How to Make a Craton

by Donald L. Blanchard

 

       
Cratons, sometimes known as shields, are sections of continents that are composed of very old metamorphic rocks, some of them - like the Canadian Shield - contain some of the oldest rocks known anywhere on earth - up to 3.5 billion years old or more. They are usually low in elevation, not rising far above sea level, and are relatively flat, having little vertical relief. Cratons form the cores of several continents, the Canadian Shield in North America, the Angara Shield in central Siberia, and the Baltic Shield in northern Europe being conspicuous examples. As they usually comprise the oldest rocks known, cratons are generally taken as the starting point in any discussion of continent formation. This author has seen little discussion of how these ancient structures came to be. The following thought exercise represents the author's thoughts on what came before where the usual discussions begin.

Start with a range of mountains. Let us make them be really big mountains, with peaks of 20,000 to 27,000 feet. Between the peaks are valleys, canyons, gorges. Place these mountains in the middle of an ocean having a depth of 13,000 feet - the average depth, curiously, of the abyssal plains of today's oceans. Because these are mountains of our imagination, we can imaging that their average elevation - the elevation if all the peaks were shaved off and deposited in the valleys to form a flat, level plateau - is 15,000 feet above sea level. Stipulate that these mountains are in isostatic equilibrium, floating in the viscous fluid of the earth's mantle. Further stipulate that the mountains are made up of typical continental rock, having a specific gravity of 2.7. (The specific gravity of the earth's mantle is approximately 3.3, and the specific gravity of the ocean is essentially 1.)

To maintain isostasy, the forces pushing down from above the fluid's surface - in this case the ocean floor - must equal the forces pushing up from below, caused by the displacement of denser mantle material by the less dense continental rock of the mountains' roots, according to the following formula:

13,000 feet (s.g. of continent - s.g. of water) + 15,000 feet (s.g. of continent)
= thickness of roots (s.g. of mantle - s.g. of continent)
or
13,000 (2.7 - 1) + 15,000 (2.7) = x (3.3 - 2.7)

(Note that the weight of the first 13,000 feet of continental material above the sea floor is buoyed by the weight of the water around it.) Thus, x (the thickness of the mountain's roots below the sea floor) equals 104,333 feet, and our total mountain range averages 104,333 + 13,000 + 15,000, or 132,333 feet thick. Now, make sure that no other continents are going to get in the way, then go away for a billion or a billion and a half years and let erosion run its course.

Sediments eroded off the mountains will be carried over the edge of the mountains' shore and deposited on the 13,000 foot deep ocean floor. The weight of the accumulating sedimentary deposits will push the deeper sediments down into the mantle, always maintaining isostatic equilibrium (or trying to). When the sediments reach a thickness of around 50,000 feet

( = (13,000 (2.7 - 1) / (3.3 - 2.7)) + 13,000 )

the surface of those sediments will be at sea level, and additional sediments will be washed over them and farther out to sea. As the mountain peaks erode away, isostasy will push the mountains' roots upward, presenting new land to erode, until only the lowest 50,000 feet of the original mountains remains, and whose surface is essentially at sea level as well, forming a peneplain. The roots of the mountains will be sitting in the middle of a flat, nearly sea level continent, ringed by flat plains of horizontal sedimentary deposits. While the total volume of continental material remains the same, the surface area will have increased by 2.65 times.

Now, put a subduction zone along side the flat sedimentary plain, with the ocean floor of the neighboring plate's ocean floor sliding down into a trench, and place a large continent on the plate behind it, taking dead aim at our recently peneplained continent. Debris and sediments accumulated on the neighboring plate's sea floor will be scraped off by the trench and form a ridge along the peneplained continent's margin, forming the beginning of a mountain range. Volcanoes will form within or behind that ridge as the subducting plate melts and its lighter and more volatile fractions rise to the surface. Eventually, the intervening sea floor will be consumed into the trench, and the invading continent will begin to press against the edge of our peneplained continent. The oceanic sediments that accumulated within the trench will be pasted onto the edge of the peneplained continent, probably to be converted into graywacke, and that section of the ocean floor that inclined downward into the trench will be sandwiched between the plates as an ophiolite, now identified as the signature of a continent to continent collision. Continued pressure by the invading continent will cause the sedimentary plain behind the volcanic ridge, which is made up entirely of soft sandstones and shales, to buckle into rather neat accordion-like folds, creating a mountain range, probably consisting of several parallel ridges, with peaks of, potentially, 20,000 feet or more. The roots of those mountains will be pushed down into the mantle to depths approaching 100,000 feet, where the heat and pressure is great enough to partially melt and deform the layers of sandstone and shale.

When all the sediments have been folded, right back to the edge of our original mountains' roots, remove the lateral pressure between the two original continents and allow them to move as a single continent. Then sit back and watch for five or six hundred million years. The soft sediments at the top of our mountains will erode away quickly, forming transverse parallel ridges with long valleys in between. As the mountain tops are removed, isostasy will push the roots upward, presenting new ridges to be eroded, until the partially melted and deformed sediments from deep within the mantle rise and cool, hardening into the metamorphic rocks, schist and gneiss. Allow the surface to continue eroding until it too becomes a peneplain, and you have a metamorphic craton, with rocks approaching a billion years old (our new mountains) sitting along side older rocks at least two billion years old (the roots of our original mountains).

What is today a 3.5 billion year old craton was, 3.5 billion years ago, the roots of a major mountain range, indicating that orogeny driven by plate tectonics was already occurring well before 3.5 billion years ago. It is also interesting to note that isostasy calculations for a 13,000 foot ocean depth (the average depth of the abyssal plains of all major oceans today) yields a 50,000 foot maximum thickness for ocean deposited sediments. In many places on the earth, geologists have documented 50,000 feet of horizontally laid marine sediments, in some cases in formations as much as a billion years old, but nowhere has this author seen any reports of marine sedimentary deposits greater than 50,000 feet. This suggests that the ocean's depth has remained relatively constant, varying by only a few hundred feet during glacial cycles, for at least the last billion years, implying that the total volume of continental material on earth and the total volume of ocean water have remained essentially unchanged as well.

 

File created January 2006.