Grade 9 Geography - Unit 3 Lesson 3 The Rock Cycle

Transcription

1 Grade 9 Geography - Unit 3 Lesson 3 The Rock Cycle What is the difference between a Rock and a Mineral? Minerals are homogeneous, naturally occurring, inorganic solids. Each mineral has a definite chemical composition and a characteristic crystalline structure. A mineral may be a single element such as copper (Cu) or gold (Au), or it may be a compound made up of a number of elements. About 2,500 different minerals have been described. Rocks are made up of one or more minerals. There are three types of rocks. Igneous Rocks - Formed from the cooling of molten rock. Volcanic igneous rocks formed from molten rock that cooled quickly on or near the earth's surface. Plutonic igneous rocks are the result of the slow cooling of molten rock far beneath the surface. Sedimentary Rocks - Formed in layers as the result of moderate pressure on accumulated sediments. Metamorphic Rocks - Formed from older "parent" rock (either igneous or sedimentary) under intense heat and/or pressure at considerable depths beneath the earth's surface. The interrelationship among the rock types is referred to as the Rock Cycle. There are four main layers that make up the Earth. Inner Core - A mass of iron with a temperature of about 7000 degrees F. Although such temperatures would normally melt iron, immense pressure on it keeps it in a solid form. The inner core is approximately 1,500 miles in diameter. Outer Core - A mass of molten iron about 1,425 miles deep that surrounds the solid inner core. Electrical currents generated from this area produce the earth's magnetic field. Mantle - A rock layer about 1,750 miles thick that reaches about half the distance to the center of the earth. Parts of this layer become hot enough to liquify and become slow moving molten rock or magma. Crust - A layer from 4-25 miles thick consisting of sand and rock.

2 The core, mantle and crust of the earth can be envisioned as a giant rock recycling machine. However, the elements that make up rocks are never created or destroyed although they can be redistributed, transforming one rock type to another. The Recycling Machine - Liquid (molten) rock material solidifies either at or below the surface of the earth to form igneous rocks. Uplifting occurs forming mountains made of rock. The exposure of rocks to weathering and erosion at the earth's surface breaks them down into smaller grains producing soil. The grains (soil) are transported by wind, water and gravity and eventually deposited as sediments. This process is referred to as erosion. The sediments are deposited in layers and become compacted and cemented (lithified) forming sedimentary rocks. Variation in temperature, pressure, and/or the chemistry of the rock can cause chemical and/or physical changes in igneous and sedimentary rocks to form metamorphic rocks. When exposed to higher temperatures, metamorphic rocks (or any other rock type for that matter) may be partially melted resulting in the creation once again of igneous rocks starting the cycle all over again.

3 Since most of the earth's surface is covered by water - molten material from inside the earth often breaks through the floor of the ocean and flows from fissures where it is cooled by the water resulting in the formation of igneous rocks. Some low grade metamorphism often occurs during and after the formation of the rock due to the intrusion of the material by the sea water. As the molten material flows from the fissure, it begins forming ridges adjacent to it. If we examine the rock cycle in terms of plate tectonics, as depicted in the figure above, we see that igneous rocks form on the sea floor as spreading ridges. As the rocks cool, and more magma is introduced from below, the plate is forced away from the spreading ridge, and acquires a sediment cover. As shown in the figure, in this case, the oceanic plate eventually "dives" under the adjacent continental plate. As the oceanic plate travels deeper, high temperature conditions cause partial melting of the crustal slab. When that occurs, the surrounding "country rock" (existing adjacent rock) is metamorphosed at high temperature conditions by the contact. The molten material is either driven to the surface as volcanic eruptions, or crystallizes to form plutonic igneous rocks. Recognizing Rocks How Do I Know What Rock I Have? Type Features Example Igneous Sedimentary Metamorphic interlocking texture of the grains the presence of vesicules (holes) in extrusive igneous rocks may be dark-colored and heavy may display two grain sizes, one much larger than the other grains cemented together the presence of fossils light-colored and light weight may display interlocking grains but is very light weight interlocking texture of large grains foliation (layering) banded light and dark colours "ching" sound instead of a "chunk" sound when tapped Granite Sandstone Shale Coal Marble Slate

4 Figure 1. Rock Cycle

5 Grade 9 Geography - Unit 3 Geological History Geological Time Scale in Terms of the 12-Hour Clock When Time Topic Era (Millions of ears) 12-Hour Clock Earth's Formation Precambrian :00 First Rocks :05 Single-Celled Organisms :36 Shellfish Paleozoic :36 Fish :45 Land Plants :55 Insects :57 Amphibians :02 Reptiles :07 Conifers :10 Dinosaurs Mesozoic :23 Mammals :27 Birds :37 Flowering Plants :40 Last Dinosaurs 67 11:50 Grass Cenozoic 25 11:56 Humans <2 11:59 (0.26%) (2.85%) (7.51%) (89.39%)

