Weather

WEATHER IS DEFINED AS THE ATMOSPHERIC CONDITIONS at a particular time and place; climate is the average weather conditions for a given region over time. Weather is assessed in terms of temperature, wind, cloud cover, and precipitation, such as rain or snow. Good weather is associated with high-pressure areas, where air is sinking. Cloudy, wet, changeable weather is common in low-pressure zones with rising, unstable air. Such conditions occur at temperate latitudes,
where warm air meets cool air along the polar fronts. Here, spiraling low-pressure cells known as depressions (mid-latitude cyclones) often form. A depression usually contains a sector of warmer air, beginning at a warm front and ending at a cold front. If the two fronts merge, forming an occluded front, the warm air is pushed upward. An extreme form of low-pressure cell is a hurricane (also called a typhoon or tropical cyclone), which brings torrential rain and exceptionally strong winds.

The atmosphere

THE EARTH IS SURROUNDED BY ITS ATMOSPHERE, a blanket of gases that enables life to exist on the planet. This layer has no definite outer edge, gradually becoming thinner until it merges into space, but over 80 percent of atmospheric gases are held by gravity within about 12 miles (20 km) of the Earth’s surface. The atmosphere blocks out much harmful ultraviolet solar radiation, and insulates the Earth against extremes of temperature by limiting both incoming solar radiation and the escape of reradiated heat into space. This natural balance may be distorted by the greenhouse effect, as gases such as carbon dioxide have built up in the atmosphere, trapping more heat. Close to the Earth’s surface, differences in air temperature and pressure cause air to circulate between the equator and poles. This circulation, together with the Coriolis force, gives rise to the prevailing surface winds and the high-level jet streams.

The ocean floor

THE OCEAN FLOOR COMPRISES TWO SECTIONS: the continental shelf and slope, and the deep-ocean floor. The continental shelf and slope are part of the continental crust, but may extend far into the ocean. Sloping quite gently to a depth of about 460 feet (140 m), the continental shelf is covered in sandy deposits shaped by waves and tidal currents. At the edge of the continental shelf, the seabed slopes down to the abyssal plain, which lies at an average depth of about 12,500 feet (3,800 m). On this deep- ocean floor is a layer of sediment made up of clays, fine oozes formed from the remains of tiny sea creatures, and occasional mineralrich deposits. Echo-sounding and remote sensing from satellites has revealed that the abyssal plain is divided by a system of mountain ranges, far bigger than any on land—the mid-ocean ridge. Here, magma (molten rock) wells up from the Earth’s interior and solidifies, widening the ocean floor. As the ocean floor spreads, volcanoes that have formed over hot spots in the crust move away from their magma source; they become extinct and are increasingly submerged and eroded. Volcanoes eroded below sea level remain as seamounts (underwater mountains). In warm waters, a volcano that projects above the ocean surface often acquires a fringing coral reef, which may develop into an atoll as the volcano becomes submerged.

Oceans and seas

OCEANS AND SEAS COVER ABOUT 70 PERCENT of the Earth’s surface and account for about 97 percent of its total water. These oceans and seas play a crucial role in regulating temperature variations and determining climate. Their waters absorb heat from the Sun, especially in tropical regions, and the surface currents distribute it around the Earth, warming overlying air masses and neighboring land in winter and cooling them in summer. The oceans are never still. Differences in temperature and salinity drive deep current systems, while surface currents are generated by winds blowing over the oceans. All currents are deflected—to the right in the Northern Hemisphere, to the left in the Southern Hemisphere—as a result of the Earth’s rotation. This deflective factor is known as the Coriolis force. A current that begins on the surface is immediately deflected. This current in
turn generates a current in the layer of water beneath, which is also deflected. As the movement is transmitted downward, the deflections form an Ekman spiral. The waters of the oceans and seas are
also moved by the constant ebb and flow of tides. These are caused by the gravitational pull of the Moon and Sun. The highest tides (Spring tides) occur at full and new Moon; the lowest tides (neap tides) occur at first and last quarter.

