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The Life in the Rocks

Dana Visalli wrote this article for the Winter 2012 issue of The Methow Naturalist, but was kind enough to lend it to us for a post. Click here to see the full Winter 2012 issue. The Methow Naturalist is published quarterly and brings readers insights on both the weird and the wonderful of Cascadian natural history.


The Life in the Rocks: Plate Tectonics & Biodiversity
Written by Dana Visalli for the Winter 2012 Methow Naturalist

Oval Peak in the Sawtooth Range; the mountain and the life upon it exist thanks to the forces of plate tectonics, which create the uplift to build mountains and cycle the nutrients necessary for life.


There is a surprising level of connectivity showing up in recent scientific studies between what we might call the living and the dead—between the exuberant vitality of the biosphere and the mechanical grindings of the geosphere. Between life and rocks. For most of human history the earth beneath our feet has seemed like it was merely a handy platform on which to stand; but one to which we bore little or no relationship. Research over the past 100 years has demonstrated that arrangements are very different than we had imagined. The planet turns out of course to be spherical rather than flat, and while it remains a handy place to stand, it has proven to be vastly more animated than previously thought. We have found that Earth has a molten core the same temperature as the surface of the sun, and that the continents themselves have been crashing around on the surface of the planet for the last three billion years, joining together at least twice into solitary land masses (Rodinia and Pangaea), only to periodically split apart and go floating off in different directions.

Our understanding of Earth’s crustal movements is quite new. As recently as 1980, 20% of all professional geologists held the theory of plate tectonics in utter contempt. Most of the world’s scientific community has now successfully weathered that paradigm shift. Still, paradigms are like oceanic crust, and constantly churn and metamorphose into new material. One of the more recent insights to spring from the study of the planetary system is that plate tectonics not only drive crustal plates and move continents, but this dynamic force may also be the single most critical factor for maintaining the diversity of life on Earth.

Energy runs downhill, and so do the nutrients essential to life. When we plant a tree we assume there will be enough phosphorus, potassium, carbon, sulfur, nitrogen and other essential elements for the plant to grow. But we must ask: why would these nutrients still be in the soil after four billion years of erosion and water moving soil to the sea? The answer is: plate tectonics. The oceanic plates that form the ocean floors slide upon the fluid upper mantle and are being pushed apart in mid-ocean by rising plumes of molten rock. As oceanic plates move east and west from their point of origin they cool, thicken, and grow heavier. Where they are forced against continental rock---which is less dense---they sink back into the earth, carrying the essential nutrients that have flowed to the sea with them.

A simplified view of the forces that formed the Methow. The heat of the mantle creates convection currents that force the earth's crustal plates to move. Denser oceanic crustal plates sink under lighter continental plates when the two are forced together. Sediments that had washed off the continents into the sea are compressed and deformed when two sections of continental crust are forced together. The Methow is largely deformed sedimentary rock.


One could think this might be the end of the story, like Frodo throwing the Ring of Power to its doom in the chasm on Mordor, but it is not. The essential elements conveniently reappear a short time later, geologically speaking (in about 100 million years), in the form of uplifted mountains and volcanic magma, both of which erode back to soil. Without plate tectonics life on land would grind to a standstill in short order.

Plate tectonics is also intricately involved with the addition and removal of carbon dioxide from the atmosphere, the balance of which is critical to maintaining earthly temperatures within the narrow range acceptable to life (roughly 0° to 120° Fahrenheit). Carbon dioxide constantly seeps into the atmosphere from volcanoes and ocean vents (there are approximately 600 currently active volcanoes and thousands of hydrothermal vents worldwide). It is also constantly removed from the atmosphere, both by photosynthesis and by a chemical reaction with continental rocks that is driven by plate tectonics.

Briefly put, carbon dioxide reacts with high silicate, continental rocks—such as granite, which there is a lot of—to form calcium carbonate (limestone) and silica (quartz), both of which are then sequestered as sediments, removing carbon from the atmosphere. The chemical reaction is faster at higher temperatures. The warmer it gets, the more carbon dioxide is removed, forming a negative feedback loop as atmospheric temperatures drop. The process requires a continual supply of silicate rocks, which is both produced and uplifted by the forces of plate tectonics. Without this removal of carbon dioxide from the atmosphere Earth would soon resemble Venus, which has a surface temperature of 900° F. Conversely, without the addition of carbon dioxide to the atmosphere from tectonic activity the surface of the earth would freeze solid.

There is another interesting twist to this story. The sun has increased in luminosity and heat output by 25% over the past three billion years. During that time the earth’s temperature has remained within that narrow range suitable for life. This has been possible because the quantity of silicate rock—which is continental rock, as opposed to the lower-silica basalt of the oceanic crust—has increased greatly over time. Evidence indicates that there was only 10% as much continental rock (and continental landmass) three billion years ago. The amount of continental rock on the planet has increased greatly over time due to the differentiation (also known as fractionation) of oceanic crust into lighter and denser rocks via the partial melting that occurs during subduction. Because there is far more continental rock now than previously, the rate of removal of carbon dioxide has increased over time, maintaining a fairly steady atmospheric temperature. (The evolution of continents was explored in detail in the previous issue of The Methow Naturalist).




Growth of continental land mass over 4 billion years.

An increase in plant biomass will also increase the transformation of atmospheric carbon into calcium carbonate, because plants pump carbon dioxide into the soil. This creates another negative feedback loop in which rising carbon dioxide in the atmosphere leads to an increase in plant growth, which then increases the rate of chemical reactions in the soil. If carbon dioxide were not continually injected into the atmosphere by natural processes, it is estimated that plants would remove the 390 parts per million of atmospheric carbon dioxide in as little as ten years. (Unfortunately, this cycle works too slowly to have a meaningful impact on anthropogenic carbon dioxide.)

The distribution and arrangement of the continents across the surface of the planet has a profound impact on the potential diversity of the biosphere. Species diversify primarily from 1) relatively stable and productive habitats, such as those in the tropics, and 2) from habitats that vary considerably over a given distance. Because of plate tectonics, our current continental configuration creates what may be the ideal conditions for diverse life on the planet.  Having what was just one continent 250 million years ago split now into six continents (Europe being little more than a hangnail on Asia), and having three of the six continents arranged along a north-south axis, optimizes temperature and climate variability along coastlines.



 

The changing positions of the continents over the past 250 million years.


What would happen if plate tectonics ceased? Once created, does high biodiversity require the continued presence of plate tectonics? If plate tectonics stopped, nutrients running to the sea would no longer be cycled back to land. Creation of continents would cease, but erosion would continue, washing all dry land into the sea within about 200 million years. While removal of carbon dioxide from the atmosphere via photosynthesis and chemical reaction with silicate rocks would continue, the infusion of that gas into the atmosphere from magma would cease. Plants would remove existing carbon dioxide within a few years, photosynthesis would cease, and the global biosphere would collapse.