lunes, 22 de noviembre de 2010


Solid substances fall into two general classes, crystalline and amorphous. Those whose atoms show long-range order, like the squares on a chessboard or the loops in a chain-link fence, are crystalline; those whose atoms are arranged in no particular, repeating pattern are amorphous. Naturally occurring amorphous solids are also termed mineraloids.
Amorphous solids are made of the same elements that produce crystalline solids, often mixed in the same ratios. For example, pure silicon dioxide (silica; SiO2) occurs both in a crystalline form (e.g., quartz); and in an amorphous form (e.g., glass). The difference between the two forms is one of atomiclevel organization. Given sufficient time, as when precipitating atom by atom from a hydrothermal solution or solidifying slowly from a pure melt, silicon and oxygen atoms assume an orderly, crystalline arrangement because it is a lower-energy state and therefore more stable, as a pencil lying on its side has less energy and is more stable than a pencil balanced on its eraser.
However, if cooled suddenly, the silicon and oxygen atoms in, for example, molten silica have no time to line up in orderly crystalline ranks but are trapped in a random solid arrangement. Natural glasses (lechatelierites) are in fact produced in large quantities when silica-rich lava is quenched suddenly in air or, as during undersea eruptions, in water.
Although few amorphous solids beside glasses occur naturally, an amorphous form of virtually any substance can be manufactured by sufficiently rapid quenching of the liquid phase or by depositing atoms from the vapor phase directly onto a cool substrate. Vapor deposition is used to build up the amorphous silicon films found in all integrated electronic circuit chips.
Most natural amorphous solids are formed by fast quenching, but not all. The precious stone opal (SiO2 [1] nH2O) is a mineraloid formed by the solidification of a colloidal solution (fine-particle mixture) of silica and water—in essence, opal is very firm silica jello. Minerals formed by solidification of colloids, like opal, are termed gel minerals. Limonite (Fe2O3 nH2O) is another gel mineral.
A crystalline solid may be transformed into an amorphous solid by alpha-particle radiation emitted by uranium or thorium atoms contained in the crystal itself. Each alpha particle that passes through the crystal strikes a tiny but violent blow against its atomic structure, slightly scrambling the orderly ranks of atoms. A once-crystalline mineral whose crystal structure has been obliterated by alpha radiation is termed a metamict mineral

