A bad year for earthquakes

At least 20,000 people died in six serious earthquakes in 1999, and many more were left injured and homeless. The worst of these earthquakes was in northwest Turkey in August, when more than 16,000 people died in the densely-populated area around Izmit. In October California was ‘lucky’ because a major earthquake occurred in the Mojave Desert rather than near Los Angles, 160 km away.

All this does not mean that earthquakes are becoming more frequent or more powerful than in the past. The difference is that more people are at risk as the world population grows. Rich countries have been able to cut the death toll from earthquakes by developing anti-quake technology and building ‘flexible’ buildings that sway during tremors. This has not happened in poorer countries, where poor-quality buildings and rapidly-growing populations have increased the danger.

The devastation caused by the Turkish earthquake was much worse than it need have been. Scientists had warned that the country’s industrial region, as well as thousands of homes, had been built in the area of highest seismic risk.

The number of potential earthquake victims has also been increased by the migration of people from rural areas to towns, where they tend to be much more crowded together. This is a particular problem in high-risk areas like the Pacific rim.

Limestone in Europe

Limestone landscapes are distinctive and widespread. The rock occurs in a number of different forms and, depending on the historical and present-day climate, it will give rise to a variety of landforms. Across Europe the various types of limestone produce spectacular scenery. The termkarst, which comes from a region in Slovenia, is often used to describe such landscapes. They are found through Slovenia, Croatia, Bosnia-Herzogovina, Montenegro and Albania.

Mountain limestones occur throughout the Alps, extending westward to France. Beyond the Alps limestone is found in the Grands-Causses region of the Massif Central, where the Tarn and Lot rivers have cut steep gorges.

Limestone also forms the underlying geology of many Mediterranean islands. Throughout southern Europe these rocks owe their origin to deposition in the ancient Tethys Sea, of which the Mediterranean is a small remnant. They have been lifted by tectonic forces and eroded by water and ice to produce the steep slopes,, gorges and caves associated with the rock.

Limestone also occurs in northern Europe, where it is the product of deposition in much more ancient seas. In the English Pennines limestone was formed some 350 million years ago in the Carboniferous period. Around Ingleton and Malham in Yorkshire we can see, on a micro scale, the karst features typical of those in eastern Europe.

Vulcanism

A volcano is an opening in the earth’s crust through which molten rock, usually called magma while underground and lava aboveground, pours forth. Because the emerging material accumulates near the orifice, most volcanoes in the course of time build up mountains with a characteristic conical shape that steepens toward the top, with a small depression or crater at the summit. Lava escapes almost continuously from a few volcanoes, but the majority are active only at intervals.

Volcanic Eruptions

A volcanic eruption is one of the most awesome spectacles in all nature. Usually earthquakes provide a warning a few hours or a few days beforehand – minor shocks probably caused by the movement of gases and liquids underground. An explosion or a series of explosions begins the eruption, sending a great cloud billowing upward from the crater. In the cloud are various gases, dust, fragments of solid material blown from the crater and the upper part of the volcano’s orifice, and larger solid fragments representing molten rock blown to bits and hurled upward by the violence of the explosions.

Gas continues to issue in great quantities, and explosions recur at intervals. The cloud may persist for days or weeks with its lower part glowing red at night. Activity gradually slackens, and presently a tongue of white-hot lava spills over the edge of the crater or pours out of a fissure on the mountain slope. Other flows may follow the first, and explosive activity may continue with diminished intensity. Slowly the volcano becomes quiescent, until only a small steam cloud above the crater suggests its activity.

Not all eruptions by any means follow this particular pattern. Volcanoes are notoriously individualistic, each one having some quirks of behavior not shared by others. In one group of volcanoes the explosive type of activity is dominant, little or no fluid lava appearing during eruptions. Cones of these volcanoes, built entirely of fragmental material ejected in a solid or nearly solid state, are very steep sided; examples are found in the West Indies, in Japan, and in the Philippines. Other volcanoes, like those of Hawaii, have eruptions characterized by quiet lava flows with little explosive activity. Mountains built by these volcanoes are broad and gently sloping, quite different from the usual volcanic structure. The most common kind of volcano is neither wholly of the “explosive” type nor wholly of the “quiet” type, but has eruptions in which both lava flows and gas explosions occur.

The chief factors that determine whether an eruption will tend to be a largely quiet lava flow or tend to be explosive are the viscosity of the magma and the amount of gas it contains. (The greater the viscosity of a liquid, the less freely it flows: honey is more viscous than water.) Magma is a complex mixture of the oxides of various metals with silica and usually has an abundance of gas dissolved in it under pressure. Like most molten silicates it is extremely viscous, and with rare exceptions lava creeps downhill slowly, like thick syrup or tar. The viscosity depends upon chemical composition; magmas with high percentages of silica are the most viscous. The presence of gas also affects viscosity; magmas with little gas are the most viscous. If the magma feeding a volcano happens to be rich in both gas and silica, the eruption will be explosive. A magma with modest gas and silica contents results in a quiet eruption.

