Convergent_Plate_Boundaries

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A.M.D.G Geography

08/07/13

CONVERGENT PLATE BOUNDARIES

·

Planet

is not getting any bigger so if new land is created other land has to be

destroyed.

·

These

boundaries are also called destructive plate boundaries.

·

This

happens where plates dive into mantle

·

Solid

rock of crust melts

·

This

is part of tectonic cycle.

3 Convergent Boundaries

·

Continental

-> Oceanic

·

Oceanic

->Oceanic

·

Continental->

Continental

·

Oceanic meets continental

·

Oceanic

material (sima) is heavier than continental (sial)

·

Oceanic

dives beneath continental and about 70km melts.

·

The

plate (lithosphere) becomes part of the mantle (asthenosphere).

·

Hence

it is called a destructive boundary

·

Stress

of this collision causes the continental plate to buckle to form mountains.

·

The

melting plate mixed with gases from the sea water may also cause violent

volcanic activity Pacific Plate North America -> Mt St Helen’s and Rockie

Mts.

·

Oceanic meets Oceanic

·

Older

heavier plate dives under.

·

This

is subduction long, deep, narrow trench developed.

·

Marianas

trench (Pacific and Philippines) about 11km at deepest.

·

Crystal

rock destroyed.

·

Magma

formed volcano activity occurs. A line of volcanic islands appear along

boundary.

·

These

are Island Arcs.

·

Philippines

and Japanese Islands

·

Part

of Pacific Rim

·

Continental meets Continental

·

Two

continental plates collide head on.

·

Light

but very thick.

·

The

lithosphere subducts but plates collide and all sediments at edges of plates

are swept up and deformed causing mountain ranges.

·

Indian

sub plate collided with Eurasian 50-20 million years ago resulting in

Himalayas.

·

Ireland:

Cadedonian mts Donegal

·

Faulting,

thrusting and folding occur.

·

Huge

amounts of deposited material from rivers.

·

They

collided recently therefore mountains are still high i.e. ->not worn down by

weathering and erosion.

·

Very

destructive earthquakes occur along fault lines within Himalayas.

Conservative Plate Boundaries

·

Plates

slide past

·

Along

fault

·

Transform

faults

·

Crust

neither created nor destroyed

·

Therefore

a passive or conservative boundary

·

Intense

earthquake activity

·

San

Andreas Fault, California 1200km long move about 5cm a year

Plate Boundary Summary

Direction

Name

Created

Moves

apart

Divergent

or Constructive

Crust

created Continents split

Move

together/Collide

Convergent

or Destructive

Fold

Mts., Earthquakes, Volcanoes, Island Arcs and crust destroyed

Moves

alongside/past

Transform,

neutral, passive

Earthquakes

Move

over hot spot

Mid

plate

Volcanic

Island

A.M.D.G Geography 12/07/13

VOLCANICITY

Intrusive activity: Magma forces itself into crust and cools. These form batholiths, sills and Dykes. Often called plutons. This activity is Plutonic activity. It is all underground.

Extrusive activity: Lava comes out at the surface. This forms Lava plateaus and volcanoes of different types. Called Volcanic Activity.

Volcano

· A volcano is a vent through which molten rock escapes to the Earth’s surface. Unlike other mountains which are pushed from below, volcanoes are built by surface accumulation of their eruptive products layers of lava, ashflows and ash.

· When pressure from gases within the molten rock becomes too great, an eruption occurs. Volcanic hazards include gases, lava an pyroclastic flows, landslides, earthquakes and explosive eruptions.

Locations for Volcanic Activity

1) Subduction zones at convergent plates (Mt. St. Helens)

2) Spreading centres Rifting/Doming (East African Rift Valley +Mt. Kilimanjaro +Giant’s Causeway)

3) Hot spots (Hawaiian Islands)

Hot Spots

· Volcanoes can be caused by mantle plumes, these so called hotspots for example Hawaii can occur far from a plate Boundaries.

· A plume of magma a volcano to occur on the surface, directly above it.

Over time lithosphere moves and a new volcano occurs over the hot spot or plume. This volcano will in time move away from the plume and another occur over the active plume.

What is erupted?

Basic lava: Low in silica, hot, runny gentle eruptions

Acid Lava: High in silica , less hot, plastic/ sticky , violent.

Gases: Carbon monoxide , sulphur dioxide , hydrogen.

Pyroclastic materials : Name for broken rock/ash/cinder bombs

Pumice: Rock with so many air spaces it floats

Lahar: A mudflow

New Ardente: Gases/ superheated steam/ dust moves very fast

Types of Volcano

Classification of Volcanoes

1. Frequency of eruption

Extinct – erupted in geological time

Dormant – erupted in historic time, not recent

Active – erupting continuously Mt Ethen

2. Shape of volcano

Types of Volcano:

· Shapes are based on type of lava/frequency and violence of eruption

· Type of lava is based on silica

· Silica is a mixture of silicon and oxygen

· More than 45%-60% silica = very acidic

· Less than 45% silica=basic

· Violent eruptions are typically acidic

· Silica traps gases

· Infrequent eruptions (dormant) tend to be very violent.

