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Rocks Chapter 4. P. Lobosco. The Rock Cycle Chapter 4, Section 1. Objectives: Distinguish between a rock and a mineral. Describe the rock cycle and some changes that a rock could undergo. Rock Forming Mineral Groups. Rock Cycle.
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RocksChapter 4 P. Lobosco
The Rock CycleChapter 4, Section 1 • Objectives: • Distinguish between a rock and a mineral. • Describe the rock cycle and some changes that a rock could undergo.
Rock Cycle • The continuous changing of rocks from one kind to another over long periods of time is called the rock cycle. The rock cycle has no definite sequence. It can follow many different pathways.
Matter and the Rock Cycle • Because granite is made of hard materials it is resistant to nature’s forces. It can be slowly worn down until bits of granite flake off and fall in streams and are eventually reduced to sand. The sand from granite, along with other sediments is carried to the sea and is deposited on the floor. The weight of layers piling on puts pressure on lower layers and with calcite the granite becomes part of a sedimentary rock. After many years, under great pressure and temperature the sedimentary rock will change to a metamorphic rock, quartzite
James Hutton • James Hutton, a Scottish physician and naturalist, discovered in 1788 that rocks undergo changes. He noticed that layers of rocks had been altered since they formed. • His findings still influence geologists today.
Igneous RockChapter 4, Section 2 • Objectives: • Recognize that magma and lava are the materials that form igneous rocks. • Contrast the formation of intrusive and extrusive igneous rocks. • Contrast basaltic and granitic igneous rocks.
Formation of Igneous Rocks • When lava or magma cools and hardens, it forms igneous rocks. • Because magma is less dense than the surrounding rock, it is forced upward to the surface. • When it reaches the surface, it is called lava.
Intrusive Rocks • Rocks that forms from magma below the surface are intrusive igneous rocks. • They are found at the surface only after the layers of soil covering them are removed by erosion.
Extrusive Igneous Rocks • Extrusive Igneous rocks are formed as lava cools on the surface of the earth. • Since is exposed to air and water, it cools quickly. The cooling rate keeps the grains from growing large.
Igneous Rock Classification • Igneous rocks are classified according to their composition and texture. • Composition refers to the minerals of which rocks are formed. • Texture means the shape, size, arrangement and distribution of the minerals that make up rocks. • Both are evident in a rock’s appearance.
Igneous Rock Composition • There are three basic igneous rock compositions which produce three colors of igneous rocks. • Dark • Intermediate • Light
Igneous Texture • There are four textures of igneous rock: • Fine (Aphantic) • Coarse (Phaneritic) • Glassy • Vesicular (Full of holes)
Glassy Igneous • Glassy igneous rocks are shiny and look like glass. They cool so quickly, the crystals do not have a chance to grow. • Obsidian is an example of a glassy texture igneous rock.
Fine-Grained Igneous • Fine-grained rocks, have crystals too small to be seen without the help of a microscope. These extrusive rocks grains cool quickly on the surface of the earth. This fine grained rock is rhyolite.
Coarse-Grained Igneous Rock • Coarse-grained rocks, such as granite, consist of interlocking mineral crystals, which are all roughly the same size and visible to the unaided eye. • Coarse grains grow as rocks cool in the earth (intrusive).
Vesicular Igneous Rocks • Vesicular Igneous rocks are formed when lava that is full of gas bubbles cools and produces a rock with open holes. • Pumice and scoria are examples.
Igneous Variety • Where and how magma cools determines the size of mineral crystals. The longer it takes magma to cool, the larger are the crystals that form. Glassy and fine-grained rocks form from lava that erupts from volcanoes and hardens on the Earth’s surface. Coarse-grained rocks form from molten rock that cools and hardens within the Earth.
Metamorphic RocksChapter 4, Section 3 • Objectives: • Describe the conditions in Earth that cause metamorphic rocks to form. • Classify metamorphic rocks as either foliated or nonfoliated.
Metamorphic Rocks • When already existing rocks are buried deep within the Earth, tremendous heat, great pressure and chemical reactions may cause them to change into different rocks with different textures and structures. The changing of one type rock into another as a result of heat, pressure and /or chemical reactions is called metamorphism.
Metamorphic Rocks • Metamorphic rocks are formed when chemical reactions from hot fluids, tremendous heat and great pressure change existing rocks into new kinds of rocks. These new rocks have chemical and physical properties usually quite different from the original rocks.
Metamorphic Rocks • The amount of heat, pressure and chemical reactions varies during metamorphism. Thus the degree of metamorphism also varies. The characteristics of the original rock also affect the degree of metamorphism. Many metamorphic rocks can be produced from more than one kind of rock.
Metamorphic Classification • Like igneous and sedimentary rocks, metamorphic rocks can be classified according to texture and composition. • There are two types of texture: foliated and nonfoliated.
