Plutons, Defined and Explained

Plutons, Defined and Explained A pluton (pronounced PLOO-tonn) is a deep-seated intrusion of igneous rock, a body that made its way into pre-existing rocks in a melted form (magma) several kilometers underground in the Earths crust and then solidified. At that depth, the magma cooled and crystallized very slowly, allowing the mineral grains to grow large and tightly interlocked - typical of plutonic rocks.   Shallower intrusions may be called subvolcanic or hypabyssal intrusions. There are a slew of partial synonyms based on a plutons size and shape, including batholith, diapir, intrusion, laccolith, and stock.   How Pluton Becomes Visible A pluton exposed at the Earths surface has had its overlying rock removed by erosion. It may represent the deep part of a magma chamber that once fed magma to a long-vanished volcano, like Ship Rock in northwestern New Mexico. It may also represent a magma chamber that never reached the surface, like Stone Mountain  in  Georgia.  The only true way to tell the difference is by mapping and analyzing the details of the rocks that are exposed along with the geology of the surrounding area. The Various Types of Plutons Pluton is a general term that covers the whole variety of shapes taken by bodies of magma. That is, plutons are defined by the presence of plutonic rocks. Narrow sheets of magma that form sills and igneous dikes may qualify as plutons if the rock inside them solidified at depth. Other plutons have fatter shapes that have a roof and a floor. This can be easy to see in a pluton that was tilted so that erosion could cut through it at an angle. Otherwise, it may take geophysical techniques to map the plutons three-dimensional shape. A blister-shaped pluton that raised the overlying rocks into a dome may be called a laccolith. A mushroom-shaped pluton may be called a lopolith, and a cylindrical one may be called a bysmalith. These have a conduit of some sort that fed magma into them, usually called a feeder dike (if its flat) or a stock (if its round). There used to be a whole set of names for other pluton shapes, but they arent really much use and have been abandoned. In 1953, Charles B. Hunt made fun of these in USGS Professional Paper 228 by proposing the name cactolith for a cactus-shaped pluton: A cactolith is a quasihorizontal chonolith composed of anastomosing ductoliths whose distal ends curl like a harpolith, thin like a sphenolith, or bulge discordantly like an akmolith or ethmolith. Who said geologists couldnt be funny?   Then there are plutons that have no floor, or at least no evidence of one. Bottomless plutons like these are called stocks if they are smaller than 100 square kilometers in extent, and batholiths if theyre larger.  In the United States, the Idaho, Sierra Nevada, and Peninsular batholiths are the largest. How Plutons Form The formation and fate of plutons is an important, long-standing scientific problem. Magma is less dense than rock and tends to rise as buoyant bodies. Geophysicists call such bodies diapirs (DYE-a-peers); salt domes are another example. Plutons may readily melt their way upward in the lower crust, but they have a hard time reaching the surface through the cold, strong upper crust. It appears that they need help from regional tectonics that pulls the crust apart- the same thing that favors volcanoes at the surface. Thus plutons, and especially batholiths, go along with subduction zones that create arc volcanism. For a few days in 2006, the International Astronomical Union considered giving the name plutons to large bodies in the outer part of the solar system, apparently thinking that it would signify Pluto-like objects. They also considered the term plutinos. The Geological Society of America, among other critics of the proposal, sent a quick protest, and a few days later the IAU decided on its epochal definition of dwarf planet that banished Pluto from the register of planets. (See What Is a Planet?) Edited by Brooks Mitchell

What Third World and Developing Countries Means

What Third World and Developing Countries Means The world is divided into those countries that are industrialized, have political and economic stability, and have high levels of human health, and those countries that do not. The way we identify these countries has changed and evolved over the years as we have moved through the Cold War-era and into the modern age; however, it remains that there is no consensus as to how we should classify countries by their development status. First, Second, Third, and Fourth World Countries The designation of Third World countries was created by Alfred Sauvy, a French demographer, in an article that he wrote for the French magazine, LObservateur in 1952, after World War II and during the Cold War-era. The terms First World, Second World, and Third World countries were used to differentiate between democratic countries, communist countries, and those countries that did not align with democratic or communist countries. The terms have since evolved to refer to levels of development, but they have become outdated and are no longer used to distinguish between countries that are considered developed versus those that are considered developing. First World described the NATO (North Atlantic Treaty Organization) countries and their allies, which were democratic, capitalist, and industrialized. The First World included most of North America and Western Europe, Japan, and Australia. Second World described the communist-socialist states. These countries were, like First World countries, industrialized. The Second world included the Soviet Union, Eastern Europe, and China. Third World described those countries that did not align with either the First World or Second World countries after World War II and are generally described as less-developed countries. The Third World included the developing nations of Africa, Asia, and Latin America. Fourth World was coined in the 1970s, referring to the nations of indigenous people that live within a country. These groups often face discrimination and forced assimilation. They are among the poorest in the world. Global North and Global South The terms Global North and Global South divide the world in half both geographically. The Global North contains all countries north of the Equator in the Northern Hemisphere and the Global South holds all of the countries south of the Equator in the Southern Hemisphere. This classification groups the Global North into the rich northern countries, and the Global South into the poor southern countries. This differentiation is based on the fact that most of developed countries are in the north and most of the developing or underdeveloped countries are in the south. The issue with this classification is that not all countries in the Global North can be called developed, while some of the countries in the Global South can be called developed. In the Global North, some examples of the developing countries include: Haiti, Nepal, Afghanistan, and many of the countries in northern Africa. In the Global South, some examples of the well-developed countries include: Australia, South Africa, and Chile. MDCs and LDCs MDC stands for More Developed Country and LDC stands for Least Developed Country. The terms MDCs and LDCs are most commonly used by geographers. This classification is a broad generalization but it can be useful in grouping countries based on factors including their GDP (Gross Domestic Product) per capita, political and economic stability, and human health, as measured by the Human Development Index (HDI). While there is debate as to at what GDP threshold an LDC becomes and MDC, in general, a country is considered an MDC when it has a GDP per capita of more than US $4000, along with a high HDI ranking and economic stability. Developed and Developing Countries The most commonly used terms to describe and differentiate between countries are developed and developing countries. Developed countries describes the countries with the highest level of development based on similar factors to those used to distinguish between MDCs and LDCs, as well as based on levels of industrialization. These terms are the most frequently used and the most politically correct; however, there is really no actual standard by which we name and group these countries. The implication of the terms developed and developing is that developing countries will attain developed status at some point in the future.