【阅读】08/18起月度寂静整理（8/27更新，50篇原始，46考古）

billyisfragile!

https://www.economist.com/babbage/2011/07/04/reducing-the-barnacle-bill

Technology monitor
Reducing the barnacle bill
At the moment, ships’ hulls are kept clean using poisonous chemicals. Alternatives would be welcome
Jul 4th 2011
by J.E. | NEW YORK

IN THE decades-long duel for naval supremacy that was fought out between Britain and France at the end of the 18th century, the British fleet had a secret weapon. It was, as secret weapons often are, hugely expensive. But it paid off, giving British ships more speed, manoeuvrability and staying power than their French rivals. It was copper.

By covering the underwater parts of their ships' hulls with copper plates, which slowly dissolved, releasing toxic copper ions as they did so, the British admirals stopped barnacles, mussels and burrowing clams from taking up residence. In fleets that were otherwise well-matched the result was decisive. France lost. The British Empire became the global superpower of the 19th century. And the world speaks English, not French.

Ship-fouling, then, can have serious consequences. Even now, when naval supremacy is less of an issue, the problem is rife. The drag imposed by a heavy infestation of barnacles may push a ship's fuel consumption up by as much as 40%. The solution usually adopted is similar to the Royal Navy's: poison. Copper is still used, though in the form of copper-containing paint. Another popular chemical is tri-butyl tin. But releasing toxic heavy metals into the sea is frowned on these days—indeed, tri-butyl tin is now illegal in many places—so the search is on for alternatives.

One possibility is to use one of a group of chemicals called avermectins. These are antiparasite agents (the most familiar is called, confusingly, ivermectin) that are used against fleas and gut worms. They also, according to Hans Elwing of the University of Gothenburg, in Sweden, prevent barnacle colonies from taking hold by stunting their growth. A barnacle that runs into the chemical finds it cannot bind as strongly to the surface. Only a tiny amount, about one part in a thousand of the paint by weight, is needed, and other marine species are not, as far as can be ascertained, affected.

Another way of discouraging barnacles is to confuse them. A formula developed by Giancarlo Galli of the University of Pisa uses polymer molecules that are water-attracting on one side and water-repellent on the other. When they are painted onto a surface this arrangement forces them into a kind of chequerboard pattern which makes it much harder for barnacles and mussels to stick, according to David Williams, who is in charge of commercialising the idea at AkzoNobel, a multinational chemical company.

P4：另一种方法：在船上刷上一种无毒无害的物质，它不能杀死这个海洋生物，但是当船速加快时，这种生物就不能继续附着在船体上了（被甩掉），现在海洋上行驶的大型船只都能够达到这一速度，但是科学界还在探索其他的方法以使小的船只可以不用行驶得那么快。

If chequerboarding does not work out, AkzoNobel has an alternative: create a surface so smooth that barnacles cannot hold onto it. This is done using a fluoropolymer—a chemical similar in structure to Teflon. The paint does not stop the animals attaching themselves to a hull in the first place, but once the vessel is moving faster than ten knots, the water sweeps them away. That is no problem for commercial vessels, which are always on the go. But for pleasure boats, which may spend a lot of time idle, Dr Williams's team is trying to improve the formula so that a boat need not be moving so fast before the paint does its job.
Small boats, particularly on inland waterways, are also the target of work by John Schetz of the University of North Texas and Robert McMahon, of the University of Texas. They have been experimenting with a mixture containing a molecule similar to capsaicin (the active ingredient of hot peppers) and another that is similar to THC (the active ingredient of cannabis).

P5:和这种情况类似的是另外一种在freshwater里生长的生物，也会附着在在freshwater里行驶的船上面，传统的方法是船在两段河道之间采取陆路运输的方式（总之就是不让船接触水，让生物干死），但是这种方法在这个生物身上不可行，因为这个生物是两栖的，不会干死。

Fouling is less of a problem for boats in fresh water, as barnacles are purely marine. But, recently, the inland waterways, docks and freshwater-intakes of North America have been overrun by zebra and quagga mussels—species that originate from the area around the Black Sea. The mixture Dr Schetz and Dr McMahon have come up with seems particularly effective against these animals, though they have yet to commercialise it.

Their aim is not just to help boat owners, but also to stop them unwittingly spreading the mussels still further. According to Dr McMahon, a big part of the problem is that both species can survive out of the water for several days, so transporting a boat overland from one river basin to another, as is common practice in North America, will not necessarly kill them. Also, boat owners are not always as diligent as they might be about inspecting their vessels for signs of infestation—and even if they do look, the mussels can be hard to see, especially when they are young, and therefore small.

