Rocks and the Rock Cycle & Rocks of the Maltese Islands PDF

Minerals, Rocks and the Rock Cycle We tend to associate minerals with precious stones, like diamonds, rubies, etc. other minerals may be very ornamental but not precious. Most minerals are very common indeed like the quartz in sand grains or common rock salt (halite). Minerals can consist of a sing...

Minerals, Rocks and the Rock Cycle We tend to associate minerals with precious stones, like diamonds, rubies, etc. other minerals may be very ornamental but not precious. Most minerals are very common indeed like the quartz in sand grains or common rock salt (halite). Minerals can consist of a single element e.g. gold, silver, and copper, diamond, graphite, sulphur etc. Most minerals consist of compounds of two or more elements e.g. salt, snowflakes, quartz, etc.. Definition: A natural inorganic solid with a specific atomic internal structure and a chemical composition that varies only within specific limits. All specimen of a given mineral share certain physical properties including: cleavage, crystal form, hardness, specific gravity, colour and streak. Minerals have restricted stability range. More than 95% of the Earth’s crust is composed of the silicate minerals. Fewer than 20 minerals form the great bulk of Earth’s crust. The most important rock- forming minerals are feldspars, micas, amphiboles, pyroxenes, olivines, quartz, calcite, dolomite, clay minerals, halite and gypsum. Rocks are normally made up of a variety of minerals. 1 Minerals, Rocks and the Rock Cycle Saviour Said Rocks A rock is any naturally formed mineral aggregate (material) that makes up a large, natural, continuous part of Earth’s crust. The term is applied to objects of various sizes from solid rock of the Earth’s mantle to sand and clay. Rocks are divided according to their origin into: igneous rock, sedimentary rock and metamorphic rock. Igneous Rocks Igneous rocks are formed when molten material (magma) moves, from deep within the earth, to a position within or atop the crust. When magma solidifies, an igneous rock is formed. Following a volcanic eruption, the lava cools quickly and the resulting rocks will often have the appearance of dark glass, such as obsidian. Huge amounts of magma never reach the surface of the Earth, instead solidifying into igneous rocks in underground passages and chambers. Through time, this melting has separated denser from lighter minerals, causing the light, continental igneous rocks, such as granite, to effectively float on denser, semi-molten rocks, or mantle. The ocean floor is made of similar, dense rocks, the most common of which is basalt. Granite is used as a building stone. Granite stones occur naturally in the highlands of Scotland, and 2 Minerals, Rocks and the Rock Cycle Saviour Said beneath the Pampas plains of Argentina. Basalt on the other hand is used in construction, especially as road stone. It is found in Northern Ireland (Giant’s Causeway), and throughout Hawaii. Scientists are interested in igneous rocks, especially volcanoes, because they create fertile soil and can be taped for geothermal energy. Sedimentary Rocks Sedimentary rocks are formed at Earth’s surface by the combined processes of erosion, transportation and deposition. Sedimentary rocks typically occur in layers or strata that cover large parts of the continents. Sedimentary rocks are subdivided according to the grain size of their component material. From the largest grain size to the smallest, these are: Conglomerates, Sandstone, Siltstone and Shale Chemical sedimentary rocks are formed by chemical precipitation. Rainfall contains weak carbonic acids which dissolve calcium carbonate. This is transported in solution and deposited during evaporation in places like caves and forms stalactites and stalagmites. Biological sedimentary rocks originate from organic remains. These include Limestone composed of the skeletal remains of marine organisms e.g. Maltese coralline limestone originate from coral reefs while Globigerina limestone originated from accumulations of foraminifera. Four major processes are involved in the formation of sedimentary rocks: Weathering, Transport, Deposition and Cimentation (lithification) During weathering, transport, and deposition, the eroded material is differentiated (sorted and concentrated) according to grain size and composition. Rocks exposed at the Earth’s surface will undergo weathering (the breakdown of rocks). This is because most rocks are formed at high pressures and temperatures, away from the influence of oxygen and water. When brought to the surface by volcanoes or Earth movements, such material is chemically 3 Minerals, Rocks and the Rock Cycle Saviour Said unstable and will break down into a more stable form. The process can be compared to iron or steel formed at high temperatures in a furnace and then left in open air, where iron oxidizes or rusts. Rocks are weathered in three ways: chemically, (ex. Oxidization and freeze-thaw weathering) and mechanically (physical weathering), which involves erosion by rivers, wind, waves, or glaciers. Sedimentary rocks are also eroded biologically. Famous sedimentary rocks include the conglomerate (made of pebbles and cobbles) that forms Uluru (Ayers) Rock in Australia. Fossil fuels derived from organic sedimentary rocks, which are formed from the accumulation and compaction of dead organic matter. Metamorphic rocks Sedimentary and igneous rocks may become buried by huge layers of overlying rock, or they may be caught up in areas where movements of the Earth’s crust are occurring. Heat is usually one of the most important factors in metamorphism. As temperatures increase, the stability limits of some minerals are exceeded, and 4 Minerals, Rocks and the Rock Cycle Saviour Said hydrous minerals break down to form a dry solid and aqueous fluid. The pore fluid helps new minerals to form. The pressure and heat produced by burial or movements