Background
rocks
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WHAT ARE ROCKS ?
It may seem a slightly odd question given we are surrounded by so many of them, but what exactly are rocks ? Rocks are made up of interlocking minerals in the form of grains or as the cement between grains. The exact types of minerals present in a rock and the ratio of each mineral to the others in the rock, is largely what creates the fundamental difference between different rocks. Although many thousands of minerals exist, only around 30 or so might be described as common. Familiar examples would be quartz, feldspar, mica, and calcite. So that begs the question, "what are minerals" ? Minerals are naturally occurring materials made up of specific ratios of certain elements, arranged in preferred shapes. For example, quartz, a simple mineral, is composed of silicon and oxygen atoms always in the ratio 1:2 (SiO2 for short). Imagine say two yellow lego blocks (representing oxygen) and one blue block (representing silicon) attached to each other in a fixed way, and then using lots of those units to build a larger specific structure (a crystal of quartz). A more complex example would be something like a mineral called orthoclase feldspar which has 8 oxygen atoms, 3 silicon atoms, 1 aluminium atom, and 1 potassium atom joined together to form the basic lego 'unit', and those units then built up in structured way to form feldspar-shaped crystals. Given enough space and time, minerals can exist as quite dramatic sized crystals of the kind you find in museums and gift shops. More commonly however, in most rocks they grow crowded together without much time and freedom to develop their full potential, so end up as much less dramatic, stunted grains. The photo opposite, taken of my kitchen top (!), illustrates a typical arrangement of quartz, feldspar, amphibole and mica minerals in a common type of granite. Granites have grains of a size visible to the naked eye or with a small hand lens. Other rocks have very small grain sizes only distinguishable with a powerful microscope. So that's what rocks are made of, but how did they form ? Rocks are classically divided into three broad groups - igneous, sedimentary and metamorphic. They are inter-related via a so-called "rock cycle" as shown opposite. IGNEOUS ROCKS Igneous rocks (meaning "from fire") are formed when molten rock (magma) cools and solidifies. A variety of minerals precipitate out of the liquid magma as it cools and the minerals weld together as they crystallise, thus forming a solid rock. Which minerals crystallise are determined by the chemical composition of the original melt and the temperature/pressure conditions under which the cooling occurs. Igneous rocks from magmas relatively rich in magnesium, iron and calcium are termed "mafic' and precipitate relatively dark coloured minerals (black, dark gray, dark green). Igneous rocks from magmas relatively rich in silicon, potassium and sodium minerals are termed "felsic" and contain more minerals of lighter colours (white, light grey, pink) - the granite in the photo above has a felsic composition. Rocks from a magma of intermediate composition have a mix of felsic and mafic composition and are termed, strangely enough, "intermediate". The size of the mineral crystals (or 'grains') that precipitate from the magma are determined by the speed of cooling - remember those old school experiments designed to grow sodium chloride crystals from a salt water solution). Magmas that erupt at surface into water or air (called "extrusive") cool very quickly and have grains so small that they are too small to be distinguished by the human eye. Magmas that cool slowly, deep in the Crust ("intrusive") precipitate coarser grained minerals, and individual minerals are visible with the naked eye - granite is an example of a slowly cooled, intrusive rock - hence the easy visibility of the grains in the photo above. So there is a continuos matrix of igneous rocks, ranging from felsic to mafic in composition, and fine to coarse grained in texture. The composition of the magma and the speed of cooling is typically determined by the plate tectonic setting in which they are generated. For example, the lavas erupting from the world's mid-ocean ridges come from very mafic magmas produced by melting the rock type in the upper Mantle (called 'peridotite'). The erupting lava cools underwater very rapidly, producing a black-dark gray, very fine-grained rock called basalt. Basalt is the most common igneous rock on the planet. (A propos of nothing, the Rosetta stone is made of basalt). During Continent-Continent collision, the rocks that melt to form magma are a pre-existing mix of sedimentary, igneous and metamorphic rocks. This mix tends to yield magmas of felsic composition. These magmas tend to cool slowly