There is thus a lot to play for. Saving fuel will save money, as well as cutting shipping's contribution to greenhouse-gas emissions. And stopping the spread of invasive mussels will make life easier for those who run the waterways of America and Canada. The prize may not, this time round, be world domination. But whoever comes up with the winning formula is at least likely to become rich.

SageWong

我真的只能感谢阅读君了～谢谢你们真的。

SageWong

第一篇我觉得考古有错，讲的是月球起源，考古链接：http://www.kekenet.com/gmat/201509/398480.shtml，看一下，应该不是阿波罗飞船！！！！！！顶！！！！

peachmoon

那个阿波罗的是不是这篇啊 我google到的
http://www.nhm.ac.uk/discover/how-did-the-moon-form.html

billyisfragile!

JOURNAL ARTICLE
DOWN GO THE DAMS
JANE C. MARKS
Scientific American
Vol. 296, No. 3 (MARCH 2007), pp. 66-71
Published by: Scientific American, a division of Nature America, Inc.
https://www.jstor.org/stable/26069192
Page Count: 6

Down Go the Dams
Many dams are being torn down these days, allowing rivers and the ecosystems they support to rebound. But ecological risks abound as well. Can they be averted?
By Jane C. Marks

At the start of the 20th century, Fossil Creek was a spring-fed waterway sustaining an oasis in the middle of the Arizona desert. The wild river and lush riparian ecosystem attracted fish and a host of animals and plants that could not survive in other environments. The river and its surrounds also attracted prospectors and settlers to the Southwest. By 1916 engineers had dammed Fossil Creek, redirecting water through flumes that wound along steep hillsides to two hydroelectric plants. Those plants powered the mining operations that fueled Arizona's economic growth and helped support the rapid expansion of the city of Phoenix. By 2001, however, the Fossil Creek generating stations were providing less than 0.1 percent of the state's power supply.

Nearly two years ago the plants were shut down, and an experiment began to unfold. In the summer of 2005 utility workers retired the dam and the flumes and in so doing restored most of the flow to the 22.5 kilometers of Fossil Creek riverbed that had not seen much water in nearly a century. Trickles became waterfalls, and stagnant shallows became deep turquoise pools. Scientists are now monitoring the ecosystem to see whether it can recover after being partially sere for so long, to see whether native fish and plants can again take hold. They are also on the lookout for unintended ecological consequences of the project.

Decommissioning dams (particularly small ones, as is the case in Fossil Creek) is becoming a regular occurrence as structures age, provide an inconsequential share of a region’s power, become unsafe or too costly to repair, or as communities decide they want their rivers wild and full of fish again. But simply removing a dam does not automatically mean a long-altered ecosystem will flourish once more. As with all things natural, reality often proves far more complex and intricate than people anticipate. Those of us who have witnessed many of the unexpected consequences of dam removals are now using that knowledge to try to minimize negative results in the future.

A Global Trend

TODAY AB OUT 800,000 dams operate worldwide, 45,000 of which are large—that is, greater than 15 meters tall. Most were built in the past century, primarily after World War II. Their benefits are clear. Hydroelectric power makes up 20 percent of the globe’s electric supply, and the energy is largely clean and renewable, especially when contrasted with other sources. Dams control flooding, and their reservoirs provide a reliable supply of water for irrigation, drinking and recreation. Some serve to help navigation, by stabilizing flow.

Their costs are obvious as well. Dams displace people and as a result have become increasingly controversial in the developing world [see “The Himba and the Dam,” by Carol Ezzell; SCIENTIFIC AMERICAN, June 2001]. The structures ruin vistas, trap sediments (needed for deltas, riverbanks and beaches), stymie migratory fish and destroy ecosystems in and around waterways. Conservationists have long history of opposing dams: John Muir tried to block the dam in Yosemite’s Hetch Hetchy Valley; Edward Abbey’s novel The Monkey Wrench Gang targeted Arizona’s Glen Canyon Dam for guerrilla demolition. In recent years, as the downsides of dams have become more widely recognized, groups made up of several interested parties—utility officials, regulators, policymakers, conservationists, native peoples, researchers and the public—have fought to decommission aging dams.