Coastlines

COASTLINES ARE AMONG THE MOST RAPIDLY changing landscape features. Some are eroded by waves, wind, and rain, causing cliffs to be undercut and caves to be hollowed out of solid rock. Others are built up by waves transporting sand and small rocks in a process known as longshore drift, and by rivers depositing sediment in deltas. Additional influences include the activities of living organisms such as coral, crustal movements, and sea-level variations due to climatic changes. Rising land or a drop in sea level creates an emergent coastline, with cliffs and beaches stranded above the new shoreline. Sinking land or a rise in sea level produces a drowned coastline, typified by fjords (submerged glacial valleys) or submerged river valleys.

Lakes and groundwater

NATURAL LAKE OCCUR WHERE a large quantity of water collects in a hollow in impermeable rock, or is prevented from draining away by a barrier, such as moraine (glacial deposits) or solidified lava. Lakes are often relatively short-lived landscape features, as they tend to become silted up by sediment from the streams and rivers that feed them. Some of the more long-lasting lakes are found in deep rift valleys formed by vertical movements of the Earth’s crust—for example, Lake Baikal in Russia, the world’s largest freshwater lake, and the Dead Sea in the Middle East, one of the world’s saltiest lakes. Where water is able to drain away, it sinks into the ground until it reaches a layer of
impermeable rock, then accumulates in the permeable rock above it; this watersaturated permeable rock is called an aquifer. The saturated zone varies in depth according to seasonal and climatic changes. In wet conditions, the water stored underground builds up, while in dry periods it becomes depleted. Where the upper edge of the saturated zone—the water table—meets the ground surface, water emerges as springs. In an artesian basin, where the aquifer is below an aquiclude (layer of impermeable rock), the water table throughout the basin is determined by its height at the rim. In the center of such a basin, the water table is above ground level. The water in the basin is thus trapped below the water table and can rise under its own pressure along faultlines or well shafts.

River features

RIVERS ARE ONE OF THE MAJOR FORCES that shape the landscape. Near its source, a river is steep. It erodes downward, carving out V-shaped valleys and deep gorges. Waterfalls and rapids are formed where the river flows from hard rock to softer, more easily eroded rock. Farther downstream, meanders may form and there is greater sideways erosion, resulting in a broad river valley. The river
sometimes erodes through the neck of a meander to form an oxbow lake. Sediment deposited on the valley floor by meandering rivers and during floods helps to create a floodplain. Floods may also deposit sediment on the banks of the river to form levees. As a river spills into the sea or a lake, it deposits large amounts of sediment, and may form a delta. A delta is an area of sand bars, swamps, and lagoons through which the river flows in several channels called distributaries—the Mississippi
delta, for example. Often, a rise in sea level may have flooded the river mouth to form a broad estuary, a tidal section where seawater mixes with fresh water.

Rivers

RIVERS FORM PART of the water cycle—the continuous circulation of water between the land, sea, and atmosphere. The source of a river may be a mountain spring or lake, or a melting glacier. The course that the river subsequently takes depends on the slope of the terrain and on the rock types and formations over which it flows. In its early, upland stages, a river tumbles steeply over rocks and boulders and cuts a steep-sided V-shaped valley. Farther downstream, it flows smoothly over sediments and forms winding meanders, eroding sideways to create broad valleys and plains. On reaching the coast, the river may deposit sediment to form an estuary or delta.