The gaseous products of volcanic activity include water vapor, carbon dioxide, nitrogen, hydrogen, and various sulfur compounds. The most prominent constituent is water vapor. Some of it comes from groundwater heated by magma, some comes form the combination of hydrogen in the magma with atmospheric oxygen, and some was formerly incorporated in rocks deep in the crust and is carried upward by the magma to be released at the surface. Much of the water vapor condenses when it escapes to give rise to the torrential rains that often accompany eruptions.

Glaciers

In a cold climate with abundant snowfall, the snow of winter may not completely melt or evaporate during the following summer, and so a deposit of snow accumulates from year to year. Partial melting and continual increase in pressure cause the lower part of a snow deposit to change gradually into ice. If the ice is sufficiently thick, gravity forces it to move slowly downhill. A moving mass of ice formed in this manner is called a glacier. Approximately 10 percent of the earth’s land area is covered by glacial ice at the present time.

Today’s glaciers are of two principal types:

Valley glaciers – found, for instance, in the Alps, on the Alaskan coast, in the western United States – are patches and tongues of dirty ice lying in mountain valleys. These glaciers move slowly down their valleys, melting copiously at their lower ends; the combination of downward movement and melting keeps their ends in approximately the same position from year to year. Movement in the faster valley glaciers (a few feet per day) is sufficient to keep their lower ends well below timberline.

Glaciers of another type cover most of Greenland and Antarctica: huge masses of ice thousands of feet thick and thousands of square miles in area, engulfing hills as well as valleys, and appropriately called continental glaciers or ice caps. These, too, move downhill, but the “hill” is the slope of their upper surfaces. An ice cap has the shape of a broad dome, its surface sloping outward from a thick central portion of greatest snow accumulation: its motion is radially outward in all directions from its center. The icebergs of the polar seas are fragments that have broken off the edges of ice caps. Similar sheets of ice extended across Canada and northern Eurasia in relatively recent geological history.

Apparently a glacier moves by internal fracture and healing in the crystals of solid ice as well as by sliding along its bed. Like a stream, a glacier carries along rock fragments which serve as tools in cutting its bed. Some fragments are the debris of weathering that drop on the glacier from its sides; others are torn from its bed when melted water freezes in rock cervices. Fragments at the bottom surface of the glacier, held firmly in the grip of the ice and dragged slowly along its bed, gouge and polish the bedrock and are themselves flattened and scratched. Smoothed and striated rock surfaces and deposits of debris containing boulders with flattened sides are common near the ends of valley glaciers. Where such evidence of the grinding and polishing of ice erosion is found far from present-day glaciers, we have reason to infer that glaciation was present there in the past.

Valley glaciers form in valleys carved originally by streams. A mountain stream cuts like a knife vertically downward, letting slope wash, slumping, and minor tributaries shape its valley walls; by contrast, a glacier is a blunt erosional instrument which grinds down simultaneously all parts of its valley floor and far up the sides as well. Effects of this erosion are best seen in valleys that have been glaciated in the past but in which glaciers have dwindled greatly or disappeared. Typically such valleys have U-shaped cross sections with very steep sides, instead of the V shapes produced by stream erosion. Their heads are round, steep-walled amphitheaters called cirques, in contrast to the small gullies at the heads of stream valleys. Tributary streams often drop into a formerly glaciated valley over high cliffs because a large glacier carves out its channel much more actively than a small one does. A tributary valley left stranded high above its main valley is called a hanging valley and is often the scene of a spectacular waterfall.

Divides between cirques and between adjacent U-shaped valleys tend to be sharp ridges because of the steepness of the valley walls. In general, since valley glaciers produce deep gorges, steep slopes, and knifelike ridges, their effect is to make mountain topography extremely rugged. The earth’s most spectacular mountain scenery is in regions (the Alps, the Rockies, the Himalayas) where valley glaciers were large and numerous several thousand years ago.

The influence of ice caps on landscapes is very different from that of valley glaciers. We cannot, of course, observe directly the effect of existing ice caps on the buried landscapes of Greenland and Antarctica, but larger ice caps that once covered much of Northern Europe and North America have left clear records of their erosional activity, which we can easily see from the rounded hills and valleys, the abundant lakes and swamps so characteristic of these regions. Like a gigantic piece of sandpaper, an ice cap rounds off sharp corners, wears down hills, and fills depressions with debris, leaving innumerable shallow basins which form lakes when the ice recedes.