In the U.S., where hydropower dams must be relicensed every 30 to 50 years, the rate of dam removal has exceeded the rate of construction for the past decade or so. In the previous two years alone, about 80 dams have fallen, and researchers following the trend expect that dams will continue to come down, especially small ones. Although the U.S. is currently leading the effort, it is not alone. France has dismantled dams in the Loire Valley; Australia, Canada and Japan have also removed, or are planning to remove, dams.

Clear successes have driven much of this activity. In 1999 engineers took apart the Edwards Dam on Maine’s Kennebec River after a long battle waged by environmentalists culminated in the Federal Energy Regulatory Commission’s denial of a renewal permit. Within years, biologists observed with some surprise the return of scores of striped bass, alewives, American shad, Atlantic salmon, sturgeon, ospreys, kingfishers, cormorants and bald eagles. They also found that the water became well aerated and that populations of important food-chain insects such as mayflies, stoneflies and caddisflies grew.

In the Loire Valley, the story is similar. Salmon were abundant in the 19th century—about 100,000 would migrate each year—but by 1997, only 389 were counted making the trip. Despite the incorporation of fish ladders and elevators, the eight dams along the Loire and its major tributaries—as well as their turbines and pumps—had decimated the salmon population. Nongovernmental organizations, including the European Rivers Network, led a campaign to bring the salmon back. In response, the French government decommissioned four of the dams—two in 1998, one in 2003 and one in 2005. Within a few months of each dam removal, five species of fish, Atlantic salmon and shad among them, began to reestablish their historic migratory pathways.

第一段说水坝造成环境问题，主要有阻拦鱼类migration（后面某道题问水坝有什么问题，其中提到阻拦Nonnative的鱼migration，是干扰答案，因为这里应该指的是native的鱼的迁移）, 造成 nonnative的鱼入侵等等。然后就后者展开描述：因为水坝把水拦起来了，水温会上升，含氧量也会变化，有些合适这种条件的非本地鱼就会过得很开心。如果把水坝拆了，水温下降了，一些喜欢冷水的鱼（如trout）就会重返这里。

In most places where dams have been eliminated, the stories of the Kennebec and the Loire have been repeated. Water clarity and oxygen levels increase as flow comes back, and aquatic insects thrive again. Warm stagnant water runs from behind the dam along with the fish, such as nonnative carp, that love it. As the water moves freely, its temperature falls and cold-loving fish species, such as trout, proliferate or return. The carp population, which tends to squeeze out others, dwindles, sometimes disappearing completely. People, in addition to flora and fauna, return to enjoy the rivers. Biologists have observed these benefits from Wisconsin—one of the U.S. leaders in small dam removal—to New South Wales in Australia. Even restoring some water to rivers without removing a dam has had positive effects [see “Experimental Flooding in Grand Canyon,” by Michael P. Collier, Robert H. Webb and Edmund D. Andrews; SCIENTIFIC AMERICAN, January 1997].

The Downsides

P1:水坝造成环境问题(ecology impact)：主要是会堵塞水道(clog/choke the waterway)（有一道题，问Dam会造成什么问题, 不要选blocking the migration of nonnative fish, 整篇文章是要保护native fish为前提）。因为水坝把水拦起来了，水温会上升，含氧量也会变化, 就会在Dam的reservoir里面繁殖出nonnative fish。如果把水坝拆了(decommission the dam)，水温下降了，环境回到原来的样子, 这些nonnative fish就会变少, 甚至消失.

A.水坝后面的淤泥(sediment behind the dam, or sediment in the reservoir)可能会堵塞水道(有题目问拆除水坝会造成什么问题)，所以很多工程师先用推土机和pipe运走淤泥再拆除水坝（这里又考了，问工程师拆除水坝的时候会干什么）

BIOLOGISTS have also recorded unexpected problems. The release of sediments trapped behind a dam’s walls can choke waterways, muddying the environment and wiping out insects and algae, which are important food for fish. This wave of turbidity can also eliminate habitat for sessile filter feeders, such as freshwater mussels. Sometimes the mud that had been held back by the structures is rife with contaminants. When engineers removed the Fort Edward Dam on the Hudson River in 1973, concentrations of PCBs rose in downstream fish and remained high for many years; even today the striped bass fishery remains closed because of high levels of PCBS.

Sediments that are not washed downstream can become problematic as well. As they dry out, they may provide fertile ground for potentially noxious exotic plants whose seeds they harbored. Eurasian reed canary grass—which homogenizes wetlands by outcompeting native plant species—grew explosively after Wisconsin’s Oak Street Dam fell, even though restoration scientists had seeded the area with native prairie plant species.