Sistema Aluvial

An alluvial system is a landform produced when a stream or river, that is, some channelized flow (geologists call them all streams no matter what their scale) slows down and deposits sediment that was transported either as bedload or in suspension. The basic principle underlying alluvial deposits is that the more rapidly water is moving, the larger the particles it can hold in suspension and the farther it can transport those particles. For example, suppose that a river is flowing across a mountainous region, eroding rock, sand, gravel, silt, and other materials from the stream bed. As long as the stream is flowing rapidly, a considerable quantity of materials such as these can be transported, either along the bottom or as particles suspended in the water column. But then imagine that the stream rushes out of the mountainous region and onto a valley floor.
As the river slows down, suspended materials begin to be deposited. The larger bedload materials (for example, rocks and stones) accumulate first, and the lighter suspended materials (sand, silt, and clay) later. Any collection of materials deposited by a process such as this is known as alluvium. The conditions under which an alluvial system forms are found in both arid and humid climates, and in areas of both low slope (river deltas or swamps) and high slope (mountain streams).Although the system mentioned above was in a mountainous setting, any river or stream is part of an alluvial system.
Many stream systems consist of several common features including channels, heads, mouths, meanders, point bars and cut banks, floodplains, levees, oxbow lakes, and stream terraces. The channel is the sloping trough-like depression down which water flows from the stream’s origin, or head, to its destination, or mouth. All channels naturally curve, or meander.At the outside of a bend in a channel meander, the flow is concentrated and so erosion causes undercutting, and a cutbank forms. On the inside of the meander, flow decreases, so deposition occurs; a sand bar, or point bar, forms.
When a stream floods, several processes naturally follow. As the water flows out of its channel, it immediately begins to slow down because it spreads out over a large area, increasing the resistance to flow. Coarser sediments are therefore deposited very close to the channel. This forms a very gently sloping lump of alluvium that parallels the channel, known as a natural levee. As the natural levee builds up over thousands of years, it helps prevent flooding. That is why humans build man-made levees—to emulate natural levees.
Finer sediments flow with the stream water out onto the flat area behind the levee, known as the floodplain. During the same flood, if the water is especially high, or the channel is highly meandering, the flood may cut a new channel, connecting two closely positioned meanders, a neck, in what is called a neck cut-off. Once the neck is cut, the channel is much straighter, and the meander is abandoned to become a part of the floodplain. This abandoned meander then forms a lake known as an oxbow.
Another common feature of alluvial systems is the stream terrace. A stream terrace is simply an old floodplain that is now abandoned. Abandonment occurred when the erosive power of the stream increased and it began to rapidly downcut to a lower elevation. The stream did not have time to erode its old floodplain by meandering over it, so it was preserved. The abandoned floodplain, or stream terrace, can be seen well above the new stream channel elevation. Multiple terraces can sometimes be seen, resembling steps in a giant staircase.
When an alluvial system operates over a long period of time, perhaps millions of years, it works to flatten the surrounding landscape, and significantly decrease its average elevation. Areas that were originally mountainous can be worn down to rolling hills, and eventually produce extensive plains composed of alluvial sediment. The sediment is eroded from highlands that may be tens, hundreds, or perhaps thousands of miles from the coast, and the alluvium serves to bury existingcoastal features beneath a blanket of sediment. During periods of lower sea level in the geologic past, coastal plains extended far out on the margins of the continents. Today, these alluvial sediments are hundreds of feet below sea level.
As a stream emerges from a mountain valley, its waters are dispersed over a relatively wide region of valley floor. Such is the case, for example, along the base of the Panamint Mountains that flank California’s Death Valley. Astream flowing down a mountain side tends to deposit heavier materials near the foot of the mountain, somewhat lighter materials at a greater distance from the mountain, and the lightest materials at a still greater distance from the mountain. Often, the flow of water ends within the deposited material itself. This material tends to be very porous, so water is more likely to soak into the ground than to flow across its surface.
Thus, there is no preferred direction of deposition from side to side at the mouth of the stream, and as the alluvium accumulates it forms a cone-shaped pattern on the valley floor known as an alluvial fan. The idealized model described above would suggest that an alluvial fan should have a gradually changing composition, with heavier materials such as rocks and small stones at the base of the mountain and lighter materials such as sand and silt at the base (toe) of the fan. In actual fact, alluvial fans seldom have this idealized structure.
One reason for the more varied structure found in a fan is that stream flows change over time. During flows of low volume, lighter materials are deposited close to the mountain base on top of heavier materials deposited during earlier flows of high volume. During flows of high volume, heavier materials are once more deposited near the base of the mountain, now on top of lightermaterials. Avertical cross-section of an alluvial fan is likely to be more heterogeneous, therefore, than would be suggested by an idealized depositional model.
Alluvial fans tend to have small slopes that may be no more than a foot every half a mile (a few tenths of a meter per kilometer). The exact slope of the fan depends on a number of factors. For example, streams that drain an extensive area, that have a large volume of water, or that carry suspended particles of smaller size are more likely to form fans with modest slopes than are streams with the opposite characteristics.
Under some circumstances, a river or stream may continue to flow across the top of an alluvial fan as well as soak into it. For example, the volume of water carried during floods may cause water to cut across an alluvial fan and empty onto the valley floor itself. Also, over time, sediments may become compacted within the fan, and it may become less and less porous. Then, the stream or river that feeds the fan may begin to cut a channel through the fan itself and to lay down a new fan at the base of the older fan. As the fans in a valley become more extensive, their lateral edges may begin to overlap each other.
This feature is known as a bajada or piedmont alluvial plain. In some regions, piedmont alluvial plains have become quite extensive. The city of Los Angeles, for example, is largely constructed on such a plain. Other extensive alluvial systems can be found in the Central Valley of California andalong the base of the Andes Mountains in Paraguay, western Argentina, and eastern Bolivia.
Alluvial fans have certain characteristics that make them attractive for farming. In the first place, they generally have a somewhat reliable source of water (except in a desert): the stream or river by which they were formed. Also, they tend to be relatively smooth and level, making it easy for planting, cultivating, and harvesting. Deltas are common alluvial features, and can be found at the mouths of most streams that flow into a lake or ocean.
When rivers and streams flow into standing water, their velocity  decreases rapidly. They then deposit their sediment load, forming a fan-shaped, sloping deposit very similar to an alluvial fan, but located in the water rather than on dry land. This is known as a delta. Deltas show a predictable pattern of decreasing sediment size as you proceed farther and farther from shore.
The Mississippi River Delta is the United States’ best known delta. Other well-known deltas are the Nile Delta of northern Africa and the Amazon Delta of South America. When Aristotle observed the Nile Delta, he recognized it was shaped like the Greek letter, delta, hence the name. Most deltas clog their channels with sediment and so must eventually abandon them. If the river then flows to the sea along a significantly different path, the delta will be abandoned and a new delta lobe will form. This process, known as delta switching, helps build the coastline out.

