Canyons, Revised Edition

Erik Hanson

Description

Canyons, Revised Edition chronicles the origins, history, and structure of the world's most breathtaking gorges, from North America's spectacular Grand Canyon to western Australia's exciting Windjana Gorge, where the Leonard River snakes its way through an ancient barrier reef. This eBook also discusses tectonic activity, undersea canyons, liquid rock, and pinpoints recent scientific studies and modern-day ecological challenges.

Sujets

• Science de la Terre
• Science
• Nature
• Formation
• Canyon
• Global
• Climate
• Geology
• Environment
• Sciences et techniques
• Young Adult Nonfiction
• Earth Sicences

Informations

• Publié par: Infobase Publishing
• Date de parution: 01 juin 2019
• Nombre de lectures: 0
• EAN13: 9781438182537
• Langue: English
• Poids de l'ouvrage: 2 Mo

Extrait

Canyons, Revised Edition
Copyright © 2019 by Erik Hanson
All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage or retrieval systems, without permission in writing from the publisher. For more information, contact:
Chelsea House An imprint of Infobase 132 West 31st Street New York NY 10001
ISBN 978-1-4381-8253-7
You can find Chelsea House on the World Wide Web at http://www.infobase.com
Contents Chapters Canyons Grand Canyon Columbia River Gorge Three Gorges of the Yangtze River Peonera Canyon Windjana Gorge Monterey Canyon Zion Canyon Fish River Canyon Chaco Canyon Canyon Diablo and Meteor Crater Support Materials Glossary Index
Chapters
Canyons

The formation of a canyon mainly requires moving water, the right types of rocks, and a climate that is relatively dry. The water in a river tends to erode the banks of the river gradually, and this can both widen the river and help establish new curves or winding turns. Rivers also tend to grind and erode in a downward direction, a process that scientists refer to as downcutting or incision. As a result of their incision, many rivers will eventually begin to act on rocky layers of land that would otherwise be covered by soil or sediment. If a river can move across an area of land that is rocky, fairly flat, and relatively free of rainfall, the river will tend to produce downward incision and to expand outward, laterally, into its banks. The river may cut out bluffs in some places where the outward erosion is limited or form a gently sloping river valley in places where incision is balanced by erosion on the banks. If the river can grind downward through rock that is especially hard or exists in special layers, the result can be a canyon that is somewhat broad at the top, very deep, and narrow at the canyon floor.
The tendency of a river to cut downward and form a canyon can also be affected by geological forces acting far beneath the canyon floor. Canyons can be more than a mile deep, but they are still just part of the outermost layer of the Earth, known as the crust. The Earth's crust is an average of about 28 times as thick as the depth of the Grand Canyon in Arizona. Compared with the thickness of the molten rock that exists beneath the crust, the crust is fairly thin and can be gradually moved upward or downward or sideways.
Most of the Earth's volume is made up of a dense and spherical core surrounded by a layer of molten rock. Scientists think of the Earth as consisting of four overall layers. The solid ground of the continents and the land under the oceans are the outer shell of the Earth. These portions of solid rock are known as the crust. Beneath the crust is the mantle, which consists of the upper mantle and lower mantle. The crust and outermost portion of the upper mantle are, together, known as the lithosphere. Between one-third and two-thirds of the lithosphere is part of the upper mantle. The other fraction is the crust. To avoid confusion, the portion of the lithosphere that is mantle will be referred to as the mantle lithosphere. The upper mantle consists of the mantle lithosphere and, beneath it, the asthenosphere. The portion of the upper mantle that is not part of the lithosphere is the asthenosphere, which extends to depths of 62 to 435 miles (100 to 700 km). The continental plates move, as continuous units of crust and mantle lithosphere, across the asthenosphere. Both the mantle lithosphere and asthenosphere are technically composed of solid rock, but the rock of the asthenosphere is relatively softer than the mantle lithosphere. The lower mantle, beneath the asthenosphere, consists entirely of molten, or semiliquid, rock. Beneath the mantle are the outer core, which is liquid, and the inner core, which is solid.
The lithosphere varies in thickness but generally ends at about 60 miles (100 km) below the surface of the Earth. The thicknesses of the crust and mantle lithosphere do not necessarily always add up to 60 miles (100 km). For example, the crust and mantle lithosphere may both be thicker in the same place. This can cause the thickness of the overall lithosphere, which is the sum of the crust and mantle lithosphere, to be locally increased. This is the case in China. These variations are not often directly relevant to canyon formation, but the thickening of the lithosphere can be accompanied by localized uplift.
Setting the Stage: Tectonic Activity
The earliest factor that contributes to canyon formation is tectonic activity. This term refers to either the movements of plates, which are distinct blocks of crust and mantle lithosphere, or the deformation of the plates by orogenic activity or fault activity. The term plate tectonics refers to the interactions of blocks of crust and mantle lithosphere. When scientists look at the entire surface of the Earth, they divide its underlying crust and lithosphere into about 15 plates. The seven largest of these are the continental plates, and the borders of the continental plates tend to correspond approximately to the borders of continents.
When two plates collide or grind against each other over time, the landscape on either plate can be deformed. These tectonic forces that occur when plates interact with each other can produce earthquakes and, over a longer time period, create mountain ranges. When one plate collides with another or undergoes subduction beneath another, the zone or line of interaction is known as a convergent plate boundary. A divergent plate boundary is formed along the boundaries between two plates as the plates move away from each other. The uplifting of one plate as another is forced to dive under is one way that mountains and plateaus are formed, and this uplift can be important for canyon formation. As you are probably aware, water tends to flow from higher elevations to the lower elevation of sea level. Put another way, even a river that appears to be flowing on flat ground is slowly moving downward to sea level. This downhill flow may or may not be accompanied by downward incision. Given enough time, the water will "find" the best and most direct path to the sea. In a river flowing downward at a fairly steep slope, the water will tend to move faster and amplify both its outward and downward erosive effects.