还有关于拆除大坝的，第一段介绍了拆除大坝的好处，第二和三段讲了拆除大坝的坏处，比如大坝拆除后的碎渣会造成河水堵塞，会成为某种threaten frog的生活地，等，但是也讲了科学家们如何采用方法来减轻这些坏处。

In some cases, dams have blocked invasive species from moving upriver and into zones above the dam. The dam at Fossil Creek, for example, halted the advance of exotic fish such as bass and sunfish, creating a sanctuary above the structure for imperiled southwestern fish, including headwater chub and speckled dace. The reservoir also provided habitat for a locally threatened species, the lowland leopard frog.

And dam removal can pose dangers for people living nearby. In places where flood control is crucial, government organizations have had to devise safety strategies before dams could come down. In the case of the Loire basin, the government computerized data on weather patterns, rainfall and river levels so flood warnings could be released at least four hours before danger arrived. Engineers also redesigned riverbeds to be wider and deeper, so the waters of the Loire Valley could move more freely without overflowing the banks.

Delicate Decommissioning

THE FOSSIL CREEK restoration project offers a prime example of the kind of planning that could help minimize the damaging effects of dam removal. Researchers carefully planned to control possible disadvantages of the operation. Their principal concerns were what to do with the accumulated sediments, whether to manage the fishery as a native one (which would mean removing exotic species) and how to protect the reservoir-resident frogs. Ultimately engineers decided to reroute water around the dam, keeping it as a barrier to exotics and permitting the frogs to survive in the backwater.

In addition, biologists decided to actively manage the native fish. They caught as many as they could from the creek itself and airlifted them to a holding tank. They then doused the creek with fish poison to kill exotic species and returned the natives to the water once the poison had dissipated. The U.S. Bureau of Reclamation built a fish barrier 12 kilometers below the existing dam to further impede exotics. Now managers are waiting to see how the Fossil Creek species do. The dam’s fate will be decided in 2010: if the leopard frog becomes established downstream and exotic fish have not reinvaded the creek, the dam will come out. If not, it will be lowered but
not eliminated.

Interestingly, restoring Fossil Creek involves the creation of many more dams—but these will be made of travertine, formed naturally as the calcium carbonate—rich water of the springs interacts with algae to form layers of limestone. These barriers create small, deep pools, the perfect habitat for a variety of fish and insects. They also trap leaf litter, a crucial food source for the river’s denizens— one that the presence of manmade dams often eliminates by trapping it permanently behind the barrier.

Wrangling Sediment

wrangle (noun) = an argument, especially one that continues for a long time

SEDIMENTS STUCK behind dams are proving crucial variables when dams are taken down. Often the biggest issue facing managers is how to contend with what can be a massive accumulation of dirt and debris. Because of the legacy of releasing PCBs downstream in the Hudson River, scientists now routinely test these materials for toxicity. If the sediments contain high levels of pollutants, the cost of removing them—especially from remote locations—has to be weighed against the ability of the waterway to wash them away. If the sediment load is very high and the river’s flushing capacity low, engineers might opt to remove the dam in stages, allowing small amounts of sediment to be released at a time. Sometimes engineers build channels through reservoirs, planting vegetation to stabilize sediments or placing physical barriers such as rocks or temporary fencing to hold the dirt in place.

In Fossil Creek, where roughly 25,000 cubic yards of sediment are trapped behind the dam, geologists and others predicted that the river would naturally flush the sediments downstream within a decade, without any ad-verse effects. So the project did not have to weigh the cost and negative environmental impacts of transporting heavy machinery into a wilderness area.

Sediments pose a much bigger problem in many other places, however. Six million cubic yards of dirt lie behind the Matilija Dam on the Matilija Creek in southern California. (So much sediment, in fact, that the dam no longer serves to store water for irrigation or drinking.) At the same time, the downstream beaches are starved of sediment: they badly need dirt and sand to stave off ongoing erosion from wind and rain.

Matilija Dam is scheduled to be decommissioned in 2009, and managers have devised an elaborate sediment plan. They intend to transport fine sediments from behind the dam through a slurry pipe to sites five to 11 kilometers downstream. From there, the river will do the work by redistributing these materials during flood events to form beaches and sandbars. The larger, or coarse-grained, sediments that have accrued upstream of the dam will be left in place, but engineers will regrade the river channel there into a more naturally sinuous one, which will better protect against flooding by allowing sediments to settle and rebuild the banks.

