Monday, August 01, 2005

(**) Earth: An Intimate History by Richard Fortey

A trip around the Earth studying geology and the deep past by a British geologist. If you don't know much about geology you will after reading this book. If you do much about geology you may find this light on technical content, but still interesting, especially because it highlights some places you may have visited or need to visit. Lastly, if you do like this genre please read John McPhee's Pulitzer prize winning 'Annals of the Former World'.

“After the 79AD debacle, which killed 2000 people and buried Hereculaneum and Pompeii, there was a great eruption that began in the morning in the morning of December 16, 1631. Within a day, ash from Naples reached Istanbul more than 630 miles distant. This eruption killed almost twice as many people as had died at the time of Roman Empire.” P 12 But no one ever really hears about this one despite its larger size and killing potency.

“Thanks to refinements in GPS, the smallest heaves in the surface of the ground can now be measured. These systems are now so sensitive that is possible to detect the depression of the surface of the earth produced by heavy snowfalls, as has been demonstrated by experiments in Japan.” P28

“There are several kinds of waves [generated during an earthquake]… P-waves are the first to arrive; that is, they travel fastest. These are push-pull waves analagous to sound waves, in which each particle moves back and forth as the waves propogate. Then come S-waves – shear waves. In this kind of wave, every particle moves up and down…P-waves can travel through liquids… S-waves cannot. Later still long period or L-waves arrive. These are surface waves, confined in their movement to the crust. They have different characteristics again, notably that they travel further more strongly than either P or S waves.” P73

“Seismic reflection profiles [using P-waves] tell us that the oceanic crust is thin; no more than 10 km thick, and in places half that thickness. By contrast, the crust under the continents is up to 40 km (24 miles), and always much thicker than under the ocean basins.” P74

“Mott Greene described the Alpine strata [the confused mountain folding and construction of the Alps] as a richly patterned tablecloth on a polished table: ‘If you should place your hand on the table and push forward, the cloth will begin to rise into folds. Push more and the folds will flop over forward and rearmost fold will progressively override those before it, producing a stack of folds [with a older rock from below lying on top an earlier more recent fold]… Take a pair of scissors and cut away at the stack from various angles, removing whole sections of folds [modeling erosion]. Having done so, push the pile again so that the segments become jumbled against each other.’” P 100 Repeat this 3 separate times over tens of millions and year, let stand and cool and voila you’ve created the Alps. This same process is happening in India, as the Himalyas grow through the collision of the Indian plate against the Asian plate. Other mountain ranges not on plate to plate collision boundaries are created through different mechanisms.

Before plate tectonics, the prevailing 19th century theory [created by Elie de Beaumont] behind mountain building was ‘That the state of our globe is such that, in a given time, the temperature of the interior is lowered by much greater quantity than on its surface, the solid crust would break up to accommodate itself to the internal mass… The idea that the earth was contracting as a result of it s slow cooling, mountains being thrown up as periodic adjustments. “ p 118 [Think of a shriveled up and drying apple.]

The source of the middle age dragon of myths? The famous Swiss geologist Eduard Suess “could not resist providing a rational explanation for legends. ‘[The Nile crocodile] lives in the neighborhood of Beirut, and it is a remarkable fact that the crocodile of the Nile still exists [in late 1880s] in the nearby estuary called Crocodile River 3 km north. Pliny [think Rome] knew of a town – Crocodilion – in this neighborhood… These facts also throw unexpected light on the numerous and circumstantial accounts of the slaughter of a scaly monster by the knight Deodat von Gozon on the isle of Rhodes in the first half of the 14th century.” P121

“For 50 years after Suess, physicists had been to work on the idea of the secular contraction of the earth, which was originally based on the concept that the earth was a cooling body. The discovery of radioactive heating [within the earth itself] upset all the calculations: it just wouldn’t work… Now there are those who thought, on the contrary, that the earth was expanding.” P123

