Fossils on Friday – Lithification

Hello and welcome back to Fossils on Friday. Today we are continuing our sedimentary theme by talking about lithification. Lithification is the process of turning an organic material into rock, also known as fossilisation, and includes all the various ways of creating a fossil, including concretion and mineralisation.

Concretion is when a creature, normally a hard-shelled creature, gets buried and entombed with a hard ‘concrete’ material. This could be limestone, mudstone, or a number of other sediments that set hard. You could even do it yourself with plaster of Paris or cement – just drop a few shells in (make sure they have no occupants first!), allow it to set, and there you have it – a fossil shell!

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A concreted fossil trilobite cast from Porthmadog Wales, UK

Mineralization is when the organic material gets replaced by minerals, as they permeate the creature/plant. There are many different forms, including petrification (the replacement with silica), permineralization (the replacement with hard crystal minerals, such as calcite), and carbonisation (often found in plants, where the carbon is compressed out into the rock).

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A pyritised timber log, from Brook Bay, Isle of Wight

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You can see some of the timber’s structure, the yellow-gold material is the iron pyrite which has replaced some of the timber structure. Iron pyrite is made from iron and sulphur. While it is wet and compacted, it holds the timber together very well. Once it begins to dry out, however, it doesn’t only just crack, it also begins to turn the timber to dust.
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The pyrite captures amazing detail, including the tree rings inside the timber as well.
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Another example of lithification – permineralization, this time in the form of a dinosaur skull we uncovered on the Isle of Wight.
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The mineralization has preserved the bone really well, but earth movement and sliding after burial has caused multiple cracks to form in the skull itself. You have to be very careful when excavating.

These two overarching lithification forms cover all the smaller categories of fossilisation. We will pull out a couple of the more unique ways in the next blog post, as we have already covered all the major ones previously, starting HERE. We are also going to play detective as well, as we look at how different fossils can help us to work out how they were buried and fossilised as well!