Wednesday, March 27, 2013

Flint: A rock made of life

Flint is one of the oldest materials used by man, shaped by knapping into an incredible variety of tools, used with iron pyrite as a fire lighter, or in early firearms. It is also found in many of England's old buildings, particularly Norman churches. The beginning of our long relationship with this rock is lost in the mists of time, but advanced tools date from at 1.75 million years ago when our ancestor Homo Erectus roamed the land. Southern England's pebbly beaches are mostly flint, with their magical tinkling noise as the waves of a North Sea storm roll in, carrying their cargoes of driftwood and the odd lump of amber floated over from the Baltic.

In Europe it is usually found within, or weathered out of, the upper Cretaceous chalk formation (deposited between 60-95 million years ago). This chalk started life as a white calcareous ooze at the bottom of a warm, shallow sea. It consists of the excreta of shrimp that ate plankton with calcium carbonate shells (called coccolithophores), and is therefore a form of very fine grained limestone with some mud and volcanic ash mixed in. Our earlier story on Beach head (link below) shows the stunning chalk cliffs found on Britain's southern coastline.

Within the chalk lie many horizons (horizontal beds) of flint nodules. Chert, another name for flint, is a mosaic of micro-crystalline quartz (crystals too small to see) and opal (amorphous silica). It is usually coloured black by organic matter/clay or orangey-brown by iron rich minerals such as haematite, and the nodules often have a thick white coating. These horizons have been used for stratigraphic correlation since Strata Smith's first geological maps in the late 18th century, and they reflect periods of silica rich deposition in the original sediments.

In our recent post on mineral evolution (link below) we discussed the importance of life/rock interactions in the creation of new minerals, here we have a similar tale about the origin of a rock. Flint formed in the complex processes of diagenesis, which transformed the ooze into the soft chalk we see today. The silica came from plankton shells and bits of sponges called spicules in the ooze. These partly dissolved in the water and were precipitated around or replaced a nucleus, often a fossil or worm burrow. It's thought that the silica was first deposited as a gel (see our story on opal, linked below), as it is often found filling sea urchin shells, creating perfect internal moulds. Over time the crystallinity increased as the gel was dissolved and re-precipitated.

The action happened some distance below the surface, where the layer of ooze containing oxygen from sea water met the anoxic layer below. Here bacteria called methanogens were busy eating by stripping oxygen from carbonate and sulphate, extracting the energy for their chemosynthetic lifestyle (some of the methane ends up in clathrates, link to recent story below). Others are feeding by decaying the organic matter in the ooze. The effect of this activity is to create complex gradients in water chemistry and acidity that end up first concentrating and then precipitating the dissolved silica into nodules. These grow layer by layer over a long time until the conditions change as the sediments are gently baked and pressed into rock.
Some scientists think the horizons correspond to radiolarian blooms linked to seasonal or other factors. Others suggest that they mark a pause in sedimentation, when sponges thrived on the sea floor. Flint formation is rarer in today's seas, which have been depleted in silica since the Eocene. The silica richness of Cretaceous seas came from the higher rate of hydrothermal activity in mid ocean spreading ridges as Pangaea broke up.

Flint is therefore made from life's remains, by the action of bacteria during the birth process of a rock. This energetic dance is good illustration of the Taoist concept of jijimuge, which means 'the interpenetration of all things'. We keep on discovering new layers to this concept as our understanding of Earth as a series of interconnected dynamic systems grows.

Image credit: Paul Frogatt

A chapter on the upper Cretaceous rocks of the UK:

You can read our piece on Beachy Head, one of England's chalk cliffs at:

on mineral evolution at:

about opal formation at:

methane clathrates at:

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