Going Forward

AT FOSSIL CREEK and elsewhere, managers and scientists are using all available information about dam removal and restoration ecology, as well as what they know of the entire watershed, to make decisions. But many gaps in our knowledge about ecosystems remain, and those working on decommissioning dams recognize they are conducting long-term experiments that may have unanticipated results. Fossil Creek, for example, was the first such project in which exotic fish were removed. If successful, this strategy could become routine, especially in smaller streams where chemical treatment is feasible.

第三段就是arizona的例子了，总之就是为了保护青蛙小鱼各种物种，搞了很多复杂的方式，又要保留水坝后面的湖，又把水改了道（可能为了疏通迁徙吧），又投毒杀死crayfish(此处我没有看太明白，但是没有影响后面的做题)。

At Fossil Creek our research team will now document how the river recovers. Among many unanswered questions we hope to focus on in the next five to 10 years are: Will native fish prosper without intervention? Will exotic fish come back? One interesting but problematic twist in the Fossil Creek story is that the chemical used to eliminate the exotic fish does not harm exotic crayfish, which are notorious for wreaking havoc on the food chain. The exotic fish had consumed crayfish, thereby keeping the crustacean’s population down. Perhaps we will have exchanged one adverse situation for another. In addition, as Fossil Creek rebounds, so do the numbers of visitors to it. With more hiking trails in place along the river, managers now need to devise rules that can allow people access but also protect the fragile ecosystem.

To supplement the in situ experiments such as the one at Fossil Creek, researchers are using computer simulations and are conducting indoor studies. The National Center for Earth-surface Dynamics in Minnesota has created a model ecosystem of miniature streams, dams and reservoirs. Investigators there use time-lapse photography to determine how sediments move downstream as dams are removed in different ways and to different extents.

Many engineers who were once dedicated to building dams now find themselves instead working on decommissioning them. US. government agencies such as the Bureau of Reclamation and the Army Corps of Engineers, as well as their European counterparts, are studying not only how to remove dams but also how to provide the benefits of the structures without their injurious effects—for instance, how to extract water from rivers without building blockades. In response to a 2000 report by the World Commission on Dams, engineers are also trying to incorporate decommissioning into the original designs of future dams.

Societies will continue to balance the pros and cons of dams, weighing their utility and benefits against their destructive costs. And scientists must continue to learn about how best to remove dams so natural ecosystems and human communities both can thrive. In the next few years the decommissioning of several large dams will provide further important knowledge. In 2009 two dams will be removed from Washington State’s Olympic National Park: the 210-foot-high Glines Canyon Dam and the 108-foot-high Elwha Dam. Scientists in both locations are now collecting baseline data about salmon and steelhead, as well as oxygen levels, insect populations and sediment loads. Japan’s Arase Dam will come down in 2010 in response to a long campaign by citizen activists concerned about poor water quality and a decline in fisheries. Australia will transform the 19,500-acre Lake Mokoan into a wetlands again when its dam is removed, while France contemplates the fall of a fifth Loire Valley dam.

In most cases, controversy about decommissioning arises—and sometimes the debate is unexpected. In the Loire Valley, a father and son ended up on different sides of the divide. The father remembered the wild rivers and the salmon runs; the son had grown up swimming and boating in the reservoir. In the case of Fossil Creek, the local community wanted to preserve components of the generating station, the Childs-Irving facility. Built by one of the few female engineers of that era, Iva Tutt, and maintained by generations of engineers who lived at the site with their families, the plant was culturally significant, and, accordingly, its preservation became part of the restoration plan.

The same proved true of the Wellington Dam in New South Wales, Australia. In 2002 the State Water Corporation ensured that a one-meter-high footprint of the structure remained (minus one gap for flow) across Bushrangers Creek so the public could still appreciate the dam that was built in 1898. With compromises such as these, along with further ecological insights and more flexible engineering, it seems possible to think of the world’s waterways as ultimately fulfilling their promise for all parties—from plants to people.

billyisfragile!

SageWong 发表于 2018-8-19 15:11
第一篇我觉得考古有错，讲的是月球起源，考古链接：http://www.kekenet.com/gmat/201509/398480.shtml，看 ...

