Stromatolites change the Earth

13th September 2012

OK, so I should warn you right now this post may be a bit geeky and may only be slightly, possibly exciting for geologists!  Today we went to have a look at some strange looking mounds rising out of the shallow waters right in the south of Shark Bay.  They are blobby, strangely pleasing little structures huddled together, but not exactly lively.  They are called stromatolites (or as Lisa calls them strolog-malagtites, which I think is a much better name) and you would be right in thinking they don’t have much going for them, they don’t move, they are dull dark brown coloured, and they probably only grow a few millimetres a year.  What could really be interesting about these little pads, about the size of foot-stools, made out of a surface layer of plant-like bacteria, that grow by sand sticking to them?  Well, they may not be much to look at, but they have been around on Earth for about three and a half billion years!  They can only live here because the water is too salty for snails and other things that would munch on them.

For two billion years this type of life dominated the Earth, and during this time they slowly changed it, without them animals could never have existed.  Each little cell takes in carbon dioxide and gives out oxygen as it stores energy from the sun.  Before these unassuming simple cells were around there was no oxygen in the atmosphere, and by the time they had come to the end of their world domination they pumped so much oxygen out it made up 20% of the atmosphere, pretty handy for us!

These little guys had a huge, but slow, impact on the Earth and are principally responsible for the massive iron deposits in the Pilbra, including the amazing formations we walked through in the gorges of Karijni National Park.  It seems strange that such small organisms could produce these massive deposits.  Before there was oxygen in the atmosphere there was lots of iron dissolved in the oceans, when this iron came in contact with the oxygen it formed iron oxide (rust) that doesn’t dissolve in water so it settled out to the sea bed.  Over millions of years all the iron in the oceans settled out, leaving the banded iron sediments on the ocean floor that have solidified into rock and make up the gorge walls we walked through.  The red bands were formed during times when lots of oxygen was produced and lots of iron oxide settled out of the oceans and the lighter bands were formed from sand or mud settling out after floods or times when there wasn’t so much oxygen around.

Thats why I think these rocks are pretty special, firstly they were formed about two and a half billion years ago, which is pretty old for most rocks and unreal in itself to imagine something being that old, they were laid down at the bottom of a shallow sea and you can still see ripple marks on some surfaces as if they were formed on a beach this morning.  On top of that they are a record of this amazing time on Earth when life was changing the atmosphere to the one that we know today and allowed animals to live.  Its pretty rare to be walking beside 2.5 billion year old rocks and then a few days later, looking over the coast at the stromatolites that created them.  Where else in the world can you do this?!

O

IMG 3451

Stromatolites in Shark Bay

The layered iron sediments at Karijini, Dales Gorge

2.5 billion year old layers of iron oxide and lighter sediments

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