The lithosphere and the soil as power equipment and hazard 11. Natural processes modifying the quality of the soils from the Equator to the polar regions. Case studies.

1. Effects of volcanism The volcanic origin soils have high level of fertility and are as a result used extensively for agricultural purposes. The components and texture of volcanic origin rocks (mostly pyroclastic rocks) are very favourable for soil formation. Very important elements and ions release from minerals during the weathering. The key characteristics of volcanic soils (andisols) for agriculture are their water retention capacity and their ability to fix elements, in particular phosphates. Other side, activity of volcanoes can destroy soils. The most importand impact have volcanic ash. During an eruption, volcanic ash is ejected from the volcano up into the atmosphere and subsequently falls to the ground in the form of ash falls. Volcanic ash is composed of fine-grained rock and mineral fragments and glass shards, commonly with acid droplets and soluble salts coating the ash-grain surfaces. The properties of the ash depend mostly on the relative proportions of their main constituents.

Agricultural area at the southern part of Mátra Mountains

2. Evolution of the Sahara The Sahara is the most well known, and largest desert on Earth. It is a land of violent contrasts and extreme conditions, where life is difficult at best. Today's Sahara is, in fact, the result of hundreds of millions of years of geologic and climatic change. Over the last 2.5 million years, climatic changes comparable to those that caused alternating ice ages and periods of interglacial warming in Europe also influenced rainfall and weather patterns in North Africa. Dry phases, when desert conditions prevailed and spread, alternated with humid periods, when great networks of rivers and lakes supported large animal and human populations. The most recent of the Sahara's wet phases began 12,000 to 10,000 years ago, toward the end of the last glacial period. The oldest of the Tassili n'Ajjer rock engravings and paintings, dating from the sixth millennium B.C. or earlier, depict hunters of a Negroid race, who wore animal skins and masks and stalked their prey with clubs, lances, and boomerangs.

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