Glacial erosion is locally very impressive, particularly in high mountains. The amount of debris and the size of the boulders that a glacier can carry are often startling. But in general, on a worldwide basis, the erosional work accomplished by glaciers is small. Only rarely have they eroded rock surfaces deeply, and the amount of material transported long distances is insignificant compared with that carried by streams. Most glaciers of today are but feeble descendants of mighty ancestors, but even these ancestors succeeded only in modifying landscapes already shaped by running water.

Minerals

Rocks are aggregates of substances called minerals, which as a rule are crystalline solids with fairly definite compositions and structures. Some rocks, for instance limestone, consist of a single mineral only, but the majority consist of several minerals in varying proportions. The different minerals in a coarse-grained rock like granite are apparent to the eye; in fine-grained rock, the separate minerals can be discerned with the help of a microscope.

What Minerals Are

It is not difficult to understand why certain substances occur as minerals and why others do not. We expect to find the more chemically inactive elements, such as gold, platinum, and sulfur, in the free state, whereas chemically active elements, such as sodium, calcium, and chlorine, are always found in combination as compounds. Compounds readily soluble in water, such as sodium chloride, sodium carbonate, and potassium nitrate, form deposits in desert regions but are rare elsewhere. Substances that tend to react with oxygen occur only well below the surface away from the oxygen of the atmosphere. Unstable compounds like phosphorus pentoxide are necessarily absent from the earth’s crust.

Silicates are by far the most abundant minerals; mica, feldspar, and topaz are familiar examples. Carbonates are another important class, its most conspicuous representative being the carbonate of calcium called calcite. Oxides and hydrated oxides include such common materials as hematite (ferric oxide), the chief ore of iron, and bauxite (hydrated aluminium oxide), the chief ore of aluminium. Various metals are obtained from deposits of sulfide minerals, such as galena (lead sulfide and sphalerite (zinc sulfide). Elements that occur free, or native, were mentioned above. Less frequent as minerals are sulfates, phosphates, and chlorides.

Unfortunately the study of minerals requires the learning of a special list of names, some of them apparently duplicates of other names. As an example, the mineral whose formula is CaCO₃ is given the name calcite instead of the chemical name calcium carbonate. For this seeming redundancy there are two reasons:

The formula CaCO₃ describes not only the composition of calcite but also that of aragonite, a less common mineral with a different crystal form, hardness, density, and so on; the chemical name calcium carbonate alone does not distinguish between calcite and aragonite.

Calcite often contains small quantities of MgCO₃ and FeCO₃ , and its composition is not precisely represented by the formula CaCO₃ because the iron and magnesium carbonates form an integral part of the calcite structure with Fe and Mg atoms replacing some of the Ca atoms in the crystal lattice.

Many other mineral formulas besides that of calcite apply to two or more distinct substances, and most minerals show a similar slight variability in composition. Hence chemical names are seldom really applicable, and the student of minerals finds necessary a new nomenclature.

Luckily, for present purposes we need only a few additions to our vocabulary. More than 2,000 different minerals are known, but most of these are rare. Even among the commoner minerals, the greater number occur abundantly only in occasional veins, pockets, and layers. The number of minerals that are important constituents of ordinary rocks is surprisingly small, so small that acquaintance with less than a dozen is adequate for an introduction to geology.

Mineral Properties

Common minerals are not only limited in number but are also easily recognizable with some experience, often by appearance alone. To distinguish the rarer minerals microscopic examination and chemical tests may be necessary, but for the minerals that compose ordinary rocks such simple physical properties as density, color, hardness, and crystal form make identification relatively straightforward.

In describing the important rock-forming minerals, two properties need special attention: crystal form and cleavage. Most minerals are crystalline solids, which means that their tiny particles (atoms, ions, or atom groups) are arranged in lattice structures with definite geometric patterns. When a mineral grain develops in a position where its growth is not hindered by neighboring crystals, as in an open cavity, its inner structure expresses itself by the formation of perfect crystals, with smooth faces meeting each other at sharp angles. Every mineral has crystals of a distinctive shape so that well-formed crystals make recognition of a mineral easy; unfortunately good crystals are rare, since mineral grains usually interfere with one another’s growth.

Even when well-developed crystals are not present, however, the characteristic lattice structure of a mineral may reveal itself in the property called cleavage. This is the tendency of a substance to split along certain planes, which are determined by the arrangement of particles in its lattice. When a mineral grain is struck with a hammer, its cleavage planes are revealed as the preferred directions of breaking; even without actual breaking, the existence of cleavage in a mineral is usually shown by flat, parallel faces and minute parallel cracks. The flat surfaces of mica flakes, for instance, and the ability of mica to peel off in thin sheets show that this mineral has almost perfect cleavage. Some minerals (for example, quartz) have practically no cleavage; when struck they shatter, like glass, along random curved surfaces. The ability to recognize different kinds and degrees of cleavage is an important aid in distinguishing minerals.