Glaciers

A VALLEY GLACIER IS A LARGE MASS OF ICE that forms on land and moves slowly downhill under its own weight. It is formed from snow that collects in cirques (mountain hollows also known as corries) and compresses into ice as more and more snow accumulates. The cirque is deepened by frost wedging and abrasion, and arêtes (sharp ridges) develop between adjacent cirques. Eventually, so much ice builds up that the glacier begins to move downhill. As the glacier moves it collects moraine (debris), which may range in size from particles of dust to large boulders. The rocks at the base of the glacier erode the glacial valley, giving it a U-shaped cross-section. Under the glacier, roches moutonnées (eroded outcrops of hard rock) and drumlins (rounded mounds of rock and clay)
are left behind on the valley floor. The glacier ends at a terminus (the snout), where the ice melts as fast as it arrives. If the temperature increases, the ice melts faster than it arrives, and the glacier retreats. The retreating glacier leaves behind its moraine and also erratics (isolated single boulders). Glacial streams from the melting glacier deposit eskers and kames (ridges and mounds of sand and gravel), but carry away the finer sediment to form a stratified outwash plain. Lumps of ice carried on to this plain melt, creating holes called kettles.

Caves

CAVES COMMONLY FORM in areas of limestone, although on coastlines they also occur in other rocks. Limestone is made of calcite (calcium carbonate), which dissolves in the carbonic acid naturally present in rainwater, and in humic acids from the decay of vegetation. The acidic water
trickles down through cracks and joints in the limestone and between rock layers, breaking up the surface terrain into clints (blocks of rock), separated by grikes (deep cracks), and punctuated by sink-holes (also called swallow-holes or potholes) into which surface streams may disappear. Underground, the acidic water dissolves the rock around crevices, opening up a network of passages and caves, which can become large caverns if the roofs collapse. Various features are formed when the dissolved calcite is redeposited; for example, it may be redeposited along an underground stream to form a gour (series of calcite ridges), or in caves and passages to form stalactites and stalagmites. Stalactites develop where calcite is left behind as water drips from the roof; where the drops land, stalagmites build up.

Weathering and erosion

WEATHERING IS THE BREAKING DOWN of rocks on the Earth’s surface. There are two main types: physical (or mechanical) and chemical. Physical weathering may be caused by temperature changes, such as freezing and thawing, or by abrasion from material carried by winds, rivers, or glaciers. Rocks may also be broken down by the actions of animals and plants, such as the burrowing of animals and the growth of roots. Chemical weathering causes rocks to decompose by changing
their chemical composition—for example, rainwater may dissolve certain minerals in a rock. Erosion is the wearing away and removal of land surfaces by water, wind, or ice. It is greatest in areas of little or no surface vegetation, such as deserts, where sand dunes may form.

Mineral resources

MINERAL RESOURCES CAN BE DEFINED AS naturally occurring substances that can be extracted from the Earth and are useful as fuels and raw materials. Coal, oil, and gas – collectively
called fossil fuels – are commonly included in this group, but are not strictly minerals, because they are of organic origin. Coal formation begins when vegetation is buried and partly decomposed to form peat. Overlying sediments compress the peat and transform it into lignite (soft brown coal). As the overlying sediments accumulate, increasing pressure and temperature eventually transform the lignite into bituminous and hard anthracite coals. Oil and gas are usually formed from organic matter that was deposited in marine sediments. Under the effects of heat and pressure, the compressed organic matter undergoes complex chemical changes to form oil and gas. The oil and gas percolate
upwards through water-saturated, permeable rocks and they may rise to the Earth’s surface or accumulate below an impermeable layer of rock that has been folded or faulted to form a trap – an anticline (upfold) trap, for example. Minerals are inorganic substances that may consist of a single chemical element, such as gold, silver, or copper, or combinations of elements. Some minerals are concentrated in mineralization zones in rock associated with crustal movements or volcanic activity. Others may be found in sediments as placer deposits – accumulations of high-density minerals that have been weathered out of rocks, transported, and deposited (on riverbeds, for example).