at depth, prevented from erupting at surface due to the sheer burden of rocks above them. Granite is the classic coarse grained, igneous rock of felsic composition formed in this way and comes in many varieties. Igneous rocks formed during subduction processes, tend to be intermediate in composition because the magma has come partly from melting oceanic crust which is mafic and partly from melting pre-existing Continental Crust which as described above, is more felsic in composition. Those lavas that erupt at surface from volcanoes above the subduction zone are in a group broadly called 'andesite' - named after the classic subduction plate setting, the Andes. They are of intermediate composition and fine grained. In the same setting, where the magma does not build up enough pressure to erupt and therefore cools slowly within the Crust, the rock is still intermediate in mineralogy but coarse grained - these are broadly called "diorites". In our story of southern Spain, we will see that the igneous rocks prevalent are extrusive ones associated with subduction and therefore fall broadly into the andesite group. They are also commonly called 'calc-alkali volcanics' because these lavas of intermediate composition contain potassium, sodium and calcium rich minerals. SEDIMENTARY ROCKS Sedimentary rocks can be broadly considered under three main groups - clastic, biological and chemical. Clastic: Clastic rocks are the most abundant type of sedimentary rock and come from the erosion of previously existing rocks, and the redeposition of the eroded fragments, or clasts, as sediments (hence the name). All of the three major rock types - igneous, metamorphic and sedimentary - can get uplifted and exposed at surface - for example during mountain building or other plate margin interactions. This results in their chemical and physical weathering by the elements (e.g. ice, rain, wind, and plant roots). The products of this weathering are fragments of rock, which can range in size from large boulders to smaller sand and mud size particles, which then get transported away from the site of weathering by a variety of agents - gravity, rain, streams, rivers, ocean currents, tides, wind, or even glaciers. This process of transportation itself further erodes the fragments into finer and finer pieces, and takes them 'downhill' to their eventual site of deposition, which happens when the agent transporting them runs out of sufficient energy to carry them any further. Typical depositional environments include deserts, glacier fronts, lakes, rivers, deltas, various coastal environments like beaches, the continental shelf (coastal waters < 200m deep) or even in the deepwater sea. Wherever the final resting place is, the fragments finally settle as unconsolidated, loose sediments. This cycle of weathering, transportation and deposition can take a while. It has been calculated that particles originally eroded from the mountains in the headwaters of the Missouri river in Montana take hundreds of years to travel the 3200km down the Missouri and Mississippi to their final place of deposition in the Gulf of Mexico. Some sites of final deposition also become sedimentary basins. These are areas where the Earth's Crust is gradually sagging, allowing a rough depression ("basin") to develop. Examples would be along the continental margins where oceanic crust is still cooling and sinking; or in rift basins where the stretched crust is thinning - like stretched toffee; or in front of advancing mountain fronts as one plate overrides another. In these sedimentary basins, new layers of sediment will continuously settle on top of previous layers. The increasing weight of successive sediments also reinforces the sinking of the underlying Crust. You could imagine it as an initially shallow plate with a rubber bottom that sinks into a deeper bowl shape as more and more sediment is poured into it. Sedimentary basins vary in their depth, but can commonly be filled with several thousand meters of sediment/sedimentary rock. As the layers of sediment are progressively buried and compacted, temperature and pressure increase and several things happen:
One basic classification of these clastic sedimentary rocks is by the dominant size of the grains that comprise the rock. The most common are (1) sandstones where individual grains are generally in the range 0,062 mm [don't ask!] to 2mm in diameter and can be distinguished with the naked eye or a simple hand lens, and (2) mudstone where individual particles are less than 0,062mm and not distinguishable with the naked eye or a hand lens. Sandstone and mudstone are often associated together in alternating layers ("interbedded"). Along with conglomerates, where by definition the individual 'particles' are larger than 2mm - and can be as large as pebbles, cobbles or even boulders - these three are the clastic sedimentary rocks of most relevance to our story of southern Spain. Biological The other major sedimentary rock associated with southern Spain is limestone. Limestone is the primary example of biological/biochemical sedimentary rocks and is composed mainly of the mineral calcium carbonate. Calcium carbonate is naturally secreted by hundreds of types of marine shellfish and other organisms which flourish in reefs, lagoons and tidal flats of shallow, warm, tropical and subtropical seas - both today and in the past. Classic modern examples are the Bahamas and Great Barrier reef. When these organisms die, they either leave behind solid structures of calcium carbonate (e.g. reefs) or their broken shells and secretions fall to the sea floor as carbonate detritus (sediment). Both the reefs and the carbonate sediment may get further broken down and the products further transported by storm, wave and current action (in this regard they are then also an example of clastic sediments). The extensive development of such shallow water, carbonate rich environments are called carbonate platforms, and as we shall see, they once covered vast areas along southern Spain (Chapter 2 - the Jurassic/Cretaceous palaeogeography of southern Spain). When buried in sedimentary basins, these carbonate rich sediments lithify into limestone rock. Chemical The final type of sedimentary rock of interest to us, is chemical. In inland lakes or ocean waters with restricted circulation, the rate of evaporation in arid climates may exceed the rate at which water from nearby rivers or open ocean can top it up again. In this case, the water precipitates "salts" as sediments - just as it would if you left a saucer of saltwater out in the hot sun to later find all the water gone and a rime of salt left. Two common salt minerals that precipitate during evaporation are gypsum (calcium sulphate) and halite (sodium chloride). The collective term for the suite of salts formed by the evaporation of seawater is "evaporites". Like clastic and biochemical sediments, as the evaporite salts are buried in sedimentary basins, they lithify into evaporite rocks. As we will see, the whole Mediterranean Sea once dried out resulting in an extensive development of evaporites (Chapter 3 - Messinian salinity crisis) METAMORPHIC Metamorphic rocks (meaning: 'change of form') result when pre-existing igneous or sedimentary rocks are subjected to temperatures and pressures high enough to change their physical and mineralogical characteristics to such a degree that they become different rocks. The source of such high temperatures and pressures is typically at plate margins during subduction or continent-continent collision, but to a lesser degree also around the fringes of magma bodies for example, or even at the base of extremely deep sedimentary basins. The physical changes of form commonly occur as the development of 'foliation' - layering and banding that happens as existing minerals get replaced by new minerals, and segregated. Also because the new rocks are getting squashed under high forces during their development, the foliation is often folded (wavy). The mineral changes of form happen as the various "lego blocks" that make up minerals happy at relatively low temperatures and pressures get reorganised into different mineral forms happier at the higher temperatures/pressures. All this happens without actually melting the rock or changing its overall composition. The exact mineral changes and degree of foliation developed depends on the parent rock and the temperature/pressure conditions it suffered. A parent rock of mudstone or mixed mudstone and sandstone for example shows a classic development of metamorphic rock types from slate through phyllite, schist and gneiss to migmatite, each with its own characteristic suite of mineralogy and foliation. Other parent rocks such as quartz rich sandstones and limestones do not develop significant foliation. Under high temperatures and pressures, quartz rich sandstone metamorphoses into quartzite; limestone becomes marble. All the metamorphic rock types shown opposite are common along southern Spain. Next: Background - Rock deformation |
Source: Plate margin images are from 'Understanding Earth' - Grotzinger and Jordan.
Rock sample images are from www.geology.com
Source: Mudstone image is from earthscienceeducation.com / Evaporite is from clastic detritus.com / Limestone is from geology.com / Sandstone and conglomerate are from flexiblelearning.auckland.ac.nz
Source: Interbedded sandstone/mudstone image (S.Peacock) /
Slate, phyllite, gneiss, quartzite, marble, limestone images (geology.com) /
Migmatite image (wikipedia.com - “migmatite”) /
Schist image earthphysicsteaching.homestead.com
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