· Subduction also converts sea floor organic remains into CO2.

Types of volcanoes or Central Vent Eruptions

1. The shield volcano -> This is a broad, shallow volcanic cone, which arises because the running lava, which is fluid and hot cools slowly.

2. This Dome Volcano -> This one has a steep, convex slope formed from thick, fast cooling lava.

3. The ash-cinder Volcano -> Throws out besides lava- much ash into air. Though this the volcanic cone is built up from alternate layers of ash cinder.

4. The composite volcano

5. The caldera volcano

1. Shield Volcano

Basic lava

Low viscosity (flows)

Runny lava

Flows great distance

Gentle slopes

Wide base

Mauna Loa Hawaii really wide base of 120km 600m below sea level and 4000m above sea level.

2. Dome Cone

Acidic lava

Viscous

Bulbous shape Like tooth paste being squeezed out of tube.

Vents block easily

Violent eruptions can therefore follow

Mt. St Helens

3. Cinder Cone

Violent Eruption

Cinder Rocks and volcanic bombs

More gentle at base

E.g. -> Parcutin, Mexico

Big Crater

4. Composite Volcano

Strato Volcano

Made up of lava flow and pyroclastic material

Lava acts as a cap so volcano does not erode and can grow to a great height.

E.g. -> Mt Etna , Fujiyama, Vesavius

5. Calder

Calderas, which are circular depressions are found on the summits of many volcanoes “Giant” calderas are the largest of these: huge craters up to many tens of miles across.

Giant calderas form by collapse in gigantic eruption that spew volcanic rocks out hundreds or even thousands of miles in all directions.

Lahar or mudflow: Mudflows set off by Volcanic activity

Examples Nevada Del Ruiz, Colombia (1985)

Destroyed city of Armero

River of mud flowed over the town caused by the heat from the volcano melting a glacier

23,000 died

Hot springs and geysers

Hot springs and geysers are also central vent emissions

Groundwater gets heated in rocks in regions of recent volcanic activity. It may return to surface as spring/ geysers.

If spring water is warmer than human body then it is classified as a hot spring.

May also contain dissolved materials

Vichy France or Blue Lagoon Iceland spas

Benefits of Volcanic Activity

Geothermal energy -> electricity is produced from water heated in pipes deep in the earth’s crust and returned to the surface to be used in houses and greenhouses.

Mineral rich soil can support more than one crop.

Tourism e.g. -> Mt Vesuvius, Canaries , Yellow stone park Iceland

Fissure Eruptions:

In this type, there is no central crater at all. Instead giant cracks open in the ground and expel vast quantities of lava that spread far and wide to form huge rocks that can cover almost everything around.

When those rocks of lava cool and solidify the surface remains mostly flat. Since the source cracks are usually buried, there is often nothing “volcano-like” to see-only a flat plane or plateau.

1. Antrim Plateau

Enormous lava Plateau

Covering 4000 square km’s

Extending to a depth of 800m

Possibly 400 metres eroded

Formed during tertiary period 65million years ago. American and European plate pulling apart.

Crust stretching , cracks appear and lava pours out.

Spreads over surface and cools quickly

Over 2 million years of this, layers build up to form a flat topped mountain called a plateau.

Layers vary in size from 5m to 40m

Edges have been eroded and undercut.

Intrusive features

Known as plutonic structures

Magma goes into the crust and eventually cools and hardens

General name for structures is Plutons.

Only revealed as rock above them is weathered and eroded.

Dykes:

Magma forces into vertical fractures.

Magma widens them further

May be a few cm’s up to several metres

Other rocks of crust get worn away over time but rocks of dyke are more resistant and weather slower

They are left as a ridge

Sills:

Magma forces it’s way into tiny gaps between sedimentary layers along the bedding planes.

It happens close to the crust where there is less pressure of over lying rock

Over lying rock may be pushed up to form laccoliths and loppoliths.

(Laccolith pushes up a dome shape loppolith)

Intrusive Features:

Dublin/Wicklow Batholiths

Stretches from Dublin to Kilkenny

Comes over 1500 square kms.

It is the largest of its kind in Ireland and Britain.

Wicklow Mts first formed about 400mya when American Plate collided with European plate (Iapetus sea began to close)

Magma masses rise into the mountains

Shale rock of mountains touches off hot magma and gets metamorphosed into quartzite .

The magma cools very slowly to form granite.

Through weathering the granite can now be seen out the surface.

Granite weathers to form rolling hills.

But in some places metamorphic rocks remain.

Lugnaquillic Mts is still capped with schist and is steep and rugged.

Sugar loaf is made up of quartzite and is also steep and sharper in outline.

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