New rocks from old

If this deposited material remains undisturbed, over time it may eventually solidify to form sedimentary rock. Similarly, some of the material that is dragged deep below the Earths surface by tectonic processes will reappear at the surface as igneous rock, recreated entirely by melting, or as metaniorphic rock, severely altered by the heat and pressure of deep burial but not remelted and so retaining some of its original nature. Lets look at each of these processes in turn, and examine the different rocks which result from them.

Sedimentary rock

For sedimentary deposits to be formed into solid rock, a cementing process called lithification must take place. In sandstone, the cement is typically iron oxide. Indeed, the silicon dioxide that makes up the grains is white – it is the iron oxide that makes the rock look yellow, brown or red, depending on how much of it is present. The iron oxide is deposited from water percolating through the sand once it is buried in the Earth.

But there are other processes at work in the process of lithification. One is the sheer pressure to which a sediment is subject as it is buried. This big squeeze is especially severe with muddy sediment. As it is compressed, the grains line up at right angles to the pressure. This is why shales and mudstones, rocks formed Irom mud, have visible layers which the mud does not have.

There are many kinds of sedimentary rock. Thinking of them in order of grain size is an intormative approach. The tinest-grained are the silts and clays. Here some geologist jargon comes into play. They will talk about sands, clays or silts when they mean hard lumps of rock made from these solt materials. In the same way among the volcanic rocks, they will describe a rock as a lava or an ash long after it has cooled and solidified.

The fact that such small-grain-size sediments can be deposited – and not washed away later – is virtually proof that the sediment formed in a low-energy environment. The larger-grained sandstones, by contrast, are made of sterner stuff, and come from a more energetic background. Sand itsell consists mainly of silicon dioxide, in its most common form, quartz. Quartz's simple composition and its presence in most rocks make it pretty much the ur-mineral in petrology, the science of rocks.

The sand dunes you see in pictures of the Sahara or other major deserts today are replicated in rocks from periods such as the Triassic, exposed across large areas of Britain. Careful fieldwork will reveal the direction and strength of the wind at the time they were deposited, and the type of rock that was eroded to form the sand.

Sandstones are common in the Earths crust because almost any rock, when it is eroded away, has the potential to produce sand. Shales are even commoner but because they are softer, they rarely form impressive cliffs or other exposures. Sandstones and shales are called clastic rocks, meaning that they are made up of clasts – the posh word for broken-dowrn pieces of other rock – which have shaken loose as weaker parts of the rock were eroded away and have now' been reassembled into new rock.

In honour of their colours, shales and sandstones usually appear on geological maps in shades of grey and yellow respectively. But on any map with a big swathe of sedimentary rocks, you will also see a lot of blue. This is the colour for limestone, which in the right light can indeed look blue and which is the main non-clastic sedimentary rock.

Limestone consists almost entirely of calcium carbonate in the form of calcite. It differs fundamentally from sandstone and shale because it is formed from living things. Limestones contain abundant fossils and it is possible to argue that they are almost fossils themselves. The most frequent fossils for them to contain are the hard parts of corals, which suggests that most have formed in warm, shallow water like those desirable holiday destinations which have coral reefs today. Other, much finer, limestones form from marine algae. Yet others are made from the shells (ooliths) of microscopic marine animals and are called oolites – pronounced oh-oh-light. Others consist mainly of larger pieces of shell. Very fine-grained limestones are called chalk, and ones with a substantial percentage of magnesium in their chemistry are called dolomite.

The grain sizes of limestones reflect their history. If coral has been allowed to die and settle to the seabed to be lithified, the result can be a rock with massive coral lumps, terrific for the head office of a bank. If the bits have been ground line to ooze, a tar-smaller-grain limestone is the result and a microscope is needed to determine the animals that make it up. (Yes, I appreciate that coral is a symbiont, not an animal, but you get the idea.)

These categories look clear enough on paper, but the Earth does not like things simple. Thus a limestone might be sandy, or a sandstone may be muddy, to the geologists eye. In addition, components that make up only a fraction of a rock often have significant structural importance. While iron oxide (hematite) is the main cement of sandstone, calcite is another chemical that sometimes takes on this role.

