Research Results For 'Lava'

LAKES

Lakes are accumulations of water in hollows on the earth's surface. When they are drained by rivers their waters are fresh, but when they have no outlet they are salty, e.g. the Dead Sea, Sea of Aral, etc.
Lakes may owe their origin to:

1. The formation of a barrier across a river.


2. Earth movements.


3. Ice erosion.


4. Volcanic action.

The nehrungs of East Prussia are sand-spits which enclose the shallow salt-water lagoons or halls, such as Kurische Haff. Earth movements cause lake formation when subsidence occurs. This is most easily seen in rift valleys. Examples of rift valley lakes are the Dead Sea, Lakes Nyasa and Tanganyika in Africa, and Lake Torrens in Australia. These are all long, narrow, and very deep lakes.
In Cheshire, the removal of underground beds of salt has caused subsidence resulting in the 'meres' of the Weaver Valley. The 'folding' of the earth across the line of a river valley may partially block a river and help to form a lake. The study of a good physical map will reveal the connection between mountain building and the formation of Lake Geneva and Lake Constance in Switzerland. Where there are large areas of depressed lowland wide and shallow lakes are formed in the lowest part of the depression, for example the Sea of Aral in Asiatic Russia, Lake Balaton in Hungary, and Lake Eyre in Australia. Ice sheets and valley glaciers may scoop out hollows to form 'rock basins'. Mountain tarns and corrie lakes in North Wales and Scotland have been formed in this way. Water also accumulates in the hollows of unevenly- distributed glacial drift. Such are the lakes of East Prussia, and also those of the Cheshire-Shropshire borders near Ellesmere. Subsidence of the land surface and consequent lake formation may be directly related to volcanic action. Lough Neagh in Northern Ireland is a shallow lake formed by subsidence of this type. Lakes are often formed by the accumulation of water in the craters of extinct volcanoes, for example the Laachersee in the Eifel region of Germany.
Research Lakes

MILLSTONE

A millstone is one of a pair of circular stones that are used for grinding grain or other substances. The surface of a millstone usually has a series of radial grooves in which the powdered material collects, and the stones are generally buhrstone or some kind of lava, such as Niedermendig, or another other firm rock with rough texture. Milling grain with stones introduces small quantities of stone into the flour, and this helps to accelerate the rate at which the teeth of consumers of such flour are worn away.
Research Millstone

PLATEAU

A plateau or tableland generally denotes a large stretch of highland which is practically the same height above sea-level, and which descends on all sides to lower ground. Some plateaux, however, such as those of Tibet and Bolivia, that are fringed by high mountain ranges which tower above the plateau level, are known as intermont plateaux. Sometimes they are so completely enclosed as to have no outlet to the sea.

A plateau might be regarded as an elevated plain, but there is often a great difference between the surface of a plain and that of a plateau. As a plateau is high, rivers are swift and carve deep, narrow valleys instead of the broad, open valleys of the lower rivers of the plains. Such plateau areas as Wales and the Highlands of Scotland, are broken by deep, narrow valleys, and are termed dissected plateaux. On reaching the top of such an area one has a long view of a series of flat-topped mountain ridges. These ridges are all of approximately the same height, and if one imagines the clouds descending until they touched one ridge, then almost every other ridge would be similarly cloud-capped. Other good examples of plateaux are Tibet in Asia, the Ecuador and Bolivian plateaux in South America, and nearly the whole of the continent of Africa.

The Deccan of India is a plateau that has been tilted so that the western edge is much higher than the eastern edge, and all the main rivers drain eastwards. In many instances plateaux are formed by the denudation or wearing down of higher mountainous areas. Ultimately, such areas may become so low that they are nearly plains, i.e. peneplains, such as the land around Hudson Bay. Millions of years ago lava was forced up through cracks in the earth's crust, and spread out over the land in great sheets which have since hardened to form plateaux of basalt.

Two well-known examples of such plateaux are in Antrim (Ireland), and on the Deccan of India to the east of Bombay. Many of the most extensive areas of plateaux in the world are composed of very hard old rock. The Guiana Highlands, most of Africa, Arabia, the Deccan of India, and the West Australian plateaux are all composed of rocks of similar age. The ancient plateau lands are principally valuable for their minerals, such as the gold of Western Australia; the iron and manganese of the Brazilian Highlands; the gold, copper, and diamonds of the African plateau; and the gold of the Lena plateau in Siberia. Where plateaux are found in tropical areas they are important because, being cooler than the neighbouring lowlands, they offer greater possibilities for successful European settlement and development. The highlands of Brazil, Kenya, and Tanganyika are illustrations of this. Much of the tropical plateau area is covered by savannah grasslands. Most of these areas are not yet developed, but offer possibilities for the production of a large variety of both animal and vegetable products when communications have been developed and further settlement has taken place.
Research Plateau

VOLCANO

A volcano is a vent in the earth's crust from which molten rock, ashes and steam are ejected. The lava tends in time to heap up a conical eminence round the vent, thus forming the crater or cup, or even a volcanic mountain.
Research Volcano

MODOCS

The Modocs or Modoc Indians are an American Indian tribe. They lived on the south shore of Klamath Lake in California and on the Oregon frontier. They began attacks against the whites as early as 1847. Hostilities continued until 1864, when they ceded their lands and agreed to go on a reservation which was not set apart until 1871. In the meantime they were placed on the Klamath reservation, and later on the Yainax reservation. A band under Captain Jack left the reservation and settled on Lost River, whence they refused to depart. Hostilities followed, Captain Jack retreated to the Lava Beds and was not finally conquered until June, 1873 when most of them died. The survivors were removed to the Klamath reservation in Indian Territory where they numbered less than 300 in 1900.
Research Modocs

AMYGDALOIDAL

Amygdaloidal is a name given to igneous rocks, usually old lava flows, full of almond-shaped cavities which have been filled up with secondary minerals, such as calcite, agate or the zeolites. These cavities vary in size up to several centimetres across and were formed while the rock was still fluid and in motion.
Research Amygdaloidal

ANDESITE

Andesite is a crystalline igneous rock, occurring mostly in lava flows, but sometimes in dykes and veins. It consists principally of plagioclase feldspar, and is often porphyritic, showing large crystals of feldspar scattered through a fine-grained mass, usually of small feldspar crystals, but often containing much glassy material. Andesite forms most of the recent volcanic rock of the Andes, and is thus named after the Andes.
Research Andesite

BRECCIA

Breccia is a rock consisting of angular fragments of any kind, united by a matrix. The shape of the components indicates that they have been produced by fracture, and have not been subjected to rounding by attrition. Fault breccia is often found between the two walls of a geological fault, and is due to the breaking down of the rocky walls when grinding on one another. Mineral veins are often formed in fissures, and are brecciated later by movement of the walls. Another kind of breccia is produced when hot molten lava enters a lake or a stream; it is suddenly cooled and solidified, being at the same time shattered by the clouds of steam that are formed. Braccias differ from conglomerates in the angular nature of their fragments, and in the method of their origin.
Research Breccia

ERUPTIVE ROCKS

In geology, eruptive rocks are those which, like lava, basalt, granite, etc, have broken through other rocks while in a molten state.
Research Eruptive Rocks

EXTRAVASATION

In geology, extravasation describes the issue of lava and other volcanic products from the earth.
Research Extravasation

	The Probert Encyclopaedia was designed, edited and programed by Matt and Leela Probert ©1993 - 2009 The Probert Encyclopaedia Southampton, United Kingdom