Plate Tectonic cycles have been happening on Earth as we know it today – with all its features – for around 3 billion years. What we currently see is just part of a cycle – a snapshot if you like – where we can see all of the features that have been created, destroyed, and recreated in the past.
What’s the evidence for Plate Tectonic cycles?
Geoscientists have recognized by studying the rocks on continents that there is evidence that plates have collided in the past – mostly through things such as:
- Ocean sediments, many containing fossils, that are on land and way above sea level
- Folded sediments that have undergone vast pressures.
- Large bodies of plutonic igneous rocks forming the cores of ancient mountain ranges.
- The remains of ocean crust (seafloor basalts) that have been folded, squashed, and welded between continental crust.
They have shown that even the oldest ocean floor crust is only around 260 million years old…and so what has happened to ocean crust for the billions of years before that?
Also by studying the alignment of small magnetic crystals that lined up with Earth’s magnetic field as molten rocks cooled, scientists have been able to work out the original location of those rocks when they formed. If you like, those crystals show us the past location of continents! As we can also date those igneous rocks, we end up with a load of evidence for where and when continents were positioned around the Earth in the past.
Based on this evidence, geoscientists have worked out that Plate Tectonics goes through a number of ‘cycles’ where continental masses have been split apart and then brought back together again.
These cycles have been described as supercycles – and have been divided up into the Supercontinent cycles and Ocean basin (Wilson) cycles)
Supercontinents and their cycles.
This term is given to those times in the geological past when all the continents have been joined together to form one large continental mass. This happens because continental crust is buoyant and resists being ‘recycled’ in subduction zones (places where the crust is forced down into the Earth’s mantle and remelted).
Evidence shows that there have been at least two times in the geological past where all the continents have been joined into a supercontinent. The first that we have evidence for existed from around 3636 to 2803 million years ago. This supercontinent is named Vaalbarra and it broke up and the continents moved around on their plates for around 100 million years. The continents all collided again and formed a new supercontinent around 2720 to 2450 million years ago. This one is called Kenorland.
Blocks of Kenorland broke off and then collided to form a chuck of continental rocks around 1820 million years ago that we call Nuna (some call it Columbia) – and this was the start of the formation of the North American/North Europe landmass.
Other blocks then collided with Nuna and eventually the landmass called Rodina was formed.
Then around 750 million years ago Rodina started to break up into three pieces – Laurasia, Gondwana and a smaller Congo craton. Around 100 million years later, Laurasia and Gondwana collided to form the supercontinent we call Pangaea.
Smaller continental blocks were also moving around the planet, occasionally colliding with the larger landmasses to form mountain ranges that we still see today – such as the Appalachians and the Alps.
Then 215 million years ago Pangaea started to split up…and continues to do so to this very day!
Ocean basin cycles – The Wilson Cycle
The Wilson Cycle, developed by Tuzo Wilson in 1968, refers to the opening and closing of ocean basins – rather than the movement of supercontinents – but must be related. It defines six stages of oceans;
(1) The rifting apart of a continent (Embryonic Ocean);
(4) The starting of a new subduction zone (Declining Ocean);