Other sedimentary rocks form because of dryness, not water. The evaporites are basically salts, formed when an area of water dries out, and later buried. The process can be seen in action today at the Gulf of Kara Bogaz in Turkmenistan, on the coast of the Caspian Sea. The Gulf is connected to the sea only by a small channel, and water evaporates faster off the Gulf than the channel can replace it, so fresh salt deposits build up. If you buy sea salt to put on your dinner, now is the time to stop. Normal salt just comes from older seas. The same process can also produce deposits of salts other than sodium chloride, such as nitrates and gypsum (calcium sulphate). If a lake dries out with no new water arriving, they precipitate out from the water in a set order reflecting their solubility.

Rocks from the fiery Earth

One important tip for would-be geologists is that you should refer to all sedimentary rocks as “soft” rocks, even if they are unbreakably hard. By contrast, igneous rocks are known in the trade as hard rocks, irrespective of their hardness. Perhaps in recognition of their deep and fiery origin, the scientists who work with them seem to have bragging rights over those who study material that might be regarded as solidilied mud.

Igneous rocks form by solidifying from a liquid state. Looked at down a microscope, they are fundamentally different from sedimentary rocks. Sedimentary rocks look like more or less rounded bits of older rocks, cemented together by some other material. Sometimes the bits are very alike, as with some sandstones, where the pieces are nearly the same size, similarly sharp or rounded according to the weathering they have had, and in some cases aligned by wind or water. In other sediments, formed in more chaotic settings, there can be a wide range of grain sizes from boulders to microscopic particles, thrown together and cemented.

But in a microscope slide of an igneous rock, something else altogether is on view. Imagine heating a chunk of rock. As it warms up, the crystals with the lowest melting point will melt first. Now imagine cooling the resulting liquid. The first crystals to emerge will be those with the highest melting point. So these crystals are the largest and most perfect. Squeezed into the spaces between them are crystals of all the other minerals that make up the rock. The last to emerge may have the atomic form of a crystal but will look nothing like your mental image of one. Instead it will be forced into an arbitrary-shaped gap between the earlier arrivals.

There are two main components to an igneous rock. The most significant are the “rock-forming minerals” These are the ones that appear in bulk, account for most of its mass and give it its properties. The rest are bit-part players. They can have economic value, as with many metal ores, or even gold and diamonds, but are present only in small amounts. The rock-forming minerals are mainly silicates, chemical compounds formed of silicon, oxygen and metals. On their own, oxygen and silicon add up to quartz. With sodium, potassium, calcium and aluminium, they yield feldspars, the commonest group after quartz. With magnesium and iron, the end product is olivine. Micas are minerals which form flat crystals. Amphiboles and pyroxenes are also silicate minerals, but form groups with more complex chemistry. To the expert, their composition is rich in data on the temperature and depth at which the
rock formed.

Igneous rocks can be categorized along two axes. The first describes where they formed and here the principle is nursery simple. If igneous rocks form at the Earths surface, they cool fast and have the smallest crystals. If they form deep down, they have bigger crystals because they have taken longer to cool, being surrounded by solid rock. Those that solidify near the surface or in smaller volumes cool at an intermediate rate with crystal sizes to match.

The second is the matter of how “acid” or “alkaline” the rock is. Lick a piece of lava (don't try this until it has solidified) and it will not seem either acidic or alkaline. This description really tells you how much quartz it contains. As quartz – the most common form of silicon dioxide – is the acid underlying silicates, more quartz means a more acid rock, while less means one that is more alkaline (or basic, essentially synonyms in this context). Because molten quartz is stickier than the other rock-forming minerals, having more quartz makes the rocks involved more gluey in their liquid state, as we shall see.

