Plate Boundaries and other Geologic structures explained with the help of animated GIFs

The belief in the plate tectonic theory has increased over the past century since the concept of sea-floor spreading was developed in the late 1950s and 1960s. Plate tectonics refers to the large scale movement of the Earth’s lithosphere, the upper most part of the Earth on which we live. The article covers various geologic structures and they are further explained with the help of animations.

Plate Boundaries

Lithosphere is broken up into various tectonic plates. The tectonic plates are around 100 km (62 mi) thick and consist of two principal types of material: oceanic crust (also called sima from silicon and magnesium) and continental crust (sial from silicon and aluminium). The composition of the two types of crust differs markedly, with basaltic rocks dominating oceanic crust, while continental crust consists principally of lower density granitic rocks . Based on their movement, there are three types of plate boundaries that exist and they are associated with different surface phenomena.

i) Transform plate boundary

Image Courtesy: plateboundaries.wikispaces.com

Image Courtesy: geology8b-9.wikispaces.com

These boundaries occur where two lithospheric plates slide, or perhaps more accurately, grind past each other along transform faults, where plates are neither created nor destroyed. The relative motion of the two plates is either sinistral (left side toward the observer) or dextral (right side toward the observer). Transform faults occur across a spreading center. Strong earthquakes can occur along fault. The San Andreas Fault in California is an example of a transform boundary exhibiting dextral motion. The first animation shows the movement of a transform plate boundary whereas the second animation demarcates the San Andreas Fault which is present along a transform plate boundary.

ii) Divergent plate boundary

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Divergent plate boundaries occur where two plates slide apart from each other. At zones of ocean-to-ocean rifting, divergent boundaries form by seafloor spreading, allowing for the formation of new ocean basin. As the continent splits, the ridge forms at the spreading center, the ocean basin expands, and finally, the plate area increases causing many small volcanoes and/or shallow earthquakes. The first animation is an example of that. The second animation is of continent-to-continent rifting, where divergent boundaries may cause new ocean basin to form as the continent splits, spreads, the central rift collapses, and ocean fills the basin. Active zones of Mid-ocean ridges (e.g., Mid-Atlantic Ridge and East Pacific Rise), and continent-to-continent rifting (such as Africa's East African Rift and Valley, Red Sea) are examples of divergent boundaries.

iii) Convergent plate boundary

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The plate boundaries occur where two plates slide toward each other to form either a subduction zone or a continental collision. The first animation shows the convergence of oceanic and continental plate. The dense oceanic lithosphere plunges beneath the less dense continent. Earthquakes then trace the path of the downward-moving plate as it descends into asthenosphere, a trench forms, and as the subducted plate partially melts, magma rises to form continental volcanoes. The second animation shows the collision that occurs at continent-to-continent boundaries.  Collision between masses of granitic continental lithosphere causes subduction of mass and plate edges are compressed, folded, uplifted.

Faults

All the stress and strain produced by moving plates builds up in the Earth's rocky crust until it simply can't take it anymore. All at once, CRACK!, the rock breaks and the two rocky blocks move in opposite directions along a more or less planar fracture surface called a fault. Faults are classified on the basis of the direction of their movement.

i) Normal Fault

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Normal faults form when the hanging wall drops down. The forces that create normal faults are pulling the sides apart, or extensional.

ii) Reverse Fault

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Reverse faults form when the hanging wall moves up. The forces creating reverse faults are compressional, pushing the sides together.

iii) Strike-Slip Fault

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Strike-slip faults have a different type of movement than normal and reverse fault.  In these faults the fault plane is usually vertical, so there is no hanging wall or footwall. The forces creating these faults are lateral or horizontal, carrying the sides past each other. Thus the slip occurs along the strike, not up or down the dip.

Folds

Geological folding involves the plastic deformation (bending, buckling) of a single or multiple (stack) strata, such as sediments and rocks, which were originally planar horizontal surfaces. Beyond plastic deformation, rocks fail structurally and faulting occurs. Folds are classified according to size, fold shape, tightness, and dip of the axial plane.

i) Anticline

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Anticlines are folds in which the oldest rock lies in the center or core.  Most often anticlines are arch shaped.  Anticlines are sought out by geologists who explore for oil and gas because the arches form natural traps for the hydrocarbons.

ii) Syncline

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A syncline is a fold with younger layers closer to the center of the structure. Synclines are typically a downward fold, termed a synformal syncline (i.e. a trough); but synclines that point upwards, or perched, can be found when strata have been overturned and folded (an antiformal syncline).

iii) Dome

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dome is a feature in structural geology consisting of symmetrical anticlines that intersect each other at their respective apices, forming a distinct, rounded, elliptical-to-circular-shaped protrusion on Earth's surface. The animation shows the how an exfoliation dome forms. Structural basins are geological depressions, and are the inverse of domes. Structural basins are often important sources of coal, petroleum, and groundwater.

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22210   05/08/2014

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