of the crust will alter the original rock so completely that a new, or metamorphic rock is formed. High pressure reduces pore spaces and can produce new minerals with closer atomic packing. Rock cycle The rock cycle is an important theory in the science of geology. This cycle was first proposed by Scottish chemist James Hutton (1726-1797). The rock cycle tries to explain how the three basic rock types known as igneous, sedimentary and metamorphic are related; that they change from one type to another through time; and that there must be a driving force behind the rock cycle. Rocks can be altered by changing the mineral proportions within the rock. Common methods of altering rocks include weathering and heating, both of which are fundamental to the rock cycle. The Rock Cycle is a naturally changing process, like the weather or the growth of plants. The difference lies in the fact that the rock cycle takes much longer to complete and is driven by plate tectonics. 5 Minerals, Rocks and the Rock Cycle Saviour Said 6 Minerals, Rocks and the Rock Cycle Saviour Said Questions on Minerals, Rocks and the Rock Cycle. 1. Define mineral. 2. Define rock. 3. Give three examples of each of: a) Rocks _____________________________________________________________ b) Minerals ___________________________________________________________ 4. Give two examples of each of the following types of rocks: a) Igneous __________________________________________________ b) Sedimentary ______________________________________________ c) Metamorphic ______________________________________________ 5. Define weathering. ___________________________________________________ ___________________________________________________________________ 6. Mention the three types of weathering and give one example of each. ___________________________________________________________________________ ___________________________________________________________________________ __________________________________________________________________ 7. When magma from the mantle erupts on the Earth’s surface it is known as ________. It solidifies and forms rocks known as ___________. An example of this type of rock is __________. Weathering wears down rocks on the Earth’s surface. Sediment is then transported by the process of ____________, and builds up layer upon layer at the bottom of the sea. Eventually, the great pressure of these sediment layers form _____________ rock. An example of this type of rock is _______________. If a rock is subjected to great heat and pressure, it is transformed into a _______________ type of rock. One example of this type of rock is ____________. 9. Define Lithification - ____________________________________________________ 10. Plate tectonics slow down the rate of the rock cycle. TRUE/ FALSE? 11. Without the rock cycle, life on earth as we know it would not exist? TRUE/FALSE? 7 Minerals, Rocks and the Rock Cycle Saviour Said Rocks of the Maltese islands Geological History of the Maltese Islands The Maltese Islands were formed during the Cenozoic Era, at a time between the Oligocene and Miocene epochs. The rocks started to form about 30 to 25 million years ago. Sediments originated on the sea bed where sediment, silt and remains of tiny sea creatures were deposited. Rock strata were formed over millions of years through continuous accumulation. With more accumulations, strata of rock were formed, and obviously each stratum took millions of years to take a shape. Then there must have been some uplift of the land below the sea. This could have been caused, possibly, by the folding of the land due to the pressure exerted by the continent of Africa crashing into Eurasia. These rock strata were uplifted for the first time 10 million years ago. This uplift was due to pressure exerted by the African plate crashing into the Eurasian plate. The Mediterranean Sea was cut off from the rest of the oceans 6 million years ago. This caused the drying up of the sea and thus a land bridge emerged connecting the Maltese Islands with Sicily and Europe. Several animals migrated to Malta and some of their remains can be found at Ghar Dalam. 5 million years ago, at the end of an Ice Age, ice melted and so the sea level of the Atlantic Ocean rose. The water spilled into the enclosed Mediterranean Sea and the Maltese Islands were again isolated from Sicily and mainland Europe. 2million years ago, the Mediterranean experienced another Ice Age. This brought a wetter climate in this region and so river valleys formed in the Maltese Islands. Wied Il-Ghasel and Wied Il-Kbir are examples of such valleys. The Ice Age ended 10 000 years ago. The changing processes have been rather slow till present day. After the last Ice-Age, which ended about 10,000 years ago, the changing processes have been rather slow, and the Maltese lands have remained islands as we know them today. 8 Minerals, Rocks and the Rock Cycle Saviour Said However, still during prehistory, there have been other factors which contributed to the configuration of the Maltese Islands by erosive action and deposition: 1. The beating of the sea against the rocks, 2. The torrential rains and river currents 3. The changing of daily and seasonal temperatures 4. The earthquakes causing faults All these ate into the rocks, formed cliffs and bays, valleys and caves, sandy beaches and rugged shores. On the other hand, new sedimentary lands were built in various bays and inlets already existing at the time. The first signs of the presence of human on the Maltese Islands can be traced to more than 5,000 years ago (the Megalithic temples – the world’s oldest free- standing buildings date back to about 3,400BC). This must have been after the primitive man’s emergence from the caves and before his discovery of the use of stone. The rocks of the Maltese Islands All the Maltese rocks are sedimentary. In total, five different rocks were formed. These were deposited throughout different geological times in a succession of various rock strata. The rocks of the Maltese islands are:  Upper Coralline Limestone (Il-Qawwi)  Greensands (Il-Gebla s-Safra),  Blue Clay (Tafal),  Globigerina Limestone (Il-Franka)  Lower Coralline Limestone (Zonqor). Our sedimentary rocks were caused by biogenic sedimentation – the gradual sedimentation and lithification of layers of shells of living organisms. 