Econstudentlog

Earth (1)

The post title reflects the fact that I consider this post to be the first ‘real’ post about the book. I didn’t read much during the last week because of social obligations – it was my birthday – but I have read the first half of the book (‘The classic text for majors in physical geology courses’) by now. I’ve written about it before and I have indicated why I probably won’t quote much from it, but because of wikipedia’s relatively poor coverage of at least some of the stuff in the book I haven’t been very happy about the ‘link-posts’. So I’ve decided to post a few standard book posts about it even though it’s somewhat hard to cover this book this way. There’s a lot of terminology and there’s a lot of stuff which only kind-of-sort-of makes sense from the text alone (sometimes illustrations and diagrams are needed to make sense of what’s going on). Have these things in mind when reading the rest of the post – there’s a lot of stuff in the book that it wouldn’t make much sense for me to cover in a post like this, and the book gave me much fewer options regarding what to include in my coverage than I’d have liked. All that said, below some stuff from the book:

i. “Civilization exists by geological consent, subject to change without notice” (the opening quote of the first chapter; the quote is by Will Durant)

ii. “iron accounts for about one-third of the mass of the Earth. […] Very early in its history, possibly in the first few hundred million years, the Earth underwent a profound reorganization after it warmed to the temperature at which iron melts. Approximately one-third of the primitive planet’s material sank to the center, and in the process a large part of the body was converted to a partially molten state. […] The molten material, being lighter than the parent material from which it separated, floated upward to cool and form a primitive crust. Core formation was the beginning stage of the differentiation of the Earth, in which it was converted from a homogenous body, with roughly the same kind of material at all depths, to a zoned, or layered, body with a dense iron core, a surficial crust composed of lighter materials with lower melting points, and between them the remaining mantle […] Differentiation is perhaps the most significant event in the history of the Earth. It led to the formation of a crust and eventually the continents. Differentiation probably initiated the escape of gasses from the interior, which eventually led to the formation of the atmosphere and oceans.”

iii. “What the pioneers of nuclear physics discovered at the turn of the century was that atoms of certain elements, the radioactive ones, spontaneously disintegrate to form atoms of a different element, liberating energy in the process. The important reason why radioactive decay offers a dependable means of keeping time is that the average rate of disintegration is fixed and does not vary with any of the typical changes in chemical or physical conditions that affect most chemical or physical processes.”

iv. “A soil takes a long time to form. Many hundreds to several thousands of years may be required for an A horizon to evolve to the point that there is a mat of decayed vegetation and organic matter with altered minerals and clays. The formation of a B layer [see previous link] takes even longer, from 10,000 to 100,000 thousand years. A soil that has evolved to maturity is in a steady state, in dynamic equilibrium with its climate. As soil is very slowly eroded from the top by natural processes – and by plowing and tilling – it is gradually deepened by chemical reaction. If erosion is rapid, chemical decay cannot keep up, and the soil is thinned […] Over the whole United States soil erosion accounts for a loss of 2 billion tons of topsoil, twice the amount of soil formed each year.” [remember here that the book is from 1986]

v. “The higher the mountains, the faster erosion wears them down. But as long as mountain building continues, tectonics prevails and altitudes increase. As tectonic movement slows – not because of any important effect of erosion but because its own machine starts running down – the mountains rise at a slower pace. For a time erosion keeps up with uplift, and the mountains do not change in elevation. Then as uplift slows further, erosion becomes dominant and the elevations begin to lower. As the lowering proceeds, the erosion slows too, the whole process eventually tapering off. The relief is constantly diminished by wearing away of the mountain tops and filling in of valleys and low spots by sedimentation of the erosional debris. Sedimentation, the consequence of erosion, acts to depress relief. […] Low areas, whether stable or subsiding, rarely persist for long because they are the natural dumping grounds for sediment.”

vi. “Water can slowly evaporate to form an invisible vapor at any temperature as well as during boiling. Even ice can “evaporate” to vapor – a process we call sublimation. Water vapor in air is not pure but is mixed with other gasses, nitrogen and oxygen. The relative humidity is the amount of water vapor that is in the air compared to the maximum amount the air could hold at its present temperature. Air can hold more vapor at higher temperatures. Conversely, air saturated with water will condense some of its vapor to water droplets when it is cooled to a lower temperature.
The white clouds of the sky or the billows of steam that we associate with vapor are actually composed of tiny droplets of liquid water formed when the air and vapor cool.”

vii. “more water evaporates from oceans than falls on them as rain. This discrepancy is exactly balanced by the return of water via runoff from the continents, which itself exactly balances the excess of precipitation over evaporation on land. […] Most of the surface runoff is transported by the large stream networks of major river systems. Despite the enormous number of streams on the continents, about half of the entire runoff from the land areas of the world is carried by only about 70 major systems.”