6 Grade 9 Geography of Canada - Unit 3 Rocks and Minerals Mineral - a natural substance which has its own distinctive structure and may have its composition expressed with a chemical formula. Some examples are quartz, calcite, and galena. Quartz has the chemical composition Si0 2. About 100 minerals are relatively common. There have been over 4000 minerals identified. Rock - usually composed of two or more minerals in some physical combination, although some rocks are composed of only one mineral. Examples of rocks are limestone, sandstone, granite, or shale. (If you wanted to think of a comparison to food, a mineral would be like flour and sugar, and a rock would be a cake.) Properties of minerals - ou can tell the differences between minerals by looking for certain properties. Because each mineral is unique both chemically and structurally, each has its own set of physical, optical, and structural properties to aid in its identification. Chemistry refers to the basic building blocks that the mineral is made of. Optical properties are the way a mineral looks and what light does when it shines on it. Physical properties such as hardness and streak can be tested easily. Physical properties Physical properties may be divided into two groups: (1) those concerned with the effects of light on the mineral, and (2) other tests. Light-dependent properties: The easiest tests to do are about how light interacts with the mineral. These are color, luster and diaphany. ou can do all of these light tests using only your eye. Color is the most obvious property of the mineral, but it is also the least used because a given mineral, like quartz, may have a variety of colors (colorless, purple, blue, pink, black, violet, green, tan, et cetera). Luster is easy to tell by looking at the mineral. Does the mineral reflect light like a piece of metal? If so, then it has metallic luster. If not, then it has non-metallic luster. Non-metallic luster is further subdivided into how brilliant the mineral reflects the light: vitreous (like glass), adamantine (brilliant, bright like metal), pearly, greasy, oily, dull, etc.

7 Diaphany means how translucent the mineral is. Can you see through it like a clear crystal, or not? Or maybe just a little. The answers for diaphany may be opaque, translucent, or transparent. This property is usually noted for a thin chip of the mineral because if the mineral is strongly colored, it may mask this property. Other Properties of Minerals Specific gravity - measure of each mineral's own unique density and how it compares to the density of water. The mineral's density is what makes the mineral heavy or light. Some minerals are very dense, like the native metals copper, silver, or gold, and some are even lighter than water and will float! Volcanic pumice will float on water. ou have to have a specially designed balance to measure specific gravity. Quartz is always a good mineral to compare another sample to, since it has a specific gravity of Minerals with a specific gravity higher than around 3.2 are considered to have relatively high specific gravity. They either contain heavy atoms, like those of the middle of the Chemist's Periodic Table (titanium, manganese, iron, cobalt, nickel, copper, zinc, lead) or have a very dense packing arrangement of the atoms (like with carbon in diamond). Streak is an easy field test to do. Interestingly, the powder of a particular mineral may or may not be the same color as the mineral! Using an unglazed piece of porcelain, like the back of a common bathroom tile, you can do a streak test. The color of the powdered mineral (the streak) may surprise you. Try it for a piece of pyrite (fool's gold). Hardness measures how hard the mineral is relative to other minerals. It is called the scratch or hardness test. Most minerals will not scratch a quartz crystal. Many years ago, a man named Moh took a lot of minerals and tried to scratch them with each other. From his experiment, he came up with chart of the relative hardness of his minerals, one to another. This chart is called Moh's hardness scale. Talc is the softest, diamond is the hardest. A chemical test allows us to determine if the mineral reacts with dilute acid. We use a drop of 10% hydrochloric acid to check for effervesence, or fizzing, and note if there is a reaction.

8 Grade 9 Geography of Canada - Unit 3 Lesson 3 Ice as a Mineral Can a Mineral be a Rock? Is ice a rock or a mineral? Are all rocks made of minerals? Can a mineral be a rock? Earth is made of solids, liquids, and gases. The atmosphere and oceans surrounding the planet are the main gas and liquid parts, although there are liquids (magma) and gas within the Earth, too. The solid part of the Earth, though, is basically rock. Now rocks are made of minerals. A rock can be made of a single mineral. Or a rock can be made of many minerals. A mineral has to satisfy the following criteria: be inorganic naturally occurring crystalline solid definite chemical formula unique physical properties Now, let's see if ice fits these criteria. Inorganic? Ice is not "organic" in the sense that it is not animal or vegetable matter. Naturally occurring? Snow, ice in glaciers, etc. are natural. Crystalline? Ice has a crystalline structure (it can be hexagonal or cubic) with atoms of hydrogen and oxygen covalently bound in water molecules and water molecules bound together in the crystal structure by hydrogen bonds. Definite chemical formula? Ice is always H 2 0. Definite physical properties "Solid"? How do you define a solid? Does ice qualify? In the true, technical sense, ice is a crystalline solid. And natural ice should be considered a mineral. Some people also consider glacial ice as rock.

9 Activity - Is It a Mineral? Indicate whether the substances listed in the left column are minerals (yes or no) and why or why not. An example is provided. Substance Mineral Why or Why Not? window glass no no regular internal structure ice water Teflon lead pencil lead gold plastic diamond chlorophyll steam coal carbon dioxide sulfur stainless steel table salt mercury tin

10 Activity - Is It a Mineral? Indicate whether the substances listed in the left column are minerals (yes or no) and why or why not. An example is provided. Substance Mineral Why or Why Not? window glass no no regular internal structure ice water N liquid mineral Teflon N not naturally occurring lead pencil lead graphite gold plastic N not naturally occurring, organic diamond chlorophyll N organic steam N gaseous mineral coal N organic carbon dioxide N gas sulfur stainless steel N man-made table salt halite mercury tin