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Tectonic Features. Contents. Folding and Fold Mountains Faulting and its associated Landforms Volcanism and the Earth ’ s Crust Ocean Ridges, Ocean Trenches and Island Arcs Earthquakes. Folding and Fold Mountains. All major mountain chains are ‘ fold-belts ’ or called ‘ orogenic belts ’.
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Contents • Folding and Fold Mountains • Faulting and its associated Landforms • Volcanism and the Earth’s Crust • Ocean Ridges, Ocean Trenches and Island Arcs • Earthquakes
Folding and Fold Mountains • All major mountain chains are ‘fold-belts’ or called ‘orogenic belts’
Features and Characteristics of Fold Mountain Areas • There are parallel belts of fold mountains separated by intermountain plateau where sedimentary strata are much less intensely folded. • They are mainly composed of thickness of sedimentary rock strata. But when the core of fold mts. has been exposed, igneous and metamorphic rocks are also found. • Young fold mts. Zones represent lines of weakness of the earth’s crust and most of the world’s recently active volacnoes lie and where the greater majority of earthqaukes originate. • The rock strata have been compressed into various kinds of folded structure.
Geometrical Varieties of Folds • Three geometrical varieties of folds can be distinguished, anticlines, synclines and monoclines.
Types of folding • The profile of a fold is its form as seen in a plane perpendicular to the axis. • Simple / Symmetrical Fold • Two limbs dip symmetrically as similar angles • Asymmetrical Fold • One limb dips more steeply than the other. • Over Fold / Overturned Fold • Recumbent Fold • The axial plane is roughly horizontal • Overthrust Fold • The sheet of rocks that has moved forward along the thrust plane. • Anticlinorium & Synclinorium • A complex of folds of different orders
Formation of Fold Mountains • According to the plate tectonic theory, fold mt originates where two plates of crust converge. • The full development of an orogenic belt requires collision between plates.
Formation of Fold Mountains • The surface is being actively eroded supplying a large amount of sediments. • An accumulation of great depths of sediments in a geosyncline (large depression) under an ocean. • Two plates move toward each other producing a great compressional force. • The sedimentary rocks are folded up • Fold mountains are formed • Magma of the mantle may also flow out to the earth’s surface forming volcanoes in this process. • Faulting is very common. • Eg. Himalayas (boundary between Eurasian plate and the Australian Plate)
Faulting and its associated landforms • Fault is a fracture of dislocation in the earth’s crust along which there has been displacement of the rocks strata. • The movement of the rocks on a fault may have been in any direction, vertical, or horizontal or some combination of these.
Fault types • Faults may be divided into several categories in relation to the movements that have taken place on them.
Normal Fault • It is caused by tension • The hanging wall is displaced downwards relative to those against the footwall along the fault plane. • ‘Downthrow’ and ‘upthrow’ for the two sides are purely relative. • Feature: Horsts (block mountain) and Graben
Reverse Fault / Thrust Fault • It is caused by comression • The hanging wall is upthrow the footwall along the fault plane. • Feature: Horsts and Graben
Tear Fault / Wrench Fault • It is called transcurrent or strike-slip faults. • It is formed where the movement was dominantly horizontal.
Block Mountain and Rift Valley • Regions which have been divided by faulting into relatively elevated or depressed blocks are said to be block faulted. • The upstanding fault blocks may be small plateaux or long ridge-like block mountains or horsts. • A long fault trough is known as rift valley. • Example: East African Rift Valley.
Volcanism and the Earth’s Crust • Vulcanicity is the process by which matter is transferred from the earth’s interior and erupted on to its surface. • Volcanoes eruption are undoubtedly one of the greatest natural hazards to life on this planet. • Below the crust, despite the high temperatures at depth, the material is usually solid because of the great pressure exerted by the superior masses of rock. • At times, a local increase of heat or/and reduction in pressure, the Basal layers of the crustal SIMA to become magma. • Magma may be able to rise to the surface of the earth through conveniently placed fissures and pipes or vents. • All such activities are known as vulcanicity. • There are two types of vulcanicity: Intrusive and Extrusive vulcanicity
Intrusive Forms of Vulcanicity • The results of the forcing into the earth’s crust of magma depend on • Degree of fluidity • The character of the planes of weakness, such as joints and faults….
Intrusive Forms of Vulcanicity • Dykes: • These are formed when magma has risen through near-vertical fissures and solidifying to form ‘walls’ of rock cutting across the bedding planes of parent rock. • Sills: • Horizontal sheets of rock solidify from magma which has been ejected between bedding planes. • Laccoliths: • These features are produced where tongue-like lateral intrusions of viscous magma have forced the overlying strata into a dome. • Batholiths: • Large masses of rock occur in the heart of mountain range. • Large scale intrusion in the great depth and magma cooled slowly to form large-crystalled rocks such as granite. • Intrusive rocks can be classified into Hypabyssal (dykes, sills and laccoliths) and Plutonic (Batholiths).