miércoles, 3 de noviembre de 2010

Como recolectar un fosíl

Si bien no existe una regla que permita conocer con anticipación el lugar donde pueden encontrarse fósiles. Para recolectar se requiere de observación y habilidad
·         Recipientes plásticos (Bolsas)
·         Agujas de distinto tamaño
·         Formones
·         Martillos
·         Pinceles de alambre de acero
·         Papel y lápiz
·         GPS 
Defina los objetivos a trazarse con dicha exploración, con el propósito de recolectar los mejores ejemplares completos y en caso de hallar fragmentos tratar de escoger aquellos que permitan su identificación.
Los Fósiles deben ser obtenidos directamente de los extractos que los contienen
El método de extracción depende de las características del yacimiento y de la naturaleza del fósil. En caso de encontrar un fósil de material muy friable se puede cubrir su superficie con una película muy tenue de goma laca. Para extraer esqueletos grandes de vertebrados es conveniente comenzar por eliminar toda la sobrecarga y a medida que se va descubriendo ir barnizando las partes visibles, luego se procede a aislarlo por los costados, de manera que los restos descansen sobre un pedestal de sedimentos. Se los cubre entonces con papel periódico y después con tiras de arpillera previamente impregnada con yeso, se deja secar completamente y se procede por ultimo a separar los restos del pedestal.
Cada fósil debe ir acompañado de los siguientes datos:  a) Ubicación exacta (coordenadas UTM) y  descripción de la localidad. b) Registro de los caracteres geológicos: Formación, nivel, asociación de fósiles, estructura del depósito.

Todo fósil si procedencia exacta de lugar y de horizonte geológico carece de valor y debe ser excluido del estudio paleontológico, como de las colecciones.

Cuando llega al laboratorio, debe limpiarse y armarse, si es necesario. Si un ejemplar está roto se puede pegar sus diferentes partes empleando celuloide líquido, cemento DuPont (cemento duco) o goma arábiga diluida. Si se necesitan hacer impresiones del fósil  un método rápido es el uso de plastilina, pero los mejores resultados se obtienen utilizando gomas sintéticas y naturales.
Existen muchos métodos empleados por paleobotánicos para la obtención de cutículas y en el estudio microscópico de estructura celular del tronco. También los paleozoologos emplean métodos especiales y, en general, los micro paleontólogos deben recurrir a ellos frecuentemente para estudiar los microfósiles.

Coleccionista habilísimo fue Carlos Ameghino. En cierta oportunidad una expedición trato y los hallazgos de Ameghino. Sin embargo, los datos eran exactos, como los comprobó otra expedición posterior que visito el mismo lugar. La dificultad estaba en el pequeño tamaño de los fósiles, cuyo encuentro requería de gran habilidad por parte del coleccionista

martes, 2 de noviembre de 2010

Sismo de 4,7 grados de intensidad sacudió a Santander y Antioquia

No se reportaron víctimas o daños, según la Red Sismológica Nacional de Ingeominas.
Mediante un comunicado, Ingeominas informó que el movimiento se produjo a la 1:18 p.m. y su epicentro fue a 5,58 kilómetros de la cabecera municipal de Los Santos (Santander).
El movimiento de tierra tuvo una profundidad de 147,8 kilómetros y la ciudad más cercana al epicentro es Bucaramanga, a 35,4 kilómetros.
El temblor se sintió en las localidades de Piedecuesta, Floridablanca (Santander) y Bello (Antioquia).
Los organismos de socorro confirmaron que no hubo víctimas ni daños por el movimiento telúrico.

Rocas Sedimentarias

 Son aquellas que se forman luego de un proceso de meteorización e intemperismo y  al igual que de un proceso de litificación según sea el tipo. Las Rocas Sedimentarias pertenecen al grupo de los Pétreos Naturales. Se forman al depositarse los fragmentos de las Rocas Eruptivas y/o de las Rocas Metamórficas, por cristalización de substancias disueltas en el agua, acumulación de restos orgánicos o productos de las explosiones volcánicas. Se presentan formando capas o estratos superpuestos, representando cada estrato un período de sedimentación.

