The Alps

sejour-2-alpes The great Carthaginian general Hannibal in 218 b.c.e. stood staringat the massive barrier of the jagged, ice-capped peaks of the Alps, weighing whether he could battle both geography and the Roman armies seeking to trap him here against the seemingly fatal wall of rock. Around him, he had gathered 38,000 soldiers, 8,000 horsemen, and 37 armored war elephants. Ahead lay a frightening wilderness of 14,000-foot (4,270- m) high peaks, most sheathed in great glaciers, gleaming brilliant white in the thin air. Innumerable rivers thundering with ice melt blocked his path, snows covered the grass that might  ustain his horses and elephants, hostile tribes haunted the passes, and the freezing cold waited to debilitate his soldiers. But the Roman armies were closing in on him here on the great plains of Europe, hoping to bring him to bay and make him fight far from the  vulnerable heart of their youthful empire. Meanwhile, the lush, rich provinces of  Rome  lay safely behind that mountain range, protected on every other side from invasion by the great Roman navy. Hannibal’s only chance for victory against the bitter rival of Carthage lay through those deadly Alpine passes. He gave the order to his army to move into the mountain and so  commenced one of the most famous military maneuvers in history. Fewer than half of his soldiers and only a handful of his elephants would emerge from those unforgiving mountains. But he fell upon the stunned Romans like a thunderbolt, using surprise and brilliant tactics to win a series of major battles. Even so, the Alps remained at his back,  cutting him off from reinforcements and resupply from Carthage, far away in North Africa. In the end, even though he defied the mountain range, geography proved decisive—as it has so often proved in the intricate skirmishes, gambles, and disasters of human history. In a real sense, the great wall of the Alps running across central Europe has shaped European history, and so the human history of the planet. The mountains protected Rome, which took advantage of its maritime dominance of the Mediterranean to lay the foundations of 3G
The Alps Europe
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2alpesWestern civilization. Rome finally fell when warlike people to the north made it past the fortress of the Alps into Italy. So this jagged upthrust of rock shaped the whole history of the continent. (See upper color insert on page C-2.) Alps Reveal Deep Secrets
In more recent centuries the Alps have also revealed the deep secrets of the Earth. Quite aside from offering the challenge that led to the invention of mountaineering in the 1800s, the Alps have become perhaps the best-studied mountain range on the planet. The Alps boast more than 50 peaks higher than 14,000 feet (4,270 m), the tallest of which is Mont Blanc, at 15,770 feet (4,810 m). The range stretches from Austria and Slovenia in the east, through Italy, Switzerland, Liechtenstein, and Germany to France in the west. Geologists have unraveled the deepest mysteries of the Earth by studying the intricate folds of ancient seabeds, great masses of fused and remelted rock thrust up from miles beneath the surface, and the bewildering jumble of rocks with dramatically different histories. The rocks of the peaks include pieces of oceanic crust, rocks from the deep mantle, and bits of crunched, buried and resurrected continents. This generations-long scientific effort to understand the complex layers and cataclysms of the Alps has made vital contributions to the understanding of the evolution of the Earth and the development of the theory of plate tectonics.