“Deep hot rocks flow upwards [think of convection or how hot less dense air rises] deforming and creeping like a contrary glacier of the underworld. There was a model for this process in the salt dome of the Middle East. Deeply buried deposits of sea salt dome upwards and pass through over lying strata, as a kind of instrusive lobe, eventually emerging at the surface – the rising tongue is called a diaper… Even odder is to find all sorts of block of rocks lying around that have been carried up with the salt as it cut through strata overlying it, including limestone blocks with fossils. Some of these salt domes have acquired commercial importance as oil traps.” P124

[The eruption that created Santorini] was the biggest eruption that Homo Sapiens has ever witnessed. It has been estimated that 30 cubic KM of material of was erupted – that is 15 times more than the 79 AD Vesuvius eruption that eliminated Pompeii. The eruption of 1700 BC also coincided with the decline of the Minoan civilization [on Crete]… The Bronze Age could have had no better source for its subsequent legends… That Thera [the progenitor island of which today’s Santorini is a mere husk] was Atlantis is given some credence by Plato’s description of the place [pre-eruption] as comprising several concentric belts of lands and lagoons – a rather volcanic configuration [think Crater lake in Oregon]. According to one scholar, the eruption of Thera may have inspired at least one passage in the story of Jason and the Argonauts. When they returned to Greece with the golden fleece, they passed by Rhodes to the eastern part of Crete; here they experienced a pall of darkness – as indeed they would have done if they were under a cloud of volcanic ash. When they fled northwards, a bronze giant called Talos pelted them with fragments of rock (this hardly requires further explanation). The eastern Mediterranean is replete with flood legends, which are not unreasonably linked with tsunamis in the aftermath of major eruptions.” P130

“Suess coined the word [Gondwana – first mega continent hypothesized by geology] - The Gonds were an ancient Indian tribe. He saw very well the connections between geology of Africa, South America and India, and appreciated that they were bound together by more than coincidence.” P131

“[Arthur Holmes in 1911] proposed that the differential effects of such heating created convection currents in a deep substratum: solid yes, but sluggishly flowing over millions of years, in an analogous way to the proverbially slow creep of glaciers. The upward limbs of the convection cells reached the lithosphere and then parted in opposite directions… The drag of the convection cell provided the motor that moved the continents. Where a cell arose beneath a continent it could split it and create a new ocean; where a cell turned down again it might pull ocean crust with it to create a [trench].” P140

“Here lay the answer to the obvious question: if new curst was being added to the mid-ocean ridges, where was it being destroyed? The downward dipping earthquake zones where the burial traces of plunging ocean crust… The ‘scrape’ of the moving crust against the adjacent buttressed margin engendered earthquakes… Where it plunged downwards it would tend to melt…Magma would find its way to the surface in explosive volcanoes: hence the ring of fire [that circumscribes] Pacific Ocean.” P154

Geology 101 Review: Where did the epochs get their names?
Cambrian: Roman Wales was called Cambria
Devonian: Devon County in England
Ordovician, Silurian: Ancient British tribes
These regions hold the ‘type area’ for rocks of these ages. “Pioneer geologists did fieldwork in areas they could reach by train or on horseback, or by foot.” P181 So the first geologic finds would tend be in and near London.

“One can find the some word in Danish, daler. In due turn, this is a variant of thaler. The thaler was the standard silver coinage across Europe in the 16th century. Each thaler weighed an ounce… Further the name thaler was itself a concraction from Joachimsthaler, referring to the mine at Joachimsthal (the valley of Joachim)… in the Czech Republic [which] was once part of old Bohemia.” P 210 The origin of the word for the thing you value so highly in your wallet – the almighty dollar.