Foliated Rocks • In the first group, the mineral crystals are arranged in parallel layers, or bands. The word foliated comes from the Latin word for leaf. It describes the layers in such metamorphic rocks, which are thin and flat. Most metamorphic rocks are foliated, like schist, slate and gneiss.
Unfoliated Rocks • In the second, smaller group of metamorphic rocks, the rocks are not banded and do not break into layers. These rocks are said to be unfoliated. Marble and quartzite are examples of unfoliated rocks.
Sedimentary RocksChapter 4, Section 4 • Objectives: • Explain how sedimentary rocks form from sediments. • Classify sedimentary rocks as detrital, chemical or organic in origin. • Summarize the rock cycle.
Detrital • The rocks that form by cementation or compaction are known as detrital or clastic sedimentary rocks. • They are further subdivided by size of the particles.
Chemical Sedimentary • Chemical sedimentary rocks form from solution • An example would be rock salt or gypsum.
Organic Sedimentary • These sedimentary rocks form from the remains of once living organisms or the processes of living organisms. • Coal is an organic rock formed from plants. • Coquina or chalk from from sea creatures.
Weathering • Rocks are broken down over by chemical weathering or physical weathering. • Chemical weathering, like acid rain, alters the structure of a rock by adding or removing an element. • Mechanical weathering. Like frost wedging, just breaks the rock into smaller pieces.
Weathering • Quartz is resistant to weathering because it is so hard. Sand on the beach is often little pieces of quartz. • Sedimentary rocks are often compacted and cemented together. This process of making a rock is known as lithification.
Cementation • Cementation occurs when water carrying minerals runs over the sediments and deposits minerals that act like glue sticking them together.
Compaction • Compaction is the pressing together of particles over time due to layers being deposited again and again. The weight and pressure pushes the small pieces together.
Erosion • Erosion occurs when the small pieces are carried away. • The agents of erosion are running water, waves, and wind.
Extrusive Rocks • Rocks formed from lava are called extrusive rocks. Because lava is brought to the surface by volcanoes, extrusive rocks are also known as volcanic rocks. Basalt and obsidian are two kinds of extrusive rocks that are quite solid. Pumice, another extrusive rock, is filled with bubbles.
Intrusive Rocks • Igneous rocks formed deep within the Earth are called intrusive. They form when magma forces its way upward into preexisting rocks and then hardens. Intrusive rocks include granite and pegmatite. Intrusive rocks are also known as plutonic rocks. A mass of intrusive rocks are known as a pluton. Plutons may produce landforms by pushing up layers of rock above them, such as domes.
Sedimentary Rocks • Igneous rocks are the most common rocks on Earth but most of them exist below the surface. 75% of the rocks exposed at the surface are sedimentary.
Classification of Sedimentary Rocks • The most widely used classification system for sedimentary rocks places them into three main categories according to the origin of the materials from which they are made. These three categories are: clastic or detrital rocks, organic rocks and chemical rocks.
Clastic Rocks • Sedimentary rocks that are made of the fragments of previously existing rocks are known as clastic rocks. Clastic rocks classified according to the size and shape of the fragments in them: conglomerate, sandstone and shale.
Conglomerates • Some clastic rocks are made of rounded pebbles cemented together by clay, mud or sand. If over a third of the rock is made of pebbles, the rock is called a conglomerate. The pebbles in conglomerates are smooth and rounded because they have been worn down by the action of water. They are also called puddingstones. If the pieces of the rock are angular then it is called breccia.
Sandstones • Clastic rocks made of small, and-sized grains are called sandstones. At least half the particles in a clastic rock must be sand sized in order for it to be considered a sandstone. Sandstones are very common rocks. They are formed from the sand on beaches, in riverbeds and in sand dunes. In a sandstone, the grains are cemented together by minerals such as quartz, calcite and hematite, that harden.
Shale • Many geologists use term shale to describe all the clastic rocks that are made of particles smaller than sand. Shale forms from small particles of mud and clay that settle to the bottom of quiet bodies of water such as swamps. Most shale can be split into flat pieces.
Organic Rocks • Organic rocks come from organisms. Limestone are often but not always organic rocks. Deposits of limestone may be formed from the shells of creatures when they die. Creatures may also cement their shells together and over time form reefs. Coal is also made from the remains of living things. It is made from plants that lived millions of years ago.
Chemical Sedimentary Rocks • Water precipitates two ways to form chemical sedimentary rocks • 1. by the process of evaporation • 2. through the life processes of marine organism
Biochemical Sedimentary Rocks • Coral is an example of a chemical sedimentary rock that was formed through the life processes of a coral polyp.
Evaporites • Halite and gypsum are examples of evaporites.
Chemical Rocks • Some sedimentary rocks are formed when a sea or lake dries up, leaving large amounts of minerals that were dissolved in water. Examples of chemical rocks formed this way include rock salt and gypsum. Some limestone rocks are formed by inorganic processes in caves. As water evaporates, a thin deposit of limestone is left behind.
