Calcium is the chemical element with atomic number 20; it has an atomic mass of 40.078 atomic mass units (amu). The chemical symbol for calcium is Ca. Calcium is a soft gray...
StreamLast Updated on 2014-11-30 22:08:37
Streams alter the Earth's landscape through the movement of water and sediment (Figure 1). Streams are powerful erosive agents moving material from their bed and banks. In mountainous regions, stream erosion often produces deep channels and canyons. Streams also deposit vast amounts of sediment on the terrestrial landscape and within lakes and ocean basins.
Geomorphologists often view streams as systems. The stream system, like almost all environmental systems, is open to both inputs and outputs of various types of materials. Water enters the stream system by direct precipitation in the channel, from runoff, throughflow, and by groundwater flow. The movement of water into a stream also carries with it dissolved and solid materials eroded from the surrounding landscape, stream banks, and the stream bed. Sediments carried by streams to lower elevations are occasionally... More »
MountainLast Updated on 2014-09-06 20:26:05
A mountain can be defined as an area of land that rises abruptly from the surrounding region. A mountain range is a succession of many closely spaced mountains covering a particular region of the Earth. Mountain belts consist of several mountain ranges that run roughly parallel to each other. The North American Cordillera, the Himalayas, the Alps, and the Appalachians are all examples of mountain belts that are composed of numerous mountain ranges.
Some mountains are volcanic in origin forming where rising magma breaks through the Earth's surface. Volcanic mountains tend to have sporadic distributions within a mountain range (Mounts St. Helens, Rainier, and Baker) or can occur alone because of a localized hot spot (Hawaiian Islands). Most mountains were created from tectonic forces that elevate, fold, and fault rock materials. Tectonic mountains can occur as a single... More »
PleistoceneLast Updated on 2014-07-02 14:12:09This article on the Pleistocene Epoch was written by P. D. P, Brian R. Speer and Ben Waggoner.
The mammoth was one of the largest land mammals of the Pleistocene, the time period that spanned from 1.8 million to approximately 10,000 years ago. Pleistocene biotas were extremely close to modern ones — many genera and even species of Pleistocene conifers, mosses, flowering plants, insects, mollusks, birds, mammals and others survive to this day. Yet the Pleistocene was also characterized by the presence of distinctive large land mammals and birds. Mammoths and their cousins the mastodons, longhorned bison, sabre-toothed cats, giant ground sloths, and many other large mammals characterized Pleistocene habitats in North America, Asia, and Europe. Native horses and camels galloped across the plains of North America. Great teratorn birds with 25-foot wingspans stalked prey. Around the... More »
SeleniumLast Updated on 2014-06-29 16:59:07
Selenium is a gray, metallic element. Its atomic number is 34 and its symbol is Se. The Swedish scientist Jons Jacob Berzelius discovered selenium in 1817. In studying the sulfuric acid produced in a particular Swedish factory, he discovered an impurity which he eventually identified as selenium. Selenium occurs in three distinct forms: as a non-crystalline, gray metal; it can form as a deep red to black powder; and it can form as red crystals. It is stable in air and in water. Selenium is actually an important trace element to mammals and some plants. Too much selenium in a mammal’s diet is poisonous and has been shown to cause deformities. When there is not enough selenium, a mammal can also have health problems. For example, sheep that graze in areas with too little selenium in the soil eventually have a problem known as “white muscle disease.” Lack of selenium... More »
Von Humboldt, AlexanderLast Updated on 2014-06-26 16:40:53
Baron Alexander von Humboldt (1769-1859) was one of the last true generalists in science. While generally considered a geographer, he contributed to most of the sciences of the natural environment found today. Born in Berlin, von Humboldt’s father was Chamberlain to the King, a royal advisor, who died when Alexander was nine years old. As a child, he received a private education and was a slow learner and sickly much of the time. On his own, though, he loved collecting local plants and animals and reading books on foreign travel and adventure. He also loved to draw, mostly landscapes. Typical of the time, science was not part of his schooling; Humboldt was generally self taught in that area. At sixteen, he attended some lectures on physics and philosophy by a local doctor and then he decided to pursue a career in science.
Humboldt... More »
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