“[Inside a mine] is the only part of the world where the cockroach is viewed favorably: their insect senses are peculiarly alert to changes in pressure in the rock… when a rockfall threatens, the cockroaches head out of their hiding places and scamble… in order to escape being crushed.” P211

“The commonest elements, silicon and oxygen [over 75% of the crust by weight], combine to form the remarkably common mineral quartz… And then quartz gets tinted by traces of other elements to produce named varieties that are commonly sold in rock shops: amethyst colored by manganese; carnelian made red because of a dash iron.” P217

“The frequent mineral corundum… is a simple oxide of aluminum. But grown deep in the underworld under special circumstances, with the right impurity added by nature to induce color, it becomes the most regal of all jewels – ruby… If another quirk of contamination happens, corundum becomes blue, rather than red, and then you have a sapphire.” P218

“Only 8 elements make up 90% of Earth’s crust – oxygen [46.6% by weight of crust], silicon [27.7%], aluminum, iron, calcium, sodium, potassium, and magnesium in decreasing abundance.” P220

“Atoms of beryllium [the second lightest of all metallic elements] and aluminum make joins between silica [quartz again] hexagonal rings [creating the mineral beryl]. Beryl is hard – it can scratch glass. When beryl crystals grow under special conditions they may be transparent, and then beryl becomes a gemstone called aquamarine. If the color happens to be yellow, it is known as heliodor; if green, it is emerald. They are all simply beryllium aluminum silicate.” P222

“Olivine is a simple silicate of iron and/or magnesium. Olivine crystallizes out very early in the igneous magma chamber… In doing so [it] effectively removes magnesium, and other elements, from the remainder of the siliceous melt… Other minerals then crystallize out in their turn… Elements that are reluctant to shuffle into a silicate mineral early on will become [more concentrated]… You can now understand one of the ways in which elements that are very rare in nature can become concentrated… In the opposite fashion, after the magma has almost completely crystallized, a fluid residue remains containing many of the most volatile elements, chlorides and flourides, and metallic elements in solutions. These seek out cracks and fissures in the in the solidified magma, there to lay down their burden of minerals, the source of silver, copper, and zinc.” P224 [This is why you often see balsalt and granite with viens, and why precious metals are found in such veins.]

“The primitive lava, freshly minted from partially melted oceanic rock, crystallizes out first with calcium rich feldspar and olivine [this is pahoehoe lave and is very viscous]. The magma held in its deep chamber evolves in composition as these early crystals are removed… Later lavas are richer in silica, which makes them altogehter more gluey; and the feldspars are pushed towards the alkali end of their range [this is aa’aa lava], which vents up into cones composed of pumic and bombs.” P225

“Diamonds are found in ‘pipes’; these are huge tubular structures, punched deep through a part of continental shield… The pipes are believed to be the product of a violent gas-charged magmatic explosion that dragged diamonds from the deep within the earth [150 KM deep].” P226

“It seems that gold is not so aloof as it appears. Under conditions of high pressure and elevated temperature, and in the presence of water, it makes temporary alliances with hydrogen sulfide [rotten egg gas] to form hydrosulfide complexes that can migrate through the crust. The gold is deposited again in due turn as its temporary partner deserts. Certain other elements, such as arsenic… can scavenge gold… When golden rock is eventually weathered at the surface, heavy and incorruptible gold remains behind, and can be concentrated into gravels in river beds.” P227

“The most durable gold-work is not 100% pure, but has upto 20% of silver in it. The silver puts some strength into a metal that is definitively malleable and ductile.” P229

“There has been one time when the streets have truly been paved with gold. In the rush of 1893 in Kalgoorlie [Australia] prospectors threw away an excess of a bulky pyritous [fool’s gold or iron sulfide] material as they delved deeper for pure gold… They used this waste to fill in ruts or to bulk up sidewalks… There is in fact a stable compound that gold forms in nature with another rare element, tellurium. The waste material used to pave the roads in Kalgoorlie in fact turned out to be this very uncommon mineral, but nearly 3 years passed before anyone realized that their streets were paved with proverbial gold.” P230

The Deccan traps were formed by a hot spot akin to Hawaii, however this spot was over the Indian continental plate. “Single flows of highly liquid lava must have flooded out on a truly enormous scale, leaving flows the thickness of a house covering more than 100,000 square KM.” p232