viii. “All of the surface storage areas for precipitation and runoff are minor compared to the water stored in the ground. […] Recent estimates are that usable groundwater amounts to much more than 90% of all of the fresh water on Earth. Yet, because of the extreme slowness of recharge and the slow rates of water movement, the groundwater in such areas as western Texas is in every sense being mined, just like coal, so that the amount left in the ground steadily diminishes as production continues. For all practical purposes, this water is an exhaustible resource that, once gone, cannot be replenished.”

ix. “Does a river do most of its work of erosion and transport during everyday stages, during small or moderate floods, or during the major events that come only once in a generation? It appears that the great bulk of the sediment in the few rivers studied is carried by floods that recur at least every 5 years—events of moderate intensity that happen often. Though the most infrequent events are very intense, they do not recur often enough to add much to the total. The amount transported by rivers in their everyday stages is too small to contribute significantly.”

x. “The streams in all drainage basins follow certain rules. All are connected in a one-way network by which smaller tributaries drain into larger ones with a definite pattern. The number of streams and their distance apart both follow a fairly orderly distribution. Most tributaries of about the same size are about the same length, and the intervals between the mouths of tributaries are fairly uniform. The larger the drainage area, the longer the stream, the ratio between the two being constant for similar terrains. Robert Horton, an American hydraulic engineer, was the first to use as a measure of the heirarchy of streams their order—that is, their position in the tributary network […] Horton’s original scheme is simple and gives the idea: A stream of order 1 has no tributaries; a stream of order 2 has tributaries of order 1; a stream of order 3 has tributaries of order 2; and so on. Thus the order is defined by the order of tributaries; we count up as we move downstream. As the order of streams increases, the following changes are systematic […]: The lenght of main streams increases, the number of main streams decreases, and the drainage area increases. These general characteristics of drainage networks are typical of many kinds of systems. For example, even the blood circulatory systems of mammals seem to have some of the same characteristics.”

xi. “Most of the geological work of the wind is done by the moderately infrequent strong winds of long duration, just as the major part of a river’s geological work is done by floods. […] Winds need chemical and mechanical weathering coupled with dryness to assist them in eroding and transporting materials. Wet materials are cohesive, the water binding the particles together enough to resist the wind’s tendency to pull them apart. By themselves, winds can do little to erode most solid rock exposed at the surface; but once there is some fragmentation of mineral particles, the wind can act.”

xii. “The desert is where the wind is best able to do its work of eroding and depositing. It does so in partnership with river action that, however infrequent, still does the major part of the work. […] though we may think of deserts as being unending expanses of ergs, only a small fraction of most desert land is covered by sand. A little more than one-tenth of the Sahara is sand covered.”

xiii. “Ice is a rock, a mass of crystalline grains of the mineral ice. That idea should not be too surprising; after all, it is a solid substance that occurs naturally on the Earth. Itis hard like most rocks, but it’s composition makes it much less dense. Like igneous rocks it originates as a frozen fluid; like sediments, it is deposited in layers at the surface of the Earth and can accumulate to great thicknesses; like metamorphic rocks, it is transformed by recrystallization under pressure. Masses of ice may creep, flow, or slide downhill, and just like other masses, they may be folded and faulted […] A large mass of ice that is on land and shows evidence of being in motion or of once having moved is a glacier. […] Glaciers are abundant on today’s Earth. It is estimated that there are between 70,000 and 200,000 glaciers of all kinds and sizes in the world, covering about 10% of Earth’s land surface. […] a wide range of ice speeds have been measured, ranging from a few centimeters to a meter per day. […] Flowing ice does just as much erosion and transportation work as running water does, and even more efficiently […] As a transporter of debris, ice is most effective because once the material is picked up by the ice, it does not settle out like the load carried by a river. Thus ice can carry huge blocks that no other transporting agent can budge.”

xiv. “The total volume of ice on Earth today is a little more than 25,000,000 km^3. Total ice volume during the height of the ice age can be estimated from the area covered by the ice sheets combined with calculations of the thickness of the ice necessary to keep the glaciers moving so many hundreds of miles from their areas of accumulation. […] the maximum volume must have been about 70,000,000 km^3. The extra 55,000,000 km^3 that came from the sea to make the additional ice lowered sea level by about 130 m […] What if the 25,000,000 km^3 of water now tied up as ice were to melt? The change in sea level would be catastrophic, for the melting of all existing ice would raise the oceans by about 65 m” [Yes, you would be right to infer from the way they write these numbers out that there are not a lot of equations in this book. But even though they perhaps didn’t need to include quite that many zeros to get the point across, these numbers are huge.]

August 5, 2012 - Posted by | books, Geology

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