Según se hayan producido estos sedimentos, se clasifican en:

ü  Sedimentación Mecánica: Formadas por fragmentos de otras rocas acumuladas por las aguas, por el viento y por los glaciares.

o   Rocas Incoherentes o Disgregadas:
o   Arena
o   Arcilla
o   Grava

o   Rocas Semi -disgregadas:
o   Terrenos
o   Rocas Sedimentarias Compactas:
o   Arenisca

ü  Sedimentación Química: Las aguas de los mares, lagos y ríos contienen disueltas ciertas sales que por evaporación, sobresaturación, descomposición e influencia de ciertos organismos las depositan, formando yacimientos de gran espesor.
o   Caliza
o   Marga
o   Dolomía

ü  Sedimentación Biológica: La acumulación de restos de animales y vegetales han formado estas rocas, las que por su naturaleza se clasifican en:
o   Caliza
o   Carbón

ü  Sedimentación Volcánica: Ciertas emanaciones volcánicas lanzan al espacio diversos productos: cenizas, puzolanas, bombas, que, acumulándose en determinados lugares y cementadas por calizas, arcilla y sílice, originan tipos de roca de diferentes sedimentaciones:

viernes, 29 de octubre de 2010

Sismo Magnitud 7.7 REGION DE LAS ISLAS MENTAWAI, INDONESIA25 Octubre 2010

Yakarta, 25 oct (PL) El servicio indonesio de geología reportó hoy un  sismo de magnitud 7,5 en la escala Richter en la isla de Sumatra. El terremoto ocurrió a las 14.42 GMT y tuvo su centro a unos 33 kilómetros de profundidad con epicentro localizado a 149 kilómetros al sur de Padang, en Sumatra, y 634 kilómetros al sudoeste de Singapur.

Las autoridades indonesias no activaron un aviso de tsunami, pero el Centro de Alerta para Tsunamis del Pacífico, emitió una advertencia a las autoridades del país, para que valoren adoptar medidas preventivas en las zonas afectadas. Expertos de esa entidad consideran que no se espera un tsunami destructivo, pero insisten en la necesidad de prestar atención a probables oleadas que afecten a las costas situadas dentro de un radio de 100 kilómetros alrededor del epicentro. Explicaron que las zonas más alejadas podrían registrar pequeños cambios en el nivel del mar o corrientes marinas inusuales.
Sin embargo, hasta el momento la agencia local de sismos no reporta víctimas ni daños, por lo que levantó la advertencia de tsunami.
Tomado de:
Para mayor información consultar sección Temas, Fenómenos Naturales, Sismos

viernes, 22 de octubre de 2010

Bienvenido(a) al universo de las rocas

El estudio de las rocas dentro de sus tres grandes grupos en existentes sobre la corteza de la tierra, se han convertido en una amplia y multifacetica ciencia cuantitativa en los ultimos años. Avances en petrologia fisica y con el apoyo de la geoquimica, han introducido nuevas teorias sobre el origen y evolucion de los magmas, su comportamiento dinamico y la forma en que son intruidos y extruidos en forma explosiva.

Desarrollos en geocronologia, la evaluacion cuantitativa de la funcion de transferencia de calor y de fluidos en rocas de la corteza y los nuevos descubrimientos de campo han ayudado a la comprension de la evolucion de los sistemas metamorficos y su interesaccion dinamica con los procesos tectonicos.La geofisica y la fisica de los minerales han proporcionado una nueva perspectiva en la naturaleza de la conveccion del manto y su papel como una maquina de calor gigante de conduccion de rocas igneas y metamorficas.

Las nuevas herramientas de todo tipo permiten nuevas formas de recopilacion de datos petrologicos, mientras que la evolucion fenomenal en equipos y programas informaticos permiten que estos datos  se almacenen, procesen, y se modelen de forma que permiten ayudar a la comprender la evolucion de ciclo de las rocas en la corteza.

Con el desarrollo de este blog espero poder compartir con todos usted los conocimientos que  cada dia voy adquiriendo, tanto en lo teorico, como las distintas salidas de campo y proyectos de indole geologico en los que voy participando, aportando  en este apasionante mundo de las rocas, que permiten admirar muchos mas detalladamente las distintas geoformas existentes a nuestro alrededor.

De antemano se hace extensiva la invitacion a todos aquellos que gusten en participar con sus valiosos aportes, vivencias y sugerencias que puedan hacer de este recurso un valuarte para los amantes de las rocas, de la geodinamica interna, la geoquimica y todo lo que esto conlleva.