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Forces and Structures Essential Questions • • • • • What is folding? What is faulting? How are mountains made? How are volcanoes made? What are the different types of volcanoes? What is folding? Fold • when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of plastic (permanent) deformation • form under varied conditions of stress, hydrostatic pressure, pore pressure, and temperature • commonly formed by shortening of existing layers, but may also be formed as a result of displacement on a non-planar fault, at the tip of a propagating fault, by differential compaction or due to the effects of a high-level igneous intrusion Fold classification • Folds are classified by: – Size – Fold shape – Tightness – Dip of the axial plane Parts of a fold • Hinge • – where the flanks join together • – the point on a limb at which the concavity reverses – on regular folds this is the midpoint of the limb Limb – the flanks of the fold • Crest – the highest point of the fold surface • Trough – the lowest point of the fold surface • • • plunge – angle that the fold axis makes with a horizontal line when the fold axis is not horizontal Hinge line – hinge points along an entire folded surface • Axial plane – the surface defined by connecting all the hinge lines of stacked folding surfaces Hinge point – the point of minimum radius of curvature Inflection point • Fold axis – is the closest approximation to a straight line that when moved parallel to itself, generates the form of the fold Types of folds • Monoclines • Anticlines • Synclines Monoclines • the simplest types of folds • occur when horizontal strata are bent upward so that the two limbs of the fold are still horizontal Anticlines • are folds where the originally horizontal strata has been folded upward • the two limbs of the fold dip away from the hinge of the fold. Synclines • are folds where the originally horizontal strata have been folded downward • the two limbs of the fold dip inward toward the hinge of the fold Classification of folds • • • • • • Symmetrical fold Asymmetrical fold Isoclinal fold Overturned fold Recumbant fold Chevron fold • Symmetrical fold – the two limbs of the fold dip away from the axis with the same angle • Asymmetrical fold – the limbs dip at different angles • Isoclinal fold – the limbs are parallel to each other • Overturned fold – the strata on one limb of the fold becomes nearly upside • Recumbant fold – an overturned fold with an axial plane that is nearly horizontal • Chevron fold – has no curvature in its hinge – straight-sided limbs – form a zigzag pattern What is faulting? Fault • is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement • large faults within the Earth's crust result from the action of tectonic forces What causes faults? • Stress – a force applied over an area – Uniform stress • a stress where the forces act equally from all directions – Tensional stress (or extensional stress) • which stretches rock – Compressional stress • which squeezes rock – Shear stress • which result in slippage and translation Types of Stress Parts of a fault • Hanging wall block – the block above the fault • Footwall block – the block below the fault • Fault line – the surface trace of a fault – the line of intersection between the fault plane and the Earth's surface • Grabens – down-dropped blocks • Horsts – uplifted blocks Slip, heave, throw • Slip – the relative movement of geological features present on either side of a fault plane – is a displacement vector • Throw – the vertical component of the dip separation • Heave – the horizontal component of the dip separation • Dip – gives the steepest angle of descent of a tilted bed or feature relative to a horizontal plane Types of faults • Dip Slip Fault – where the slip on the fault plane is approximately vertical • Strike-Slip Fault – where the slip on the fault plane is approximately horizontal • Oblique-Slip Fault – has non-zero components of both strike and dip slip Types of Dip Slip Faults • Normal Fault • Reverse Fault • Thrust Fault Types of Dip Slip Faults • Normal Fault – where the hanging-wall block has moved down relative to the footwall block – result from horizontal tensional stresses Horst and Graben Normal fault Due to the tensional stress responsible for normal faults, they often occur in a series, with adjacent faults dipping in opposite directions A normal fault that has a curved fault plane with the dip decreasing with depth can cause the downdropped block to rotate Types of Dip Slip Fault continued… • Reverse Fault – the hanging-wall block has moved up relative the footwall block – result from horizontal compressional stresses in brittle rocks – the dip of a reverse fault is relatively steep, greater than 45° Types of Dip Slip Fault continued… • Thrust Fault – a reverse fault – the dip of the fault is less than 15o – can result in older strata overlying younger strata – typically form ramps, flats and fault-bend folds Strike-Slip Fault • where the relative motion on the fault has taken place along a horizontal direction • result from shear stresses acting in the crust Types of Strike-Slip Fault • Sinistral – Strike-slip faults with left-lateral motion • Dextral – Strike-slip faults with right-lateral motion • Transform fault – such faults form a plate boundary Types of Strike-Slip Fault continued… Oblique-Slip Fault • a fault which has a component of dip-slip and a component of strike-slip • some occur within transtensional and transpressional regimes • others occur where the direction of extension or shortening changes during the deformation but the earlier formed faults remain active How are mountains made? Mountain Building & the Growth of Continents Earth Revealed Series • http://www.learner.org/vod/vod_window.ht ml?pid=318 • Complete the question sheet provided • 30 mins Types of Mountains • • • • • Fold Mountains (Folded Mountains) Fault-block Mountains (Block Mountains) Dome Mountains Volcanic Mountains Plateau Mountains Fold Mountains (Folded Mountains) • formed when two continental tectonic plates collide and their edges crumble to form mountains • the crust is uplifted forming folds on top of the other Fault-block Mountains (Block Mountains • created when faults or cracks in the Earth's crust force materials or blocks of rocks upward or down • block mountains break up into chunks or blocks and move either up or down • usually have a steep front side and then a sloping back side Dome Mountains • formed when large amounts of