“Increased accuracy in radiometric dating techniques has shown that the eruptions were bracketed by an interval as short as 1.5 million years. Considering the volume of basalt erupted – an estimated 2.5 million cubic KM – and the vast area over which it extended, this was an extraordinary outpouring of magmatic floods.” P239

“The Deccan Traps erupted 66 million years ago… The same period saw the extinction of the dinosaurs… Currently, rather more evidence favors the impact of a huge meteorite in the Yucatan peninsula…[But] the Deccan Traps is a remarkable temporal coincidence, and the eruption scenario is a hypothesis waiting in the wings.” P240

“Granite intrusions are found in mountain belts… They appear at the center of those most dramatic upheavals of the crust, where one plate collides with another.” P252

Granite is formed “deep within mountain belts. It was shown that natural granite magma should be the first to ‘melt out’ under the ambient conditions at depth if there were water present. Since water is well nigh ubiquitous in the crust, this was not an unreasonable assumption.” P259

“The image perhaps being more appropriate to denistry: mountainous molars have downward extensions into the tissues of the earth. [Remember the tablecloth analogy for moutain folding. Imagine some of those fold diving down into the table, and then melting.] While this happens the temperature gradient can be depressed for a long time, pushed downwards along with the folded sedimentary pile to a region of high pressure at depth. But, given time, the heat flow from the interior of the earth will be restored… Then as additional heat sweats out the granite magma from the already metamorphosed rocks, ready to mass up and rise as plutons through the developing mountain chain.” P261

“Now we can create a vision of the Archaean world [over 2.5 billion years ago]. Heat flow from the young earth was greater than it is now…There may have been more numerous convection cells within the mantle… The nascent masses of continental crust had not yet congealed to their present size. Instead smaller rafts of lighter rocks formed the nuclei of what would become more stable continental areas… Unprotected by any cloak of plants, the wind and rain worked fast upon the naked rocks. Slabs of oceanic rock were covered with sediments derived from the rapid weathering…” p304

“The [Archaean] atmosphere was initially almost without oxygen, while gaseous hydrogen sulfide and methane were abundant…Bacteria could thrive in the absence of oxygen – in fact oxygen is a poison [to many] types of them. Three billion years of photosynthesis…added oxygen molecule by molecule to the air… Life changed the air of the world – scrubbed out the early poisons – and in the process even modified the way rocks decayed, since oxygen is crucial to all sorts of chemical weathering.” P306

“If Pangea (when all of the earth’s continents were connected) split apart a couple hundred million years ago, but was itself assembled from dispersed continents, then is it not likely that there might be a still older ‘Pangea’ when the continents were married on a previous occasion?... There is now evidence for no less than 4 Precambrian supercontinents.” P310

“They were carbonate sedimentary rocks – dolomites and limestones. We knew that they were laid down under the warmth of a tropical sun… However, between two layers of warm-water rocks something very curious happened… For a brief interval the carbonates disappeared, and in their stead some utterly different rocks took the ground. Prominent among them was a reddish rock… full of boulders and blocks of various kinds and sizes, all suspended in a pink mud… When icebergs have calved from the glacier to drift out to sea, they deposit just such… a collection of stones, along with the ground up waste derived from the terrain they scoured… So here is the paradox: a glacial event extending so far into low latitudes as to drop its debris where, not long before [and after], tropical algae and bacteria has flourished on a limy sea floor… [The theory of] ‘Snowball Earth’ had been born. There was a vision of the whole planet engulfed in a shimmering ice sheet, from pole to pole [about 590 million years ago]… Furthermore, since the Cambrian biological explosion occurred not so very long after alleged great freeze had relented [it relented because greenhouse warming from volcanic gases was unabated by the freeze, and eventually the earth overheated to melt the glaciers], it was tempting to link the two as cause and effect.” P315

“As with many big ideas, the theory is not without its critics. They point out that more than 2/3 of the earth’s surface was covered with ocean, and since all 590 million year old ocean crust has long been since subducted to oblivion, there will never be proof that every part of that vast hinterland became deeply frozen.” P316 But in any case, it must have been damn cold near the equator for at least some parts of the earth. That’s not being debated.

