Can a greenstone belt be used as a key to the past?

Greenstone belts record the Earths early evolution. They are geological structures of volcanic and sedimentary rock deposited on the surface long ago. The volcanic rocks have over time transformed into green schist which gives these rocks its characteristic colour. By studying the green stone belts of northern Fennoscandia we can find a more than 500 million year long history.

The oldest rocks found in northern Fennoscandia are rocks of Archaean age. In the Archaean eon (4 to 2.5 billion years ago) the earth had cooled enough to form a crust. Most of the Fennoscandian crust was created 3.0 to 2.8 billion years ago. After that nothing really happened for the next 400 million years.

The continent started to break apart 2.45 billion years ago. Magma ascended through the crust, resulting in a catastrophic volcanic event covering most of the continent with flood basalts. The remains of these volcanic rocks are found at the base of our greenstone belts and define the start of their record. This continued for the next 40 million years until the continent stopped breaking apart.

The crust below remained stable for the next 200 million years, but the surface went through several dramatic episodes. 2.4 billion years ago, cyanobacteria started to alter the atmosphere by producing oxygen. The change to an oxygen rich atmosphere led to a dramatic drop in global temperature and the earth turned into a gigantic snowball. The increase in oxygen led to our first known mass extinction of almost all anaerobic lifeforms. There is no direct record of this mass extinction in our greenstones, but we can find glacial sediments from this period.

Pillow lava found outside Lakselv, northern Norway. The rock is the remains of a 2.2 billion year old submarine volcano.

The continents started to drift apart again 2.2 billion years ago and the Kola Ocean started to form. The new shallow ocean was ideal for Paleoproterozoic lifeforms, which built large carbonate banks that we now can find as marble. The volcanic activity increased along with the rifting. Large submarine volcanoes were depositing thick units of pillow lava on the ocean floor and hydrothermal systems poured out sulphur rich water that forced most lifeforms to more peaceful parts of the planet.

Around 2.0 billion years ago the surrounding continents started to move towards each other and the Kola Ocean started to close. As the surrounding land mass squeezed together, the ocean floor started to deform by folding and faulting. Eventually, the ocean disappeared, and the remains were incorporated into the mountain chains that formed as the continents collided.

Today, the ancient mountain ranges that were towering the northern Fennoscandia are long gone. But, for those who are willing to search, the history of a young earth still lay hidden in their old roots.

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Harald Hansen

PhD candidate working precambrian rocks and tectonics at UiT The Arctic University of Tromsø.

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