Tectonic Features

Contents

Folding and Fold MountainsFaulting and its associated LandformsVolcanism and the Earth’s CrustOcean Ridges, Ocean Trenches and Island ArcsEarthquakes

Folding and Fold MountainsAll 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 FoldsThree geometrical varieties of folds can be distinguished, anticlines, synclines and monoclines.

Elements of Fold

Types of foldingThe 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 FoldRecumbent Fold

• The axial plane is roughly horizontalOverthrust Fold

• The sheet of rocks that has moved forward along the thrust plane.

Anticlinorium & Synclinorium• A complex of folds of different orders

Types of folding

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 MountainsThe 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 upFold mountains are formedMagma 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.

Elements of fault

Fault typesFaults may be divided into several categories in relation to the movements that have taken place on them.

Normal FaultIt is caused by tensionThe 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 FaultIt is caused by comressionThe 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 ValleyRegions 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.

Cross-section of East African Rift Valley.

Volcanism and the Earth’s CrustVulcanicity 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 VulcanicityThe results of the forcing into the earth’s crust of magma depend on

Degree of fluidityThe character of the planes of weakness, such as joints and faults….

Intrusive Forms of VulcanicityDykes:

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 materialsGases:

Include gaseous compounds of sulphur and hydrogen with carbon dioxide.Stream is the most important factor affecting the eruption.

Solid:Pyroclastic Debris / Pyroclasts / TephraInclude: • Country rocks• Finer materials: (lapilli, dust, ash)• Volcanic bombs: lava are thrown into the air solidify

before reaching ground.

Extrusive materialsLiquid:

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.

Structure of Volcanoes

Three Types of Volcano

They are classified by eruption typeExplosive eruption• Cinder cones and composite cones

Quiet eruption / Fissures eruption• Shield Cones and lava plateaux

Cinder ConesThey are the smallest volcanoesThey are formed by Pyroclastic Debris, tephra and volcanic ashesCinder cones rarely grow to more than 150 to 300m in height.

Basic Lava Cones / Shield ConesThey 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 ConesLight colour and viscous acid lava flows very slow for high content of SiO2.It produces a steep dome.

Volcanic PlugWhen the vent of lava cone was solidified by acid lava.When the country rocks were removed, the solidified vent is called volcanic plug.

Volcanic Plug

Composite VolcanoesMost 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.

Composite cones

CalderasVolcanic 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.

Calderas

Calderas

Active, Dormant and Extinct VolcanoesActive 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 VolcanoesThere 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. AmericaAconcagua (7021m), Guayatiri (6060m)

Distribution of VolcanoesAfrica:

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, Ocean Trenches and Island Arcs.

Ocean RidgesThey 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

Ocean TrenchesThe 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.

Island ArcsIsland arcs are mostly found around western margin of the Pacific Ocean and in the northeast of the Indian Ocean.They formed on the subduction zone.

Island Arc - Japan

Island Arc - JapanJapan is the largest area of land formed by island arc.MT. Fuji reaches nearly 4000m, and several other peaks on Honshu top 3000m.Honshu is largely a pile of basalt and andesite lying between the Japan Trench and the Sea of Japan.It seems that the subducted ocean-floor becomes mobilized as magma when it reaches 120km depth.When the descending plate reaches over 100km in depth partial melting takes place, magma rises to form a pile of volcanic rocks and the island arc begins to rise / form.

Earthquake

Earthquakes

ContentsIntroductionSeismic WavesDistributionEarthquake and Plate TectonicEffects of Earthquake

http://www.nstm.gov.tw/earthquake/A_2.htm

Introduction

Earthquakes - natural vibrations within the earth’s crust.Some of these are violent and are responsible for large scale death and destruction.Most earthquakes are twoo small to be felt by man and only be detected by seismographs.

Seismic WavesThe investigation of earthquakes and the transmission of earthquake waves is known as seismology.Earthquake generates elastic vibrations or ‘waves’

Four kinds of seismic wavesPrimary Waves (P waves)Secondary Waves (S waves)Love waves (L waves)Rayleigh waves

P and S wavesP waves

It is also called compressional waves which consists of longitudinal vibrations.These waves propagate very rapidly through both solids and liquids and are usually the first indication that an earthquake has occurred.The speed of P-waves

• Crust: 5.5 km/s; Upper mantle: 8 km/s; Lower mantle: 13.5 km/sS-waves

It is also called Shear waves which are transverse vibrations with an movement at right angles to their path.The speed of S-waves is 60% of P-waves.It cannot propagate through the liquid (core)

The P-waves and S-waves are body waves.

P-waves and S-waves

L-waves and R-wavesL-waves:

It is also called Love waves which travel around the periphery the earth.

R-waves:It is also Rayleigh waves which travel similar to wind-driven ocean waves.

L-waves and R-waves are surface waves.P-waves is the fastest and L-waves are the slowest, but L-waves have the greatest amplitude and are those that do the most damage.

DistributionIt is very similar with the distribution of volcanoes.

Distribution80% originate in the ‘Circum-Pacific Ring of Fire’.15% in the Mediterranean and Trans-Asiatic zone.5% in other parts of the world.

Formation

Refers to the theory of plate tectonic.

http://www.nstm.gov.tw/earthquake/A_2.htm

Effects of EarthquakeShattered buildingsDisplaced roads and railwaysCollapsed bridgesGreat cracks in groundChanges in sea levelTsunamisFire and floodFamines and diseaseLoss of life