Today we learned from Sheldon Cooper,
“Silicon — one of the most common elements in the Earth’s crust”
True. But “one of”? Silicon is merely the second most common element in the Earth’s crust, weighing in at 28%. We are here to resolve the suspense. What is actually the most common element in the Earth’s crust?
Oxygen makes up nearly half (47%) of the Earth’s crust by weight. Not oxygen the gas, but oxygen in the form of silicates, that is minerals with an atom of silicon bound tightly to oxygen atoms, constituting a long list of minerals found in the Earth’s crust. Content in knowing this fact, you may be forgiven the feeling of walking on air.
For those of us that live and breathe on Earth we are accustomed to finding our oxygen in the atmosphere. But that is a relatively late arrival. Oxygen came to earth mostly already bound up in minerals and and in water. Even as early life formed, any excess oxygen produced by early photosynthesis such as by cyanobacteria would quickly bond to other minerals it would encounter, leaving the atmosphere mostly free of oxygen. Only about 2.5 billion years ago did photosynthesis produce oxygen faster than it could be absorbed, causing the fraction of free oxygen to rise, known as “Great Oxygenation Event”. Within the last billion years, oxygen had little place to go and its portion of the atmosphere increase even further. Oxygen in the atmosphere has only been common for the last half of the Earth’s history.
So what is this crust? Originally the Earth formed from the coalescing debris of the solar system. Dense elements, metals such as iron, fell to the center, or core while less dense, rocky materials floated on top of it. The very surface, just the outer 3-6 miles of the land below the oceans, or outer 10-60 miles of continent, have a particularly low density compared to the rest of the Earth. That may sound deep but the outer skin of an apple is relatively a bit thicker than most of the crust is to the size of the Earth.
The only reason geologists define a crust at all is that there is a clear separation in density between it and the denser rock in the region below it, called the mantle. The separation of this region form the denser rocky mantle below is called the Moho. So named probably because “Moho” is easier to say than “the Mohorovičić discontinuity“. In 1909, a Croatian geologist, Andrija Mohorovičić, was studying the speed of and direction of seismic waves and deduced that such a division must exist.
So if so thin, why not drill down and go look for the transition? Over fifty years ago, geologists thought this could shed light on the controversy if continental drift existed. (Nowadays, the largest arrays of radio telescopes can not only detect continents moving within about a week, they will not work if they do not correct for it.) Such a deep hole was the grand aim of one of the world’s first “Big Science” projects, “Project Mohole“. Starting in 1961, geologists found one of the thinnest spots in the Earth’s crust, from a floating platform in the Ocean off of Mexico, and started drilling.
The geologists never made it past 601 feet before the project was canceled. Winning a battle of the cold war, subsequently the Soviets managed to drill down nearly 7 miles on land creating the Kola Superdeep Borehole. But since this was on the continental shelf, it was nowhere deep enough to reach the Moho. The Soviet record was later beaten by a whole 100 feet, but for good reason: to extract oil from the Persian Gulf.
A Japanese project Chikyū aims to drill in a thin spot in the ocean where the mantle could be just over 4 miles below the surface. Their drill rig was damaged by an Earthquake and the project was delayed.
To this day no scientific effort has reached the Moho. It would be a lot easier if all this oxygen were not bound to silicon. But then we would have other problems. Let’s get our moho workin’.