escala de tiempos geológicos

«Continentes arcaicos»

Límites de placa La superficie de la Tierra está dividida en placas rígidas. Estas placas son de 100 a 120 kilómetros de espesor e incluyen la corteza y una pequeña parte de la parte superior del manto. Muchas placas contienen tanto la corteza continental y oceánica. Los científicos a menudo se refieren a esta corteza / capa del manto superior como la litosfera. Las placas se sientan encima de una capa de plástico más suave del manto llamada astenosfera (de la palabra griega "débil"). Hay 12 grandes placas, además de un número de menores. Las placas llevan el nombre de las regiones donde se ubican. La placa de América del Norte, la placa del Pacífico y la placa del Caribe son ejemplos. Todas las placas están en constante movimiento, algunos moviéndose más rápido que otros. Se mueven en diferentes direcciones más o menos al mismo ritmo que el crecimiento de las uñas, de aproximadamente cinco centímetros cada año en promedio. Eso puede parecer lento, pero a lo largo de millones de años, las placas y los continentes que montan en ellos mover un largo camino. Los científicos no entienden completamente por qué las placas se mueven. La teoría más aceptada es que las corrientes de convección en la astenosfera ellos arrastran. Las corrientes de convección se producen cuando una parte de un fluido es más caliente que la otra parte. Piense en una olla de agua que se calienta mediante una sola llama en su centro. El agua en la parte inferior de la olla en el centro se calienta y se eleva a la parte superior. El agua más fría en la parte superior se mueve hacia los lados de la olla y se hunde hasta el fondo, creando una corriente de convección. Del mismo modo, las columnas de material caliente del manto profundo de la Tierra se elevan hacia la superficie. Cuando el material del manto alcanza litosfera delgada de la Tierra, se mueve a lo largo de la superficie lejos de la columna de material caliente. A continuación, se enfría y se hunde en el manto profundo. El rojo es el terremoto y zonas calientes.Fíjese en el mapa cómo la mayoría de los terremotos y los volcanes ocurren a lo largo de bandas estrechas. Estas bandas corresponden a los límites de las placas. Pangea: The North America - Africa Connection The Supercontinent Pangea during Late Triassic time. All of the major continents of a today were assembled into a single landmass surrounded by a large ocean. Note that southern North America was located at the Late Triassic equator. Modified from Olsen (1997). Reconstruction of North America and Africa during the Late Triassic, showing the distribution of outcropping rift-basin rocks (black), subsurface rift-basin rocks (dark gray), and possible rift basins (light gray). The Triassic equator (gray line labelled 0°) passed through the Dan River basin. The Argana basin of Morocco, Africa, and the Fundy basin of Nova Scotia and New Brunswick, Canada, lay adjacent to one another on this reconstruction. The Newark rift basin, part of which is located in New Jersey, lay adjacent to West Saharan Africa. The inset map at lower right shows a simplified version of Pangea during Triassic time, and shows the approximate locations of the Fundy and Argana rift basins. Modified from Olsen et al. (2000). The Breakup of Pangea. This animation shows the progressive breakup of Pangea from the time period 150 Ma to the present. By 150 Ma, the central North Atlantic Ocean had already opened somewhat. Animation from the websitehttp://www.odsn.de/odsn/services/paleomap/animation.html, which allows users to generate a plate reconstruction for any time in the past 150 million years using a variety of map projections and views of the globe. The Supercontinent Pangea during Late Triassic time. All of the major continents of a today were assembled into a single landmass surrounded by a large ocean. Note that southern North America was located at the Late Triassic equator. Modified from Olsen (1997). Topographic map of the Earth's surface. Yellow, orange, and red colors represent progressively higher elevations; green, light blue, and dark blue represent progressively lower elevations. The middle of the Atlantic Ocean (see closeup map below) contains a topographic high (light blue curvilinear feature) surrounded by the deep ocean (dark blue). This feature is the Mid-Atlantic Ridge, and marks the boundary between the North American and European or South American and African tectonic plates. Images from NOAA's National Geophysical Data Center. Top: Schematic cross section of a mid-ocean ridge. The center of the ridge contains the youngest basaltic crust. The