Extrusive Forms of Vulcanicity • A volcano consists of vent or opening at the surface of the crust through which material is forced in eruption. • This may accumulate around the vent to form a hill, or it may flow widely over the country rock as an extensive level sheet. • Three types of extrusive materials
Extrusive materials • Gases: • Include gaseous compounds of sulphur and hydrogen with carbon dioxide. • Stream is the most important factor affecting the eruption. • Solid: • Pyroclastic Debris / Pyroclasts / Tephra • Include: • Country rocks • Finer materials: (lapilli, dust, ash) • Volcanic bombs: lava are thrown into the air solidify before reaching ground.
Extrusive materials • Liquid: • Lava • The form of a volcanic cone and the nature of eruption depend on a large extent upon the nature of lava. • Acid lava: • High silica (SiO2) content, high melting-point, viscous, solidify rapidly and flowing very slow. • It builds high, steep-sided cones. • They may solidify in the vent and cause recurrent explosive eruption. • Basic lava • Relatively poor SiO2 content, low melting point and flows faster, and produces much flatter cone / shield cone. • Its eruption is quiet.
Three Types of Volcano • They are classified by eruption type • Explosive eruption • Cinder cones and composite cones • Quiet eruption / Fissures eruption • Shield Cones and lava plateaux
Cinder Cones • They are the smallest volcanoes • They are formed by Pyroclastic Debris, tephra and volcanic ashes • Cinder cones rarely grow to more than 150 to 300m in height.
Basic Lava Cones / Shield Cones • They are built by basic lava. • Characteristics of basic lava (Basalt) • Dark Colour with Low content of SiO2 • Low viscosity, Low melting-point and fast flowing. • Volcanoes with gentle slope (4o to 5o) • Eg. Hawaiian Islands
Acid Lava Cones • Light colour and viscous acid lava flows very slow for high content of SiO2. • It produces a steep dome.
Volcanic Plug • When the vent of lava cone was solidified by acid lava. • When the country rocks were removed, the solidified vent is called volcanic plug.
Composite Volcanoes • Most of the world’s great volcanoes are composite cones. • They are built pf layers of cinder and ash alternating with layers of lava. • Many composite volcanoes lie in a great belt, the circum-Pacific ring (ring of fire) and Mediterranean group of volcanoes. • The eruption of large composite volcanoes is accompanied by explosive issue of steam, cinders, bombs, ash and by lava flows. • The crater may change form rapidly. • Example: Fujiyama in Japan.
Calderas • Volcanic explosive so violent as to destroy the entire central portion of the volcano. • There are remain only a great depression, a caldera. • Example: • Krakatoa (in Indonesia), exploded in 1883, leaving a great caldera. • 75cubic km of rock disappeared during the explosion. • Great tsunamis appeared and kills thousands of people of Java and Sumatra.
Active, Dormant and Extinct Volcanoes • Active Volcanoes: • Eruption periodically in historic times. • Dormant Volcanoes: • Renewal of eruption activity is possible. • Eg. Vesuvius had been dormant so long before its eruption of AD 79 that it was thought to be extinct. • Extinct Volcanoes: • They were formed in long-past geological times and with no sign of any volcanic activity.
Distribution of volcanoes Pacific Ring of Fire
Distribution of Volcanoes • There are many known active volcanoes all over the world. 80 active and ten calderas locates here. • Pacific Ring of Fire • 2/3 world’s volcanoes occur here. • The chain of volcanoes near 3200 km • America: • St. Helens is very active in recent years. • Highest peaks in S. America • Aconcagua (7021m), Guayatiri (6060m)
Distribution of Volcanoes • Africa: • They are found along the East Africa Rift Valley. • Mount Kenya (5195m), Kilimanjaro (5889m) • Asia and Europe: • Alpine-Himalayan belt which forms the most tectonic features (folding, faulting, volcanoes…..) • Others: • Volcanic activity is widespread in Iceland.Several Atlantic islands, which have many eruption .
Ocean Ridges • They connect through all the oceans to form a worldwide feature nearly 60000 km long. • They are all composed of basaltic lava. • They are formed by fissure eruptions with a fairly uniform rate of lava emission. • Iceland is built astride the Mid-Atlantic Ridge system.
Ocean Trenches • The deepest parts of the oceans are elongated troughs descending to depths of over 10000 metres. • Sediments accumulating on the trench floors. • Nearly all the trenches occur around the margins of the Pacific Ocean and arc of volcanic islands is commonly present on the continental side of the trenches.
