The Earth Story

The Xenolith Flow These barren rocks have sat on this site since 1802, when they arrived here after pouring out of Hualalai, one of the volcanoes that compose the main island of Hawaii.

Cutting through Gneisses of the Grand Canyon

Diquís Spheres

The Diquís River Delta in Costa Rica is dotted with about 300 of these large, sculpted spheres. The largest of these stones is over 2 meters in diameter, making it larger than most average humans, with a mass of nearly 15000 kilograms.

The spheres are made mostly of local igneous gabbro, with an occasional sphere of other local materials such as limestone. Since the rocks are much older than the carving, geochemical techniques struggle to date the time that they were carved. Instead, scientists estimate that they were carved between 600 and 1400 AD based on the layers of sediments that surround some of the boulders.

The original reason for their carving is unknown. When they were first recognized as artifacts in the 20th century, several of them were destroyed or even dynamited due to rumors that they may hold gold. These boulders and other carved rocks in the region were added to the UNESCO World Heritage list in 2014.

-JBB

Image credit: http://bit.ly/29aYEfQ

References: http://whc.unesco.org/en/news/1160 http://news.bbc.co.uk/2/hi/science/nature/8593717.stm http://bit.ly/29a5aT3 http://bit.ly/29ad43s http://whc.unesco.org/en/list/1453

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

The Geology of Cemetery Ridge

152 years ago today a small hill in rural Pennsylvania was the scene of a battle that would change the course of American history. This is a page on Earth science, and every good geologist (and military commander) knows that the geomorphology can be your best friend. Ridges don’t just exist by chance; their existence says something about the bedrock and evolution of the area.

The small ridge seen in the background of this image, rising less than a hundred feet above the surrounding plains and marked by military memorials is known as Cemetery Ridge. It sits just outside of the small Pennsylvania town of Gettysburg.

On July 1, 1863, a battle began in this area. Union forces retreated from several surrounding locations and gathered their armies on this ridge. On July 2, 1863, Confederate forces attempted several attacks on hills to the south of Cemetery Ridge, attempting to take positions which would make the Union defenses on Cemetery ridge untenable.

On this day, July 3, 1863, Confederate forces under General Pickett charged directly at the Union lines on Cemetery ridge and were almost totally destroyed.

Cemetery ridge is actually not a very good place to put a defensive position. It isn’t just high ground, it’s also very rocky; whatever the rocks are, there is very little soil developed on them, making it very difficult to dig fortified positions or trenches. The only defense on Cemetery ridge comes from fences and any other fortifications built on top of the rocks; soldiers just can’t dig in.

The rocks of Cemetery ridge played a key role, therefore, in this fight. The area around Gettysburg is made up of sedimentary rocks formed during the Triassic. At this time, the mountain ranges to the east were growing due to collisions between North America, Africa, and Europe. Those collisions built mountains that were shedding sediments into basins to the west. The plains around Gettysburg are made of these rocks; layers of sandstone and shale that are easily eroded and do not form high ground.

The high ground in the Gettysburg area comes in the form of long, linear ridges like Cemetery Ridge. To the south of Cemetery Ridge sits other high ground, including features such as Round Top and Little Round Top which sit on the same line as Cemetery Ridge.

These features are made of a type of igneous rock known as diabase. Diabase is a term for a rock that has a composition like basalt or a gabbro, but has a grain size in-between those 2. There can be some crystals large enough to see, but not many. The rocks are made up of solidly-intergrown, small crystals with little space for water to intrude. The tight packing of these crystals makes the rocks difficult to erode, leading to the formation of high-ground and thin soils.

The diabase in the Gettysburg area forms a type of structure called a sill. A sill forms when magma rises up through the crust but reaches a level of neutral buoyancy – the magma is too dense to rise through the lighter rocks above and instead pushes its way into the rocks. The magma forms a sheet in-between other layers of rock, becoming part of the stratigraphy.

This magma was formed nearly 200 million years ago, during the Jurassic period. It formed as a consequence of the end of Pangaea; as North America and Europe tore apart, cracks formed throughout the Eastern U.S. and magma flowed up into those cracks. One of those cracks formed the sill which created Cemetery Ridge and the other high-ground in the Gettysburg area.

Finally, the rocks in the area were tilted some as the continent continued to evolve, and erosion exposed the rocks seen today.

Several hundred million years ago, a normal series of geologic processes, including deposition of sedimentary rocks and intrusion by ordinary magmas, created the ground on which the American Civil War’s key battle was fought.

-JBB

Image Credit: National Park Service found here: http://npsgnmp.wordpress.com/2012/05/10/the-cavada-brothers-two-soldiers-two-wars/

Geology and the Gettysburg Campaign: http://www.dcnr.state.pa.us/cs/groups/public/documents/document/dcnr_014596.pdf

Volcanic Rocks of Pennsylvania: http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Places/volcanic_past_pennsylvania.html

The Cuillin Mountains

Myths tell the story of how the Cuillin Mountains on the Island of Skye were formed when the Sun flung its spear into the ground. A huge blister appeared and began to swell where the spear had struck the earth. Eventually the blister burst and discharged glowing molten material forming the Black Cuillin. This glowing molten material is the black and ragged gabbro, the type of igneous rock that composes the steep Black Cuillin. The bursting blister concealed the smoother, much lower-lying, snow-covered Red Cuillin, which consists of much lighter colored granite.

Interestingly, this myth correctly describes how a volcanic dome would burst and spew out hot magma. The Cuillin are indeed the remnants of an ancient volcano, eroded by rain and wind. They formed 50-70 million years ago when the Atlantic Ocean widened and the area experienced extreme volcanic upheaval. In this process a chain of volcanoes formed along the west side of the Scottish Highlands. The lava (basalt) welled up from fissures in the earth’s crust and rapidly cooled. Eventually the basalt was injected with dark gabbro, a type of rock which has larger crystals and cools slower because of the surrounding basalt. This is how the rugged Black Cuillin were formed. The gabbro and molten basalt also came into contact with the lower lying crust. The intense heat caused the lower crust to melt and the lava was injected with red/pinkish granite,creating the Red Cuillin.

Sometimes, geological history and creation myths do not differ that much.

-OW-

Image: Copyright Arpingstone. The main ridge of the Black Guilin.

Further reading: Major, Adrienne. 2004. Geomythology. Encyclopedia of Geology. http://bit.ly/1LHAF5O