molten rock or magma push the earth’s crust from underneath • before it can erupt the source of magma goes away leaving the pushed up rock as such – rock then cools and forms a mountain Volcanic Mountains • created when magma pushes its way from beneath the earth to the crust, and when it reaches the surface, it erupts as lava, ash, rocks and volcanic gases • erupting materials build around the vent through which they erupted Plateau Mountains • formed by erosion • large areas of high levels of flat land, over 600 meters above sea level formed due to earth’s internal activity • found near Fold Mountains Ranges versus Belts • Ranges are groups of mountain peaks or ridges that form areas bordered by valleys or rivers. • Mountain belts are long chains of mountain ranges that can extend across continents or along their edges. Ranges • Most mountain ranges are uplifted, erode to low elevations, and are uplifted again before they become stable Major Mountain Ranges • Appalachian Mountains • Rocky Mountains • Ozark Mountains • In Nova Scotia – Appalachian Mountains – Cape Breton Highlands – Cobequid Hills Mountain Ranges of Canada Cape Breton Highlands • an extension of the Appalachian mountain chain • 'highland' or 'plateau' of ancient rock across northern Cape Breton Island, Canada • comprise the northern portions of Inverness and Victoria counties • Elevations average 350 metres at the edges of the plateau and rise to more than 500 metres at the centre • Highest peak – White Hill Cobequid Hills • are considered part of the Appalachians • range stretches from Cape Chignecto in Cumberland County in the west through to Pictou County in the east • are composed of a combination of sediments, granites, and volcanic rock all of which has been crushed and folded by continental drift • the highest peaks – – – – Nuttby Mountain (360 m (1,181 ft)) Higgins Mountain (355 m (1,165 ft) Dalhousie Mountain (335 m (1,099 ft) Mt. Thom Belts • Typically 1000 of kilometers long and 100 kilometers across (wide) • Parallel to continental coastlines • The taller the mountains the younger – WHY? Major Mountain Belts • North American Cordillera – western edge of North and South America • Andes – South America • Appalachians/Caledonides – spanning continents • Pyrenees – Europe between France and Spain • Alps - Europe • Urals - Russia • Himalaya – Asia and includes Everest and K2 How are volcanoes made? Volcanism Earth Revealed Series • http://www.learner.org/vod/vod_window.ht ml?pid=324 • Complete the question sheet provided • 30 mins Parts of a volcano • Vent – an opening at the Earth's surface. • Pipe – a passageway in the volcano in which the magma rises through to the surface during an eruption • Crater – a bowl-shaped depression at the top of the volcano where volcanic materials like, ash, lava, and other pyroclastic materials are released • Cone – made of solidified lava, ashes, and cinder – layers of lava, alternate with layers of ash to build the steep sided cone higher and higher. Parts of a Volcano continued… • Magma Chamber – an underground lake of hot liquid What are the different types of volcanoes? Types of volcanoes • Composite Volcanoes – also called strato volcanoes • Shield Volcanoes – also called shields • Cinder Cones & Spatter Cones • Complex Volcanoes – also called compound volcanoes Composite Volcanoes • Dimensions: – Height as much as 8,000 feet – Diameter 1000sq.km – Volume 400km3 • formed by alternating layers of lava and rock fragments • constructed along subduction zones • are often so quiet they seem extinct • form impressive, snow-capped peaks • usually large and conical Types of Composite volcanoes • • • • • • • • Concave Pyramidal Convex-concave Helmet-shaped Collapse caldera Nested Multiple summits Elongated along a fissure Composite Volcano eruption • usually erupt in an explosive way – – – – – usually caused by viscous magma magma rises to the surface it clogs the craterpipe gas in the craterpipe gets locked up the pressure will increase resulting in an explosive eruption • emits gases, ash, pumice, and a small amount of stiff, silica lava • can have eruptions accompanied by lahars -deadly mudflows Shield Volcanoes • Dimensions: – Height often exceeding 1,500 to 2,000 feet • Mauna Loa, the largest of the shield volcanoes its top is over 28,000 feet above the deep ocean floor – Diameter ~5 - 6.5 km • huge in size • built by many layers of runny lava flows • produced by hot spots which lay far away from the edges of tectonic plates Shield Volcano eruption • low-explosivity • lava-fountaining • forms cinder cones and spatter cones at the vent Cinder & Spatter Cones • Dimensions: – Height rarely exceed 250m – Diameter 500m • formed above a vent – formed by Strombolian eruptions • • • • usually a bowl-shaped crater at the top forming steep-sided accumulations grow up in groups they often occur on the flanks of strato volcanoes and shield volcanoes Cinder Cone eruption • lava flows through a single vent that is usually only up to about 1,000 feet tall • lava fragments are ejected from a single vent and accumulate around the vent when they fall back to earth • eruptions usually don't cause any loss of life Spatter Cone eruption • hot erupting lava contains just enough explosive gas to prevent the formation of a lava flow • the lava is torn by expanding gases into fluid hot clots, ranging in size from 1cm to 50cm across Complex Volcanoes • it is because we mean the "system" of those volcanoes is not "simple" • consists of a complex of two or more vents • form because changes of their eruptive characteristics or the location of multiple vents in an area Complex Volcano types • Caldera complexes – often have a large caldera with many subsidiary vents and deposits – some of which could be considered "volcanoes" in their own right. Complex Volcano eruption • the most explosive volcanoes • after an eruption, the result is a caldera (crater) caused when the area around the vent collapses Three Stages of Activity • Active Volcano: – A volcano that is erupting. – Also, a volcano that is not presently erupting, but that has erupted within historical time and is considered likely to do so in the future. • Dormant Volcano: – Literally, "sleeping." The term is used to describe a volcano which is presently inactive but which may erupt again. – Most of the major Cascade volcanoes are believed to be dormant rather than extinct. • Extinct Volcano: – A volcano that is not presently erupting and is not likely to do so for a very long time in the future.