WEATHERING

Frost Shattering Frost shattering soon reduces bedrock to a litter of angular rock debris. Here 3000 feet (950m) up Beinn Eighe in Wester Ross the quartzite is exposed to frequent cycles of freeze-thaw action. Water penetrates into the cracks and joints of the bedrock, freezes, expands and prises the rock apart. The frost-shattered rock fragments then fall or slide downslope the produce scree. scree - (frost shattered debris) quartzite

Here in Coire Mhic Fhearcair at the back of Beinn Eighe lies the ‘Three Buttresses’, a huge cliff of Torridonian Sandstone. The rock is strongly bedded and jointed, which makes it susceptible to freeze-thaw action. Below the cliff are three talus cones (talus is the correct term for scree) which have formed where frost-shattered debris has accumulated below gullies in the cliff face. A B C

Weathering & Rock Hardness Quartz is the hardest common mineral there is. Rock formed almost exclusively of quartz is called quartzite. Yet here in Beinn Vair and in Beinn Eighe the white quartzite has been reduced to rubble by frost shattering. This is because of the numerous fine cracks that run through quartzite, along which water can penetrate and freeze. Weathering & Rock Hardness Beinn Eighe Beinn Vair Glencoe Liathach Beinn Alligin

Chemical Weathering If rock hardness plays little part in affecting rates of physical weathering, the same is equally true of chemical weathering. Here, the rock is dolerite to the right and basalt below. Both basic igneous rocks are extremely hard to break with a hammer. Yet chemical breakdown of the minerals in the rock rapidly reduces it to a soft gritty mush along the joints, leaving core stones. The most important factor here is the chemical composition of the minerals within the rock. In dolerite or basalt the main minerals include two Fe-Mg rich minerals, olivine and pyroxene, both of which break down rapidly through oxidation, hydration and hydrolysis. The weathered rock spalls off like skins of an onion.

Chemical Weathering of Limestone Limestone is mainly composed of calcite (a calcium carbonate mineral). Rainwater picks up CO2 from the atmosphere to form weak carbonic acid. The carbonic acid reacts with the calcium carbonate in the limestone to produce highly soluble calcium bicarbonate which the rainwater removes in solution. Solutional weathering is greatest where the limestone is jointed and over time forms grikes. The joint-free rock is left upstanding as clints. The whole area of clints and grikes is called limestone pavement. grike clint limestone pavement

Chemical Weathering of Limestone (contd.) The solutional weathering of limestone continues deep underground as well. Here at Malham Tarn the stream disappears down a swallow hole and flows through a huge cavern to re-appear almost two miles downstream. The top photo shows the swallow hole of Gaping Gill and the photo opposite is of the cavern directly below Gaping Gill.

A B Biological Weathering In biological weathering plants or animals are responsible. Tree roots prise rock apart (Photo A) and humic acids from rotting vegetation act chemically on the rock beneath (Photo B). Tree roots prising rock apart Humic acids from decaying plant material leach down to rock below A B