Chapter 12 Groundwater

Chapter 12 Groundwater - 2009 Allan Ludman and Stephen...

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1 CHAPTER 12 GROUNDWATER AS A LANDSCAPE FORMER AND RESOURCE PURPOSE To understand how groundwater infiltrates and flows through Earth materials To explain why groundwater erodes and deposits differently from streams and glaciers. To learn to recognize landscapes formed by groundwater and to interpret groundwater flow direction from the topographic features To understand how geologists carry out groundwater resource and pollution studies MATERIALS NEEDED Specimens of four different materials for infiltration exercise Tracing paper and colored pencils 12.1 INTRODUCTION Some rain and snow that falls on the land runs off into streams, some evaporates into the air, and some is absorbed by plants. The remainder sinks into the ground and is called groundwater . Groundwater moves much more slowly than water in streams because it has to drip from one pore space to another. It therefore has much lower kinetic energy than stream water and cannot carry the particles with which streams erode bedrock by abrasion. As a result, groundwater can only erode chemically and groundwater erosion is most effective in soluble rocks– usually limestone, dolostone, or marble containing carbonate minerals like calcite and dolomite. If groundwater erodes underground, how can it form landscapes at the surface? As groundwater erodes rocks from below, it undermines the support for the surface and land may collapse to produce very distinctive landscapes (Figure 12.1a). Landscapes in areas of extreme groundwater erosion are among the most striking in the world, with narrow, steep-sided towers unlike anything produced by streams or glaciers (Figure 12.1b). Groundwater-eroded landscape is called karst topography after the area in Yugoslavia where geologists first studied it in detail but spectacular examples are found in Indiana, Kentucky, Florida, southern China, Puerto Rico, and Jamaica. © 2009 Allan Ludman and Stephen Marshak W.W. Norton & Company
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2 Figure 12.1 Landscapes carved by groundwater (Karst topography) a. Karst topography characterized by sinkholes and b. Karst towers, Guilin area, southern China underground caves, southern Indiana Groundwater is also a vital resource, used throughout the world for drinking, washing, and irrigating crops. Strict rules govern its use in many places because it flows so slowly that renewal of the groundwater supply takes a long time. In addition, its slow flow and underground location make it difficult to purify once it has been polluted. We will examine both roles of groundwater in this chapter – landscape former and resource. EXERCISE 12.1 FACTORS AFFECTING INFILTRATION AND GROUNDWATER FLOW Let’s look first at factors that control the flow of water underground. Your instructor will provide four containers screened at the bottom and filled with materials that have different grain shape and size (Figure 12.2).
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Chapter 12 Groundwater - 2009 Allan Ludman and Stephen...

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