“Joe Kirshvink [my old geology professor!] from Caltech, has recognized up to four ‘Snowball Earths’ between 900 and 590 MYA, and has recently added another far older one at about 2 BYA.” P316

“A study was carried out upon minute metamorphosed carbon specks within crystals of ancient [3.8 BY old] Greenland rocks… Cooked though it was, the stamp of life was there… If life was there 3.8 BYA, it would have experienced a day only 5 hours long, as a result of the earth spinning faster, and the moon would have hung large and close in the sky. [The moon is moving away from the earth at 2.5 inches a year, and the earth slows down its rotation in the process.]” p319

“The Grand Canyon was not always perceived as one of the 7 natural wonders… The first western visitor to reach it was Spaniard Garcia Lopez de Cardenas in 1540… Horrified by its impassableness, [he] returned to Mexico with no treasure for his pains… In 1858, the surveyor Joseph Ives was to report to his superiors that the ‘region is altogeher valueless. Ours has been the first, and will doubtless be the last, party of whites to visit this profitless locality.” P323

“[A] mnemonic told to me by a receptionist at the White Angel Lodge: Kissing Takes Concentration. However, Sex Requires Manoeuvring Between Tempting Variables: Kaibab, Toroweap, Coconino, Hermit, Supai, Redwall, Muav, Bright Angel, Tapeat, and Vishnu in that order. The succession of rock formations in the Grand Canyon.” P325

“A 1000 years of erosion has been calculated to wear back the walls by no more than 1 meter…”

“The center of the earth is some 6370 KM from mean sea level. The earth’s core extends to 2900 KM and takes up less than 20% of the total volume of the planet. The inner (solid) part of the core begins at 5120 KM below the surface.” P347

“It is worth remembering that the magnetic field is very weak – more than 100 times weaker that the field between the poles of a toy horseshoe magnet.” P349

“The first suggestion tha the world did behave like a magnet was made by William Gilbert before 1600; it took science nearly 400 years to investigate Gilbert’s insight in detail.” P349

“ Thermal convection in the outer (liquid) core is one possibility [for the magnetic dynamo’s energy]: a kind of deep, simmering turnover of the molten layer providing a motor of magnetism. Another possibility is that the inner core grows by liquid iron ‘freezing’ onto to it at its outer boundary. When this happens, it leaves behind a ‘light fraction’ in the other core that then rises, leading to another, but compositionally driven, form of convection.” P349

The magnetic poles have reversed many times in earth’s history. “The switchover from one polarity to another is completed within 4000 years – a mere blip in geological time… In fact, switching poles is a comparatively easy thing for a dynamo to do, and the flip may be controlled by relatively small changes of the fluid motions in the core.” P351

“If you were asked on a game show: ‘What is the most abundant material on earth?’ you should answer: ‘The perovskite phase of the lower mantle.’ The applause would be resounding. A simple calculation tells you why your answer is true. The layer of the earth containing perovskit stretches from 660 KM below the surface (The Moho discontinuity for you Geologists) to the boundary layer at the outer edge of the earth’s core, 2900 KM below the surface. It is a vast slice of the earth. Since perovskite comprise more than 70% of this layer, it is an easy calculation.” P355

“The Pacific ocean will be subducted away in a 100 million years or so, producing a new Pangea, when Asia and America finally conjoin.” [Few oceans live beyond 200 million years from birth to death. This lifespan is called the Wilson Cycle after the famous geologist J. Tuzo Wilson. ]

“The Amazon is no less than 200 miles wide at its mouth. More water passes through it in a day than does in a whole year through the River Thames.” P388


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