Epeirogenic Universe

March 22, 2011

On Earth, geologists can understand the geologic events of the past by analyzing the different rock types and using clues to discern what forces creating them. The rock cycle explains how the three different rock types come to be via natural processes of volcanic eruptions, weathering and erosion, and heat and pressure (and time). Igneous rocks are created when lava on the surface cools and hardens into a solid mass. The density, composition, and texture of igneous rocks can tell a geologist where and how it formed; granitic rocks (rocks that contain a higher proportion of silicate materials) tend to be less dense than basaltic rocks (rocks with a higher percentage iron, calcium, or magnesium metals and less silicates). A geologist can usually tell what an igneous rock is made out of by coloring at the color of the rock; darker means more basaltic, and lighter hues suggest a granite-type. And lastly, the texture of a rock tells a geologist how fast the rock cooled; rocks with a glassy texture cooled too quickly to form grains, which means the rock formed on the surface, where the different between the molten interior of the earth and the room temperature outside meant that rocks solidified rapidly. Large grains mean the rock had time to form slowly, which suggests that it formed under the Earth, when higher temperatures allowed mineral grains to slowly coalesce.

Igneous rocks are classified by their composition and their rate of cooling

What kind of rocks are the Earth’s crust made out of? The Earth’s crust is made up of silicates, minerals like quartz or olivine, and Areios shares the same basic bulk composition as Earth, with some minor chemical differences that we won’t need to worry about. The crust is made up of plates that are pushed by the convection of the underlying mantle at a snail’s pace; only a few centimeters a year. There are two different kinds of plates that vary by their age and composition. The oceanic plates overlie the Earth’s ocean and are made up of denser basaltic rocks and the continental plates which are less dense make up our continents and some of the underlying mantle. When these jostling plates come into contact with one another, they create volcanic eruptions, earthquakes, tsunamis and other natural hazards. Because oceanic plates are denser, when they collide with continental plates, they sink into the mantle and are partially melted and become part of the mantle. When two oceanic plates collide, the denser one will sink below the mantle. But when two continental plates collide, typically they ram up against one another to form mountain ranges like the Himalayan Plateau (a collision between the Indian and Asian plates). Plates can also move apart from one another in an event called rifting. Rifting occurs beneath the Atlantic Ocean along mid-ocean ridges where the material from the mantle rises and punches a hole through the crust, allowing magma to well up to the surface, where it hardens to form new oceanic crust. As more magma rises up it pushes older rockers farther out from the ridges, creating new crust material. Oceanic crusts will eventually get pushed into subduction zones where old crust gets recycled into the mantle. This system of continental drift drives the movement of tectonic plates.

Areios has tectonic plates and despite the many differences in Terroan and Areiosan geology, the processes on both planets are analogous. Areios has roughly the same volume of land area covered by ocean as the Earth, but because Areios’ land area is larger, oceans make up a smaller percentage of the total land. This is important because water acts as a lubricant for subduction and this process that creates and destroys crust is responsible for the regulation of carbon dioxide. The biggest change in in the tectonic system from Earth to Areios is that the process on Areios happens much quicker, so it voraciously scrubs any carbon dioxide out of the atmosphere as quickly as it can be released from mid-ocean ridges. This makes it harder for photosynthesis to occur on Areios because CO2 is the fuel for photosynthesis. Along with the high levels of sulfur compounds that bleach chlorophyll and the lower levels of sunlight, photosynthesis doesn’t appear until later on in Areiosan history and the plant kingdom never arises on Areios at all. We’ll discuss how this omission impacts the biosphere later on, but for the time being it suffices to say that the animal kingdom on Areios would have to adapt to a world without green.

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