Onion Weathering at Causeway EarthCache

The Earthcache

           ► Local geological context

The geology of the Antrim Plateau is dominated by important igneous rock areas.

Around 60 Ma (in early Tertiary times), great masses of molten rock were rising from the depth of the earth’s mantle.
These nation-sized ‘lava-lamps’ are collectively considered to be a ‘hot spot’, now known as the ‘Iceland Plume’.
They split the earth-wide continent of Pangaea apart in great cracks that were aligned roughly northwest to southeast. This was sufficient to split the land apart on a vast scale, beginning the opening of the Atlantic Ocean and creating an increasingly wide gulf between, on the one side, the Americas and, on the other, Europe/Africa.

The rising magma spreads over great areas of what is now Northern Ireland, Scotland and the Faroes.

           ► Characterization of magmatic rocks

The mantle of the Earth is very hot, but it remains solid because it’s under intense pressure. Sometimes, though, the pressure may ease. When this happens, the mantle melts and becomes magma, or hot, liquid rock.

During techtonic events, mantle lava can enter the Earth's crust; it will then form two types of solid magmatic rocks, of identical composition but different crystallization according to their formation conditions.

- On the one hand, the magma can remain trapped in depth, it cools slowly.
This gives crystalline rocks very hard and very grainy, usually light in color (we speak of leucocratic or mesocratic rocks) like granite.
They are called intrusive plutonic rocks because they form beneath the ground.

- On the other hand, cracks or fissures form to the surface, creating paths to liquid magma through the crust.
The rock will then rise to the surface (whether in the open air or under water) and spread over large areas. With this sudden drop in pressure and temperature between the mantle and the surface, the liquid rock will crystallize very rapidly, forming a very hard and ungreened rock (crystal size invisible to the naked eye), basalt.
Basalt is a melanocratic rock with a holomelanocratic (dark to very dark) microlithic structure.
Mainly bluish in color, it sometimes shows more green colorations due to the presence of olivine.

The interior of a basalt flow consists of a superposition of sub-layers, sometimes separated by accumulations of small bubbles.

During cooling, the "withdrawal slots" (more or less flat and vertical breaks) and the boundaries between the flow units (rather horizontal zones of weakness) broke up the flow into roughly parallelepipedal blocks.

On this outcropping, breaks (or diaclases) appear as slits when they intersect a side of the trench and as bumpy planes when they are parallel.

In humid climates, the infiltrated water follows all these plans and wets the rock for long periods. Slowly, hydrochemical processes alter the lava in contact with water and "rot". The intersections of the diaclases (vertices and edges of the parallelepipeds) are the most affected areas, the corners round out.

The alteration continues towards the center of the blocks and gives scales ("onion peels") which delimit "balls" and giving the appearance of onion layers..

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