#ferricrete

Lava? Or maybe not...

Though it is dark, it has holes that resemble bubbles and has the shape similar to what you often see on the pictures from Hawai’i, believe me it is not lava. Actually, what you see originally was a shale, very contrasting to basalt lava which it resembles.

What you see is called ferricrete. Ferri-, refers to iron and -crete to concrete, as it consists of material cemented together by the iron oxide. It forms beneath the soil by accumulation of iron particles that first form nodules (reddish spots in the upper part of the profile) which slowly grow until they eventually join together to form something like this (lower profile).

This spectacular exposure is attributed to it proximity to the sea, as this beach side cliff is splashed on by the waves every day, which remove the clay that is among the nodules leaving this porous texture well exposed.

Step-by-step laterite

Not long ago I have shown you a ferricrete from Dee Why Beach in Sydney. After receiving many questions regarding its formation, I thought I give you a step by step insight into the process.

1) First, minerals breakdown to form clay and in the process release iron oxide (rust) which precipitates around some particle (organic matter, gravel, sand etc.) or forms a patch (photo 1).

2) This promotes further nucleation of iron oxide around that centre which results in growth and iron oxide nodule formation. With time as water sips through this soil profile more and more rust is added onto the nodule and so they grow bigger and bigger (photo 2).

3) Nodules eventually get so big that they start to touch and so they connect to each other and form horizon (broad horizontal chunk) as shown in the lower photo.

These form within soil under our feet when conditions are right, so you don’t usually get such 3D exposure. However, here are at the beach constant wave action has washed off all the dirt and clay leaving just the hard ferricrete behind.

P.S.

Ferricrete beneath the golf field.

Ferricrete forms by slow accumulation of iron element within the soil profile. It first forms small nodules that progressively grow bigger, forming a layer, as the one exposed here.