Fossils

FOSSILS ARE THE REMAINS of plants and animals that have been preserved in rock. A fossil may be the preserved remains of an organism itself, an impression of it in rock, or preserved traces (known as trace fossils) left by an organism while it was alive, such as organic carbon outlines, fossilized footprints, or droppings. Most dead organisms soon rot away or are eaten by scavengers.
For fossilization to occur, rapid burial by sediment is necessary. The organism decays, but the harder parts— bones, teeth, and shells, for example—may be preserved and hardened by minerals from the surrounding sediment. Fossilization may also occur even when the hard parts of an organism are dissolved away to leave an impression called a mold. The mold is filled by minerals, thereby creating a cast of the organism. The study of fossils (paleontology) can not only show how living things have evolved, but can also help to reveal the Earth’s geological history—for example, by aiding in the dating of rock strata.

Sedimentary rocks

SEDIMENTARY ROCKS ARE FORMED BY THE ACCUMULATION and consolidation of sediments. There are three main types of sedimentary rock. Clastic sedimentary rocks, such as breccia or sandstone, are formed from other rocks that have been broken down into fragments by weathering, which have then been transported and deposited elsewhere. Organic sedimentary rocks—for example, coal—are derived from plant and animal remains. Chemical sedimentary rocks are formed by chemical processes. For example, rock salt is formed when salt dissolved in water is deposited as the water evaporates. Sedimentary rocks are laid down in layers, called beds or strata. Each new layer is laid down horizontally over older ones. There are usually some gaps in the sequence, called unconformities. These represent periods in which no new sediments were being laid down, or when earlier sedimentary layers were raised above sea level and eroded away.

Igneous and metamorphic rocks

IGNEOUS ROCKS ARE FORMED WHEN MAGMA (molten rock that originates from deep beneath the Earth’s crust) cools and solidifies. There are two main types of igneous rock: intrusive and extrusive. Intrusive rocks are formed deep underground where magma is forced into cracks or between rock layers to form structures such as sills, dikes, and batholiths. The magma cools slowly to
form coarse-grained rocks such as gabbro and pegmatite. Extrusive rocks are formed above the Earth’s surface from lava (magma that has been ejected in a volcanic eruption). The molten lava cools quickly, producing fine-grained rocks such as rhyolite and basalt. Metamorphic rocks are those that have been altered by intense heat (contact metamorphism) or extreme pressure (regional metamorphism). Contact metamorphism occurs when rocks are changed by heat from, for example, an igneous intrusion or lava flow. Regional metamorphism occurs when rock is crushed in the middle of a folding mountain range. Metamorphic rocks can be formed from igneous rocks, sedimentary
rocks, or even from other metamorphic rocks.

Volcanoes

VOLCANOES ARE VENTS OR FISSURES in the Earth’s crust through which magma (molten rock that originates from deep beneath the crust) is forced on to the surface as lava. They occur most commonly along the boundaries of crustal plates; most volcanoes lie in a belt called the “Ring of Fire,” which runs along the edge of the Pacific Ocean. Volcanoes can be classified according to the violence and frequency of their eruptions. Nonexplosive volcanic eruptions generally occur where crustal plates pull apart. These eruptions produce runny basaltic lava that spreads quickly over a wide area to form relatively flat cones. The most violent eruptions take place where plates collide. Such eruptions produce thick rhyolitic lava and may also blast out clouds of dust and pyroclasts (lava
fragments). The lava does not flow far before cooling and therefore builds up steep-sided, conical volcanoes. Some volcanoes produce lava and ash eruptions, which build up composite volcanic cones. Volcanoes that erupt frequently are described as active; those that erupt rarely are termed dormant; and those that have stopped erupting altogether are termed extinct. As well as the volcanoes themselves, other features associated with volcanic regions include geysers, hot mineral springs, solfataras, fumaroles, and bubbling mud pools.

