Plate Tectonics

Why are plate tectonics - the gradual sliding and shuffling of Earth's continents - important for intelligent, land-based life?

Well, first of all, consider erosion - the effect of water on land.  Water is constantly breaking down soil and rock,  then washing it into the rivers, lakes, and oceans.  It's generally a slow process.

But even a slow process can have giant effects given enough time.  And over billions of years, the vast amounts of water on our planets would erode the continents away entirely, until the earth was a near uniform flat surface, entirely covered in dirty salt water.  Not a very nice place to raise the kids.  Plate tectonics counteracts the problem of erosion by putting constant, minute pressure on the continents, which over time builds mountains and land mass, keeping the Earth's land/ocean surface ratio at a near constant rate, counteracting the constant erosion with constant uplift.^[1]

Another important part of plate tectonics is in the Earth's carbon cycle.  CO₂, a greenhouse (heat trapping)  gas, is dissolved into the oceans over time.  If too much were absorbed by the ocean in relation to the amount being produced on land, the planet's temperature would drop dramatically.  On the flipside, if the ocean's couldn't absorb anymore because they lacked a means to release it and it was still being produced on land, the temperature would rise dramatically.  Because of Plate Tectonics, however, the carbon dioxide is removed at subduction margins^[1][2] and later released through hot springs and volcanoes.  The Earth does, of course, experience cyclical weather changes, but never to the extreme that all life perishes.

Another reason plate tectonics help life along is because without them, the crust would have no way of releasing built-up heat.  Radioactive decay and other processes inside the core and mantle of our planet produce copious amounts of heat, and therefore pressure.  This pressure, and heat, are relieved through hydrothermal vents and volcanoes, both of which form in areas where the plates scrape against each other.  It is a continuous, slow, process - fortunately for us.

If we had no plate tectonics, this heat and pressure would have nowhere to go, and would continue to build over hundreds of millions of years, until one of two things happened.

Either (a), the Earth would form massive, stacked volcanoes like Olympus Mons on Mars, that would spew ridiculous amounts of carbon dioxide, methane, and water vapor - all greenhouse gases that would wreck havoc with our climate - as well as ash and soot, that would block out sunlight, rendering photosynthesis impossible, killing most plant life, further interrupting the carbon cycle, and keeping more carbon dioxide in the atmosphere, creating even more problems with the climate, or (b), the entire surface of the Earth would resurface itself like Venus has in the past.  Either outcome is devastating for life, and option (b) would likely destroy all life, even microbial.

Lastly, plate tectonics have played an important evolutionary role in Earth's history, by isolating different population segments of the same animals, and putting selective pressure on those populations because of climate and habitat change, both for land and ocean dwelling creatures.  It's not a stretch to say that evolution would've taken an entirely different course had it not been for the break-up of super continents like Pangea, and the migration of the hospitable continents to different temperate zones.

1. - http://www.lifeinuniverse.org/noflash/Platetectonics-05-02-01.html

2. - http://www.spacedaily.com/news/life-01x1.html