讲一个一个太空员取得了一个moon的样本，然后说以前的三个理论都有问题，
第二段讲为什么有问题
第三段讲了一个新理论 collision theory

origin of moon (condensation, capture, fission and collision theory)


《第二篇》看到就有点崩溃了。。记不太全 好像是说什么天体形成的原因，一开始有三个学说，后来阿波罗登月计划实施以后，分析了 rocks，又提出了collision theory

Pmoment_Peter

今晚会更新吗？？我明天上午考慌的一批。。。跪谢

NicoleJHL

感谢！825二战，许愿分手

Hailey0105

跪求今天晚上再更新一次啊

billyisfragile!

SCIENTIFIC AMERICAN
May 1988
Volume 258, Issue 5


Ancient Magnetic Reversals: Clues to the Geodynamo
Is the earth headed for a reversal of its magnetic field? No one can answer this question yet, but rocks magnetized by ancient fields offer clues to the underlying reversal mechanism in the earth's core

By Kenneth A. Hoffman


For well over a century geophysicists have observed a steady and significant weakening in the strength of the earth's -magnetic field. Indeed, if this trend were to continue at the present rate, the field would vanish altogether in a mere 1,500 years. Most investigators are inclined to think that the decay is merely an aspect of the restlessness inherent in the field and that the field will recover its strength. Yet one cannot dismiss out of hand the possibility that the weakening portends a phenomenon that has recurred throughout geologic time: the reversal of the geomagnetic field.

Which of these two scenarios is correct? The answer lies concealed 3,000 kilometers below the earth's surface within the outer core, a slowly churning mass of molten metal sandwiched between the mantle of the earth and the solid inner core. It is now generally accepted that the earth's magnetic field is generated by the motion of free electrons in the convecting outer core. This theory supposes the core behaves like a self-sustaining dynamo, a device that converts mechanical energy into magnetic energy. In the geodynamo the earth's rotation, along with gravitational and thermodynamic effects in and around the core, drives the fluid motions that produce the magnetic field [see "The Source of the Earth's Magnetic Field," by Charles R. Carrigan and David Gubbins; SCIENTIFIC AMERICAN, February, 1979].

Although the basic principles of dynamo action are well established, geophysicists do not yet understand the thermodynamics, fluid mechanics and electrical properties of the earth's interior well enough to construct a universally accepted model of the geodynamo. Yet its workings can be glimpsed indirectly by observing the present-day field. These measurements yield many details of the short-term behavior of the field, such as its shape and "secular variation," or ordinary fluctuation. To study the activity of the dynamo over aeons one must turn to the paleomagnetic record-the ancient magnetism frozen into rocks from the time of their formation.

https://forum.chasedream.com/forum.php?mod=viewthread&tid=1327298&pid=24243563&page=1&extra=#pid24243563

第三篇: 讲地球的magnetic field的reversal也不怎么的，就记得一个关键词：P（P开头的一个很长的单词）evidence。题目有问1906年前是什么情况以及两个主旨题。

Indeed, paleomagnetic evidence led to the first proposal that the earth's field has reversed itself, put forward in 1906 by the French physicist Bernard Brunhes. Brunhes was intrigued by the discovery of rocks that were magnetically oriented in the direction opposite to the earth's field. His startling suggestion was furiously debated for more than five decades. It was not until the early 1960's, at about the time J. S. B. Van Zijl and his colleagues published the first detailed study of a paleomagnetically recorded field reversal in lavas from South Africa, that the idea was accepted by the scientific community at large. Today it is a fundamental tenet of geophysics that the earth's magnetic field can exist in either of two polarity states: a "normal" state, in which north-seeking compass needles point to the geographic north, and a "reverse" state, in which they point to the geographic south.

In the 1960's studies of radiometrically dated lavas yielded a consistent log of past polarity changes, including no fewer than nine major reversals in the past 3.6 million years, the most recent of which occurred 730,- 000 years ago [see "Reversals of the Earth's Magnetic Field," by Allan Cox, G. Brent Dalrymple and Richard R. Doell; SCIENTIFIC AMERICAN, Febru ary, 1967]. The time scale of polarity transitions has since been extended back nearly 170 million years.

Paleomagnetic records show that the geomagnetic field does not reverse instantaneously from one polarity state to the other. Rather, the process involves a transition period that typically spans a few thousand years. Hence for perhaps 98 percent of the time the field is stable and its shape is well understood. But for the remaining 2 percent of the time the field is unstable and its shape is not obvious. The foremost task for geophysicists in my field has been to chronicle the behavior of the reversing field―its shifting shape and fluctuating intensities―based on the sometimes faint and complex record of past events, imprinted in stone. The findings provide an invaluable probe into the hidden mechanisms of the geodynamo.