age of the crust increases away from the center of the ridge. The thickness of sediment overlying the basaltic crust increases away from the ridge, as does the age of the sediments directly above the basaltic crust. Modified from Hamblin & Christiansen (1998). Bottom: Thickness of sediment in the oceans. This work-in-progress shows that the thinnest sediments are present along the mid-ocean ridge in the south Atlantic, and that the thickest sediments are present adjacent to South America and Africa. Modified from an image produced by NOAA's National Geophysical Data Center: http://www.ngdc.noaa.gov/mgg/image/sedthick.jpg. Top: Age of the oceanic crust. The youngest oceanic crust (red, orange) is always found along the mid-ocean-ridge systems. The age progressively increases away from the ridge and toward the continents (gray). In the central North Atlantic Ocean, the oldest oceanic crust (purple) is ~200 million years, and occurs adjacent to eastern North America and northwest Africa. Bottom: Detail of the age of oceanic crust in the Atlantic. Modified from an image produced by NOAA's National Geophysical Data Center:http://www.ngdc.noaa.gov/mgg/image/images.html#crustage Top: Schematic cross section of a mid-ocean ridge. A mid-ocean ridge marks the boundary between two plates that moving apart. As the plates move apart, the mantle rises upward to fill the gap between the diverging plates. Partial meltingof the upper mantle then yields magma, some of which is erupted on the seafloor to produce the basaltic crust. As the two plates pull apart, normal faults form to accommodate the stretching. Modified from Hamblin & Christiansen (1998). Bottom: Evolution of oceanic crust formed at a mid-ocean ridge. The processes described above are repeated continuously. New oceanic crust is created at the mid-ocean ridge, and half becomes part of the tectonic plate containing continent A, and half becomes part of the plate containing continent B. Over time, the width of the oceanic crust gets larger, as does the distance between continents A and B. Modified from Marshak (2001). Schematic diagram showing how to restore two continents separated by oceanic crust. Continent A is shown in green; continent B is shown in olive. Oceanic crust is shown in shades of brown. The age of the oceanic crust is labelled. The present-day location of the mid-ocean ridge is given by the black line labelled 0 Ma. On this and subsequent diagrams on this page, fracture zones (black lines) are used in the restoration for the diagrams on the right; they are not used in the diagrams on the left. The yellow areas are regions adjacent to the continents where the age of the oceanic crust is not unknown; the orientations of fracture zones are also not known. Step 1 in the restoration involves "removing" oceanic crust that is 0 Ma to 50 Ma old. The white areas on this diagram correspond to the regions where this crust was "removed."  Step 2 in the restoration involves squeezing the blocks together to eliminate the white gaps produced by "removing" the oceanic crust that was 0 Ma to 50 Ma old. For the diagram at left, the blocks where squeezed together in a direction perpendicular to the trend of the "bands" of oceanic crust. In the diagram at right, blocks where squeezed together in a direction parallel to the fracture zones.  Step 3 in the restoration involves removing crust that is 50 Ma to 100 Ma old. The results are shown above. Step 4 in the restoration involves removing crust that is 100 Ma to 150 Ma old. The results are shown above. The next step involves removing the "yellow" crust. For the diagram at left, after removing the "yellow" crust, the blocks are simply squeezed together in a right-left direction. For the diagram at right, the blocks can be squeezed together in a number of different directions (see arrows for possible examples), but the exact direction is not known because the orientation of fracture zones is unavailable for the "yellow" crust. The final restorations are shown above. The restorations differ because that on the left always moved the blocks from right to left, whereas that on the right moved the blocks parallel to the fracture zones for all stages except the last, which used independent evidence to determine the restoration direction (the circular features are required to be adjacent to one another in the final restored state). In the restoration at right, notice that the region of the present-day equator (0°) on continent B is adjacent to the region which at the present day lies at a latitude of ~13°. In the restoration at left, present-day and restored latitudes much one another.