Clastic means dried and broken. Picture the case of a stream leaving behind a puddle of mud. It dries in the open sun. A sudden storm occurs causing flash flooding. The creek which passes by this mud gets scoured deeper and the mud is undercut and collapses and breaks up, the clastic pieces being carried a short distance before being dumped along with sand
grains to form a clastic sandstone. On the north headland at Bondi Beach, Sydney, the cross sections of the Upper Triassic Hawkesbury Sandstone have deposits of grey clasts in sandy channels scoured from the original soft sand deposits. It opens up a tiny picture into a world that is now 280 million years old.
2. An evaporite is simply a mineral that forms from aqueous solution. One that springs to mind is calcium sulphate, another is calcium carbonate. The natural minerals are gypsum and calcite. Evaporation means that there is some access to the atmosphere for the evaporation to occur and the excess water to be taken away. Water dissolves sulphur, oxygen and calcium. As the solution evaporates, the elements can combine and crystallise and can actually be scooped up by hand from shallow sandy deposits.
3.The continental igneous rocks can weather both chemically and physically. Their feldspars can chemically change into clays, which release quarts grains that can be deposited by water from rain into beds which ultimately form sandstones. Finer grains can form siltstones, mudstones, claystones, etc. The state of the earth's crust is in in constant change. Sedimentary rocks also get weathered and redeposited. Igneous rocks from new eruptions can intrude through sedimentary rocks. The net result is that the sediments form a veneer on the original igneous rocks. But that WAS the case 4,500 million years ago. Sometimes, the sedmentary deposit gets laid over a large area, which is perfectly normal, and it just keeps on being deposited. In order for this to happen the deposit area must be sinking. This sinking develops into a geosyncline, and the sediments will continue to accumulate for a depth of eight or more kilometres. At this depth, natural heat from within the earth and the pressure of all the mass above will convert those deep sediments into metamorphic rocks, schist, slate, gniess.
This is a process that will take many millions of years to develop and another five million to get carried out. Any Longer, say another five million, then that gniess can be changed back to a granitic crystalline mush will harden when sedimentation above stops and erosion from above reduces the load. This enables the granite mass to start to become uplifted to surface, give another ten million years.
Fantastic, isn't it? This process has been happening and reoccurring everywhere all over the earth's surface. And still is going on.
4. Sedimentary rocks are typically identifiable because they occur in stratified beds. A fracture surface on a sedimentary rock will not have the shining, glistening crystalline appearance of an igneous rock. But the geologist recognises a sediment as aindeed any rock type by the particular accumulation of minerals it contains.
5. When sand grains get carried along in a strong stream current, they will get dumped when that current loses energy. i.e. it enters a lagoon or lake. Also carried along with the current are minute grains of clay, most of which get carried along in the current when the sand has been dumped. It will slowly settle amongst the sand grains and even from thicker lens of its own deposit in the centre of the lagoon. But it is this clay which bonds the sand grains together over a very long time to form a solid sandstone rock. In fact, it forms a cement which bonds the particles together. This process, lithification, which is called after the Greek word "lithos" meaning rock, also takes a long period of geological time to complete, but a sandstone is more weather resistant than is solid granite.
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