Starting at the top, the rocks formed at the Earth's surface are called lavas. They reflect the full range of chemistry within. The least acidic lavas are basalts, which contain little silica and are very liquid. In some parts of the Earth such as India they have formed massively thick deposits. So an alkaline volcano is the one to live next to. Better to avoid are those emitting andesite (a bit more acid) and rhvolite (the most acid). These sticky lavas produce volcanoes that erupt rarely but with extreme violence. Mount St Helens in the US is a case in point. The violence of such eruptions is enhanced if the lava contains a lot of gas, which expands as the pressure comes off it. In addition the gas can be poisonous (as with hydrogen sulphide) or might block the oxygen from reaching people and animals, as happens with carbon dioxide in incidents which have involved severe loss of life.

Once it reaches the surface, lava still has some options. It can flow along until it solidifies. Or it can be produced as ash, which can settle miles away. Rocks formed from the fusing of layers of volcanic ash are called ignimbrites.

In addition, lavas can be erupted under water, where they produce the distinctive shape of pillow lavas. Lavas that solidify from very gassy liquid can contain large volumes of holes and are termed pumice. And when lavas are cooled at very high speed, the solid that results can be a glass such as obsidian, not a crystal at all.

The biggest crystal sizes are encountered in rocks such as granite, a rock whose origin was a twentieth-century epic tale of geological science. It obviously cools deep in the Earth, as we can tell from its large crystals, and tends to be found in the exposed roots of big, old mountain chains. But it appears in such bulk that the question that needed answering was where the rocks had gone that must have had to make room for it. In a tale told beautifully in Richard Forteys The Earth, it is now known to be produced by the melting of crustal material at depth. Granite consists of at least 20 percent quartz. The most common coarse-grained rock with a less acid chemistry is called diorite. Again, nature laughs at these neat categories, so there are intermediates like granodiorites.

The rocks with medium grain size are far from being a point of detail. Many are formed when magma – liquid rock – is squeezed into lines of weakness between sedimentary rocks, where they crystallize out to form sills (more or less horizontal) or dykes (more or less vertical). Their alignment reveals the centres of action of old volcanic episodes, and they are also beautiful – see tor example the Whin Sill in northern England, which has produced such lovely sites as Holy Island. These medium-grained rocks have self-explanatory titles such as microgranite or microdiorite – same chemistry as granite or diorite, smaller grain size.

The Whin Sill in Northumbria. The Romans found it a handy barrier against barbarians - Hadrian's Wall runs along the top of it (see foreground).

Rock restructuring

It a mass of molten rock is thrust into a pre-existing sediment, there is bound to be a rude thermal awakening for the soft rock concerned. The changes that ensue produce the last category of rocks we shall look at, the metamorphics.

Metamorphism can happen on a number of scales. If a sill or dyke is injected into a sedimentary rock, the result can be a few metres or centimetres of cooking. But when large masses of rock are dragged to the roots of a continent, cubic kilometres can be remade at a time. For example, limestone hardens to marble (at least to the geologist – builders and architects use the term much more loosely), while shales turn into rocks called pelites. If they are not too heavily metamorphosed, these can retain the layered structure they had upon their original deposition. If it is possible to split them along these planes, the rock is slate.

The basic ingredients of metamorphism are temperature and pressure, but the way they work is not straightforward. On their own, these two forces can turn the crystal structure of a rock formed near the surface into a higher-pressure version. But if the stress is aligned in a specific direction, the crystals will realign in sympathy. Another metamorphic effect is the separating-out of the chemical components of a rock, to give a new rock with a banded appearance.

The new minerals that form under metamorphism reveal information about the temperature and depth at which the change occurred. The temperature can be anything from about 300 to 1000°C, and the pressure can be equivalent to depths as great as 50km below the Earths surface.

Most major types of sedimentary rock are found in metamorphic guise. The most highly metamorphosed are called gneiss (pronounced “nice”). These are the most banded, folded and coarse-grained of metamorphics, and it can sometimes be tough to work out what rock they are descended from. But igneous rocks are not guaranteed remission from metamorphosis. Their altered forms have names such as metagranite.

Don't forget that once rocks have been removed to these mind-numbing depths, they have to be pushed back up to the surface for us to see them. In the next chapter, on the deep Earth, we shall learn more about how this happens.