Saturday, February 13, 2016

Once Upon a Time...

This essay was written to accompany a geologic cross section which I unfortunately don't have anymore. Imagine that there is an outcrop of rock along the side of the road, with several distinctly different beds. Some are thick, thin, light, dark, crumbly, solid, fossil filled and fossil poor. This essay traces the history of this location from the bottom to the top.

Originally written November 28th 2015

Once Upon A Time...
by Aradia Farmer, Geo 201 Fall 2015
Once upon a time, there was a forest. The forest was lush, dripping with plants and deafened by the songs of newly evolved insects. Ferns and other water loving plants proliferated, as the forest was often inundated with a few feet of standing water. Sometimes, the rains would not come and the forest floor would emerge, covered in a coat of sticky mud that dried and cracked into plates, but the rains always came again and hid these jagged breaks beneath a fresh coating of silt. Woody fern fronds were captured between these layers, encased away from the elements.  
Then came a time when the rains never failed, and mud-cracks were a thing of the past. The forest grew ever taller, ever more alive, ever more densely packed with all manner of plants. Several feet of decaying plant matter built up in the anaerobic environment beneath the surface of the swamp, becoming denser as time passed and the overlying weight grew greater.
In the blink of a geologic eye, everything changed. The ocean advanced so far inland that the forest died, unable to cope with the load of salts borne by the new way of things. For a time, a coral reef and attendant ecosystem of clams and fish dominated the area. Near-shore currents changed their directions periodically, and the accumulation of shells large and small continued unabated.
Again change rocked the ecosystem, this time as the water retreated. A vast area of decreasing depth harbored the last corals and fish, who then died as the temperature and salt content of the water became intolerable. Still, the water continued to evaporate, ultimately leaving behind huge sheets and crystals of gypsum blanketed over the reef’s graveyard.
The retreat of the sea allowed the return of the forest, much as it had existed before, though a bit drier. As the ocean retreated, it drew with it a strand of one of its tributaries, pulling a river channel into the forest floor. Occasional flooded periods still  came and went, leaving mud-cracks behind, but there were times when even many rainfalls could not flood the forest floor, allowing the impressions of raindrops to be preserved in the silty soil.
The next advance of the sea was less dramatic, turning the forest into an estuary instead of a reef. The gentle flow from river to ocean pushed ripple marks into the muddy bottom where clams and their predators burrowed in the endless quest to eat or be eaten. Plants adapted to the saltier water, and thrived in the low-energy currents.
Gentle or dramatic, the sea cannot be resisted when it chooses to change its bounds. The estuary was eventually flooded over completely, returning to its earlier reef state, but this time with more ecological diversity. Shells ranging from the size of a human hand to a blood vessel were deposited in large swaths. The remnants of the river’s current still pushed into the ocean as it could, laying down distinctive crossbeds that were unapologetically disturbed by burrowing creatures seeking food and shelter.
Beneath the day-to-day drama of the reef, woody ferns were crushed ever more tightly into their silty tombs, the remains of the swamp forest were compacted into a rock that would one day be able to burn, the older reef was squashed into limestone, and mud became rock. The upper reef eventually also succumbed to the passage of time and sediment accumulation, and was subsequently changed into limestone and chert. Then erosion exhumed the rocks, and I wrote an essay about them.      

The End

Thursday, February 4, 2016

Himalayan Mountains - A Convergent Boundary

Originally written 5/1/2015

The Himalayan mountains are one of the most impressive terrestrial landforms on present day Earth. They are also relatively young, which means that they are very geologically active. This activity, while exciting to geologists, has tragic consequences for the people that live in the region.

The Himalayan mountains are actually only a symptom of a much larger geological process. Starting about 71 mya the Indo-Australian continental plate began subducting under the Eurasian plate in a classic convergent plate boundary, in which the denser oceanic crust sinks below the lighter continental crust. Normally this process creates volcanic island arcs in the overlying plate, but this time something different was in store. As the ocean that had separated the continents disappeared the continents themselves started to collide, each refusing to sink beneath the other. Around 50 mya the last bits of the ocean basin, squeezed like Play-Doh in your fingers, was elevated to create mountains of limestone.

As the rocks closest to the actual boundary are pressed ever tighter together by the formidable forces of plate tectonics, their individual mineral identities are distorted, folded, and interwoven into the geologic equivalent of a suture in flesh. Hundreds of millions of years from now, when those tortured rocks are exposed by erosion to a new generation of geologists, they will display distinctive narrow stripes of minerals in tight folds, and it will be impossible to tell which mineral grains came from which plate.

Convergent boundaries are always marked by earthquakes, and usually also by volcanoes. This particular boundary does not have volcanoes, since there is no actual subduction occurring anymore, and thus no melting rock to provide magmatic fuel for explosions. The Himalayas are therefore built entirely of pre-existing rock, not magma as the Hawaiian islands are. Despite the lack of magma, the Himalayas are far from benign. On April 25th, 2015, India jerked a little farther into Asia, a regular spasm in the ongoing collision. The resulting loss of life and property on the surface has yet to be totally added up, but as of May 1st, 2015, the death toll is over 6,000 people, and many historical sites and temples have been flattened. The last earthquake of comparable strength was about 80 years ago, a long time ago to humans but a mere eyeblink ago to plate tectonics.

Essentials of Geology, section 2.12