Mineral features

MINERALS CAN BE IDENTIFIED BY STUDYING features such as fracture, cleavage, crystal system, habit, hardness, color, and streak. Minerals can break in different ways. If a mineral breaks in an irregular way, leaving rough surfaces, it possesses fracture. If a mineral breaks along well-defined
planes of weakness, it possesses cleavage. Specific minerals have distinctive patterns of cleavage; for example, mica cleaves along one plane. Most minerals form crystals, which can be categorized into crystal systems according to their symmetry and number of faces. Within each system, several different but related forms of crystal are possible; for example, a cubic crystal can have six, eight, or 12 sides. A mineral’s habit is the typical form taken by an aggregate of its crystals. Examples of habit include botryoidal (like a bunch of grapes) and massive (no definite form). The relative hardness of a mineral may be assessed by testing its resistance to scratching. This property is usually measured
using Mohs scale, which increases in hardness from 1 (talc) to 10 (diamond). The color of a mineral is not a dependable guide to its identity as some minerals have a range of colors. Streak (the color the powdered mineral makes when rubbed across an unglazed tile) is a more reliable indicator.

Minerals

A MINERAL IS A NATURALLY OCCURRING SUBSTANCE that has a characteristic chemical composition and specific physical properties, such as habit and streak. A rock, by comparison, is an aggregate of minerals and need not have a specific chemical composition. Minerals are made up of elements (substances that cannot be broken down chemically into simpler substances), each of which can be represented by a chemical symbol. Minerals can be divided into two main groups: native elements and compounds. Native elements are made up of a pure element. Examples include gold (chemical symbol Au), silver (Ag), copper (Cu), and carbon (C); carbon occurs as a native element in two forms, diamond and graphite. Compounds are combinations of two or more elements. For example, sulfides are compounds of sulfur (S) and one or more other elements, such as lead (Pb) in the mineral galena, or antimony (Sb) in the mineral stibnite.

The rock cycle

THE ROCK CYCLE IS A CONTINUOUS PROCESS through which old rocks are transformed into new ones. Rocks can be divided into three main groups: igneous, sedimentary, and metamorphic. Igneous rocks are formed when magma (molten rock) from the Earth’s interior cools and solidifies. Sedimentary rocks are formed when sediment (rock particles, for example) becomes compressed and cemented together in a process known as lithification. Metamorphic rocks are formed when igneous, sedimentary, or other metamorphic rocks are changed by heat or pressure. Rocks are added to the Earth’s surface by crustal movements and volcanic activity. Once exposed on the surface, the rocks are broken down into rock particles by weathering. The particles are then transported by glaciers, rivers, and wind, and deposited as sediment in lakes, deltas, deserts, and on the ocean floor. Some of this sediment undergoes lithification and forms sedimentary rock. This rock may be thrust back to the surface by crustal movements or forced deeper into the Earth’s interior, where heat and pressure transform it into metamorphic rock. The metamorphic rock in turn may be pushed up to the surface or may be melted to form magma. Eventually, the magma cools and solidifiesbelow or on the surface-forming igneous rock. When the sedimentary, igneous, and metamorphic rocks are exposed once more on the Earth’s surface, the cycle begins again.

Earth’s physical features

MOST OF THE EARTH’S SURFACE (about 70 percent) is covered with water. The largest single body of water, the Pacific Ocean, alone covers about 30 percent of the surface. Most of the land is distributed as seven continents; these are (from largest to smallest) Asia, Africa, North America, South America, Antarctica, Europe, and Australasia. The physical features of the land are remarkably varied. Among the most notable are mountain ranges, rivers, and deserts. The largest mountain ranges— the Himalayas in Asia and the Andes in South America—extend for thousands of miles. The Himalayas include the world’s highest mountain, Mount Everest (29,029 ft/8,848 m). The longest rivers are the Nile River in Africa (4,160 miles/6,695 km) and the Amazon River in South America (4,000 miles/ 6,437 km). Deserts cover about 20 percent of the total land area. The largest is the Sahara, which covers nearly a third of Africa. The Earth’s surface features can be represented in various ways. Only a globe can correctly represent areas, shapes, sizes, and directions, because there is always distortion when a spherical surface— the Earth’s, for example— is projected on to the flat surface of a map. Each map projection is therefore a compromise; it shows some features accurately but distorts others. Even satellite mapping does not produce completely accurate maps, although they can show physical features with great clarity.