felipe valley and the Gulf of California; one of those faults sits where the high ground in this image ends. The basin in front of you is opening as part of the movement between the Pacific and North American plates. As the plates move, this area is pulled apart and subsides, creating a dry valley, surrounded on all sides by mountains that today are being pulled apart. -JBB Image credit: Wikimedia Commons http://en.wikipedia.org/wiki/File:San-Felipe_Sierra-SanPedroMartir_BajaCalifornia_Mexico.jpg