Block diagrams showing the three different types of convergent boundaries: ocean-ocean, ocean-continent, and continent-continent. An ocean-ocean convergent margin produces an island arc similar to the Aleutian Islands of Alaska. An ocean-continent convergent margin produces a magmatic arc similar to the Andes Mountains of South America. A continent-continent convergent margin produces an orogeny similar to the Himalayan Mountains of southern Asia.
Source: Infobase Learning.
In the convergence between the continental plate and oceanic plate, in the diagram, the volcanic arc activity has not yet migrated inland. As this type of plate convergence progresses, the volcanic arc activity can migrate in an inland direction. This can create a backarc zone, backarc basin, and backarc foreland basin, also known as a retroarc foreland basin. Some canyons, including Zion Canyon and the Grand Canyon, were carved out of sedimentary rocks that built up in these low-lying basins. Other canyons were incised out of foreland basins that had been formed by the convergence between two continental plates. These canyons include Fish River Canyon and Peonera Canyon.
Cooperative Effects of Rocks, Water, and Tectonic Activity
The water in a river, the small pieces of rock on the bottom of a river, and tectonic events can all interact to help a canyon form. The downward slope that a river follows is known as the river's gradient. A river with a steep gradient might have a lot of tiny waterfalls over rocks, and the downhill movement causes rocks on the river's bottom to grind downward more. Water that is moving very fast produces more grinding, by causing rocks to strike both one another and the river's bottom. Rivers also carry very small sediment particles, which are essentially very small pieces of rock, suspended in the water. These sediments can have an abrasive effect, akin to the effect of sandpaper, on the bottom of the river. Rocks that are larger but still small enough to be moved by the current of the river also contribute to incision. The buoyancy of objects in water can make submerged rocks effectively "lighter," because the rocks displace a certain amount of water. This allows rocks to move more easily along the bottom of the river. Fast-moving water can also carry away more of the fine fragments that are ground up.
Incision can also be driven by local increases in the gradient of a river, and some of these local changes can result from sediment deposition. If a river receives a large input of water and sediments at one site, the local deposition of sediments can actually intensify the gradient. This local increase in gradient that results from the deposition of sediments, known as alluvial sediments, is known as aggradation. Aggradation can promote incision along portions of the riverbed that are downstream of the site of aggradation. The downstream incision further increases the gradient and speeds up the flow at the site of aggradation, and this can erode the locally deposited sediments that produced the aggradation. In some cases, however, aggradation may be prolonged and may lead to prolonged intervals of incision. In the Grand Canyon, for example, lava dams are thought to have contributed, at times, to the striking differences between the incision rate in the western Grand Canyon and the rate in the eastern Grand Canyon.
Rocks that fall off the walls of a canyon can either help or interfere with the ongoing formation of a canyon. On the one hand, a canyon wall can become less steep when its rocks break off and