The Alps owe their complicated existence to a head-on collision between two great crustal plates—one carrying Europe and Asia, the other carrying Africa. Just to complicate matters, the sediments of a small ocean covering an oceanic crustal plate got caught in between. This threeway collision created a slow-motion cataclysm and the complex    geology of the modern Alps. The ultimate rise of the Alps started more than 250 million years ago as the supercontinent of Pangaea began its inexorable breakup. Currents deep in the molten core of the Earth transmitted outward to the molten and semimolten mantle and then along to the crust caused the constant shifting of the crustal plates. The lighter, thicker, continental crust floated atop the dense rock of the plates, like icebergs. The great convection currents of the mantle forced the overlying plates to constantly bump up  against one another, dragging the high-riding continents along with them. These crustal plates are constantly created anew along the great system of undersea fractures that cause a global network of undersea mountain chains, like the Mid-Atlantic Ridge. But since the surface of the Earth does not get any larger, the creation of new plate material at one edge demands the destruction beneath the great undersea trenches of a similar  amount of material at the other end. So the plates move like conveyor belts, carrying the floating bits of continents along. Sometimes, the lighter rocks of the continents are pulled down into the trenches, but more often they get scraped off the descending plate. That scraping process has built most of the great mountain chains, like the Alps, the  Himalayas, and the Andes. It also accounts for why rocks on the continents are often 1 or 2 billion years old, while the oldest oceanic crust is merely 200 to 300 million years old. The oceanic crust is constantly created and consumed—endlessly recycled. But the light crustal rock floats along, surviving the destruction of one plate to be scraped off and carried away by a new plate. Studies suggest a certain pattern in all this crustal plate destruction and continental movement. Evidence suggests that about once every 500 million years, this movement winds up smashing together most of the light continental rock. So instead of having seven scattered continents,1 during such periods a single great continent accounts for most of the land area. In fact, projections of continental movements suggest that we are currently about halfway through one of those cycles and that the continents will reconvene in another 250 million years or so. So the roots of the Alps go back to the last time the continents gathered together in the supercontinent geologists have dubbed Pangaea, which was formed from the merger of two earlier massive continental masses—Gondwana and Laurasia. This great continent spanned the equator of a planet without ice caps, a land of vast inland seas, mild temperatures, great interior deserts, and limitless marshes. It proved the perfect setting of a great proliferation of life, including the rise of the dinosaurs and other life-forms. The earlier collision between Gondwana and Laurasia had raised a great chain of mountains. In such a collision, sometimes the plate edges buckle and fold, raising up great piles of rock from each  continent. Sometimes, one crustal plate rides up over the other, scraping rock off the top of the subducting plate but driving the smaller, over-ridden plate deep down into the mantle. The descending plate begins to melt miles beneath the surface. Geologists have named the great mountain chain raised by that first collision the Hercynic range. Once Gondwana and Laurasia were fused into Pangaea and the crustal plate motions shifted, the Hercynic range stopped rising and started eroding. Its fused, metamorphic rocks were reduced to gravel and sand and washed down into the lowlands, where they would eventually be reconstituted to become the distinctive sandstones and conglomerates of the later Alps. So these surviving rocks in the current Alps bear witness to the entire birth and death of an older mountain range. Moreover, the shallow seas that covered much
of the area during this period resulted in the formation of great layers of limestone in the seabeds. Made from the skeletons of tiny creatures that died and settled to the bottom of these shallow seas, limestone is one of the most revealing of rocks. The steady rain of these calcium-rich remains formed layers that compressed under their own accumulating weight to eventually form limestone. Such thick, level limestone layers mark the
one-time existence of an ocean bottom, even when eventually lifted to the top of Mount Everest or the lofty peaks of the Alps. Some 180 million years ago at the start of the Jurassic period, the crustal plates once again shifted in their movements. At that time, the
oceans brimmed with fish, squid, ammonites and great sea-going dinosaurs like the Ichthysaurs and the long-necked Plesiosaurs. On the land, the giant plant-eating dinosaurs were harassed by the small ancestors of the great meat-eating dinosaurs, and the ungainly, batlike Pterosaurs dominated the skies. During the 70-million-year-long Jurassic period, marshes bordered by gigantic ferns and palmlike cyads began accumulating the organic sediment that would eventually be buried, pressurized, heated, and turned into the oil and coal deposits that sustain modern economies.
The Jurassic period actually draws its name from the Jura Mountains on the edge of the Alps between France and Switzerland, where much of the study of modern geology had its start. At the onset of the Jurassic, a narrow new ocean began to open up along a fissure that began to split open Pangaea. As a result, what would become North America, Europe, and Asia moved north while the future Africa and South America moved south. The dense oceanic crust fused from the magma that forced its way to the surface along this rift system formed a growing mass of oceanic crust labeled the Piemont-Liguria Ocean, an arm of the much larger Tethys Ocean further east. Because no continental landmasses existed near the North or South Poles in the Jurassic, the planet lacked ice caps and sea levels were much higher. Shallow seas connected these ocean basins where limestone continued to form. The Piemont-Liguria Ocean never grew as large as the Atlantic Ocean, which also began forming in this period. Instead, it was more like today’s Red Sea, a narrow rift between continents.