9 Minerals, Rocks and the Rock Cycle Saviour Said The most important biogenic sedimentary rock is limestone (calcium carbonate), formed from skeletons and shells of marine organisms. In the Maltese Islands, only sedimentary rock is found. Sedimentary rock is that rock which has been formed through millions of years of deposition and accumulation of material derived: 1. Either from sea – which originated in the sea by deposition and accumulation of algae and dead sea organisms (which abound in calcium carbonate). 2. Or from the land – which originated on the surface of the land, and then carried to the sea by the action of rain, wind and other agents. Such is, ex. Blue Clay The rocks of the Maltese Islands have been built up gradually (i.e. through millions of years) and they were built in strata (layers). These layers are always found in the same sequence, even though in some places one or more layers may be missing. Besides, any one of these layers may, for some various geological causes, appear on the surface. Upper Coralline Limestone Upper Coralline Limestone (UCL) is the youngest rock strata to form (about 10 mya). This limestone rock is hard and therefore is used as spalls (zrar) for road surfacing and concrete mixtures. It is a porous layer and the rock is weathered into Terra Rossa soil. The whole surface of Comino and Northern part of Malta is made up of U.C.L. Greensands This rock strata is found underneath the Upper Coralline Limestone. It has a greenish-yellow colour and when it is exposed to air it turns into orange. It is the thinnest layer and reaches a maximum thickness of 11 metres at Il-Gelmus in Gozo but the average thickness is 1 metre. It is friable and therefore pulverizes easily into sand. It contains many fragments of fossils and is highly porous and so holds vast reservoirs of water in it. In Maltese it is known as ‘il-Gebla is-Safra’ (because of its colour) or ‘Ramli’ (because it erodes into sand). 10 Minerals, Rocks and the Rock Cycle Saviour Said Blue Clay This rock is named after its bluish colour. It is very soft and erodes quickly giving rise to cone-shaped slopes. This rock can be found in the north western part of Malta (Ghajn Tuffieha) and north east of Gozo. These slopes are turned into terraced fields with rubble walls to hold the soil in place. Since it is an impermeable rock water that seeps down from upper layers, collects and stay there. Blue Clay is impermeable and so it holds water which can be used for irrigation purposes. Soil formed from Blue Clay is very fertile. Blue Clay supplies material for the making of pottery. Globigerina Limestone Globigerina Limestone (known as Franka) is the second oldest rock and outcrops over approximately 70% of the area of the islands, eroding to give a broad, gently rolling landscape. Variations in the thickness of this formation are considerable, ranging from 23m near Fort Chambray, Gozo to 207m around Marsaxlokk, Malta. This rock consists of yellow to pale-grey limestones comprising tests of planktonic globigerina foraminifera. The formation is divided into Lower, Middle and Upper Globigerina Limestone by two beds of phosphorite pebbles. This is the thickest layer and can be found in central and the south of Malta. It is extracted from quarries and used for building purposes. Lower Coralline Limestone Lower Coralline Limestone is the oldest exposed rock in the Maltese Islands, outcropping to a height of 140m in the vertical cliffs near Xlendi, Gozo. It is mainly composed of the tests of coralline algae indicating deposition in a shallow gulf environment. It is generally found exposed on cliff sides facing the sea, such as Ta’ Cenc (Gozo) and Dingli Cliffs. It is a very hard rock and thus forms a rugged and sharp pointed land surface. When it is found on the surface further inland the rock is rugged and sharp pointed (xaghri) as such it is known as Karst. This type of karst can be seen along the Bugibba-Burmarrad-Bahar-ic-Caghaq coast. 11 Minerals, Rocks and the Rock Cycle Saviour Said Because of its hardness it is difficult to quarry. However, when it is quarried it is used for pavements, building of breakwaters, covering of facades of buildings near the sea, and spalls or for concrete mixtures. 12 Minerals, Rocks and the Rock Cycle Saviour Said Questions on the Geology of the Maltese Islands 1) Define the term biogenic sedimentation. 2) List the five layers of rocks found in the Maltese Islands, starting from the oldest layer. a) ___________________________________________________________________ b) ___________________________________________________________________ c) ___________________________________________________________________ d) ___________________________________________________________________ e) ___________________________________________________________________ 3) State whether each of the following statements about the rocks of the Maltese islands are true or false: a) The Greensands layer is very common in the Maltese Islands ________ b) Globigerina Limestone was formed after the Upper Coralline layer _________ c) Blue Clay is the only impermeable layer ________ d) Upper Coralline Limestone is found in the surface of the SE part of Malta __________ e) The most extensive Greensands layer is found at Ta’ Gelmus, in Victoria _________ 4) Approximately how old is the : a) oldest layer of rock found in the Maltese Islands? _______________________ b) Youngest layer of rock found in the Maltese Islands? _____________________ 13 Minerals, Rocks and the Rock Cycle Saviour Said