Most know about continental drift. What not as many know about, is the greater plate tectonics. The Earth’s crust is divided in 8 – 10 pieces at least the size of continents. (Two of the pieces are on the way to split in two each.) In addition there are some smaller ones which number is uncertain. The point is them moving in relation to each other. The pace they move by is counted in centimetres (or inches) per year. These movements cause most earthquakes and volcanic eruptions. Otherwise they are not noticeable over historical times.
In order to understand plate tectonics one has to understand the seafloors. Outside many coasts there are areas less than 200 meters (656 feet) deep. These are called continental shelves and geologically consist a continuation of adjacent land areas. Their border zones to deeper seas are called continental margins. Deeper parts of the seas consist of ocean floor. Most of the oceans are 3 – 6 kilometres (9,800 – 19,700 feet) deep. They are crossed by mountain chains called mid-ocean ridges. There new ocean floor forms and spreads out from there. When it is pressed down into the interior of the Earth elongated oceanic trenches form. They are 6 – 10 kilometres (19,700 – 32,800 feet) deep. The deepest ocean trench is circa 11 kilometres (36,000 feet) deep.
Ocean floors consist of rock types which are relatively heavy. The continents and continental shelves have a layer of lighter rock lying on top of it. Same applies to islands lying on the continental shelves. Some other larger islands have this extra layer. Since this layer is lighter it will always be higher. It never gets pressed down into the interior of the Earth as the ocean floors do. Instead, the amount of lighter rock has accumulated trough thousands of millions of years.
When the Earth’s crust first solidified it was only ocean floor. Within 200 million years water vapour condensed into seas. This time period was long enough it did not have to be pouring down all the time. The sea still came to cover the Earth which yet lacked continents. There is uncertainty on how plate tectonics stated. For certain there were originally just volcanic islands. Even if they did not move lighter rock could form. As the volcano island got bigger the lowest parts were pressed down into the interior of the Earth and melted. The lightest minerals were the ones melting first. Such ones were transported up by volcanoes and settled on top of heavier rock. It was possible for large meteoritic impacts to partially melt the Earth’s crust. When it later again solidified lighter minerals settled on top. Both processes gave rise to small areas of lighter rock.
When plate tectonics was well underway considerably larger areas of lighter rock could form. Older ocean floor was pressed down under younger in ocean trenches. When longer pieces melted down more of lighter minerals could rise. Long volcanic island chains formed consisting of lighter rock. Moreover, the Earth’s mobile crust meant islands could collide with each other. Volcanic island chains fused not only with each other. They fused with smaller areas of lighter rock formed earlier. Over the course of up to 2,000 million years the cratons formed this way. These now consists the cores of the continents.
It has been suggested plate tectonics was originally not as reliable. That would explain why so little other than the volcanic cratons have survived until today. People talk about the “boring billion” when not much seem to have happened. This would then have been 1,800 – 800 million years ago. Rock layers seem to be almost entirely missing from this time period. I think it is several lost eras which people have mixed up. With patchy or slower plate tectonics erosion would have gotten more time. Most sedimentary rock types would then have eroded away.
Today’s plate tectonics have existed for at least 800 million years. Not only have volcanic mountain chains added to the continents. When they eroded sediments have ended up on continental margins and extended the continents. Also, life has contributed to the continents’ build-up. Some organisms form shells from calcium dissolved in water. When they die their shells can accumulate on the bottoms of lakes and seas. If they don’t erode away first these shells give rise to limestone.
All mountain chains are not volcanic. Many have formed when areas of lighter rock have collided. These are called fold-mountains since the rock gets crumpled up. Such mountain chains consist mostly of sedimentary rock types. When the rock stops being squeezed together it can only be eroded down. Hundreds of millions of years old mountain chains only remain as spread-out groups of tall hills and rocks. Parts of them can disappear entirely. All today’s tall, step mountains are at most a few tens of millions of years old.
Finally, one has to remember continents can split too. This is why the west coast of Africa matches so well with the east coast of South America. Africa is now on the way of splitting creating its highland in the east. This is dotted with volcanoes forming proper mountains. However, not all cracks do develop into great oceans. When South Asia splintered of from Africa Madagascar originally followed. Later a new splintering formed between Madagascar and South Asia. The splintering between Africa and Madagascar then stopped to extend. The result is a wide strait no longer growing. New Zeeland has been splintered from Australia in a comparable way.
Uploaded on the 28th of June 2024.
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