Rocks that have been melted and then solidified are known as igneous rocks. They can form underground, but also on the surface as a result of volcanic action. The earth is constantly changing, inside and out. Underground rock formations are constantly being transformed by high pressures and temperatures. On the surface too, the form of rocks is being changed all the time. Rocks are broken down by the heat of the sun, frost, flowing water, shifting ice and wind, and the pieces are transported elsewhere. Rocks become pebbles; pebbles become sand; even smaller grains are called clay. When the lumps, flakes and grains come to a halt, they form layers or ‘strata’. The process is called sedimentation.
Formed deep underground
A piece of granite is constructed of crystals. Granite is an igneous rock that forms deep underground and under great pressure as a result of a slow process of cooling and solidification. It is composed of a mixture of minerals. It may contain grey, glassy quartz crystals or pink and greyish white feldspars. These are the most commonly occurring minerals in the Earth’s crust. In addition, granite often contains small, shiny leaf-shaped fragments of mica, biotite or muscovite.
Fresh crust in the middle of the ocean
In the mid-oceans, the Earth splits apart. The fault lines where the Earth’s tectonic plates are moving apart are called the mid-ocean ridges. This is where the lava that wells up from the depths of the planet cools to become heavy, olivine-rich rocks like basalt. Deeper down, the cooling magma becomes gabbros. Because magma cools much more slowly under the ground than on the surface, the crystals in gabbros have time to grow quite large. In a gabbro, crystals of minerals like pyroxene, plagioclase, feldspar and sometimes olivine and amphibole can be seen with the naked eye.
Solidified lava
Volcanoes spew out molten rock in the form of lava. As lava cools, it forms extrusive rocks. Heavy, olivine-rich lava is known as basalt. Granitic lava is called rhyolite or porphyritic quartz. Porphyry is a collective term for igneous rocks with an uneven, grainy texture. Phenocrysts are larger crystals embedded in a more finely-grained groundmass of smaller crystals with the same chemical composition. Porphyritic quartzes contain quartz phenocrysts. As in the case of granite, the groundmass is composed of microscopic crystals of quartz, feldspar and micas.
Rocks eroding into sand
All rocks are eventually ground down into smaller pieces by the action of water and weather (temperature, rain and wind). We know the very smallest pieces as sand, silt and clay, and bigger pieces as gravel or stones. All these materials are eventually transported by water, wind or ice and deposited elsewhere. The process of deposition is called sedimentation. Rivers and seas deposit sand or clay in countless places. The process can continue for millions of years, building up layer upon layer. Here and there, layers may erode to expose a cross-section of strata. The material that has been eroded is deposited elsewhere. When sand is transformed deep under the ground and under great pressure into rock, we call the result sandstone.
Zeebodem als begraafplaats
Koraaldiertjes bouwen koraalriffen langs de kusten van tropische zeeën. Deze riffen bestaan uit kalk. Stormen en vloedgolven slaan deze riffen kapot. In de zee dwarrelen ook voortdurend kleine kalkdeeltjes en skeletjes naar de bodem. Ook zeelelies, verwanten van de zeesterren, maar dan met een stengel en een soort ‘bloem’ van kalk aan de top en schelpfragmenten komen daar regelmatig terecht. Als de kalkmodder op de zeebodem versteent, ontstaan lagen kalksteen waarin veel brokstukjes koraal en fossieltjes zitten.
Extreme makeover
Heavy pressure and heat can completely change the nature of rocks. Minerals are transformed or restructured. Existing rocks can change their composition and new minerals can develop. When mineralogical composition has been changed in this way, we call the result metamorphic rocks. The word metamorphic means changed. Marble is composed almost entirely of crystallised calcite. The original limestone or dolomite has been transformed by great pressure and high temperatures. In the process of metamorphosis, fossils have become unrecognisable.
Flink onder druk gezet
Gneis is ontstaan door een metamorfose van graniet of sedimentaire gesteenten. Er zijn gneissoorten waarin duidelijk te zien is dat de oorspronkelijke gesteenten flink zijn gekneed tijdens de vorming. Deze gesteenten vertonen lijnen en slierten en soms heel lang uitgerekte kristallen. Een mooi voorbeeld is de ogengneis. De witte ogen die erin zitten, zijn veldspaatkristallen die in diepe aardlagen in het gesteente zijn gedraaid en uit elkaar getrokken.