GLOBAL DISTRIBUTION OF TECTONIC HAZARDS

Tectonic hazards include:Seismic activity (earthquakes)

TsunamiVolcanoes

EARTHQUAKES

Most earthquakes occur in clearly defined linear patterns which generally follow plate boundaries

Why?

THE MID-ATLANTIC RIDGE There is a clear line of earthquakes

associated with the Mid-Atlantic Ridge This is because of the build up of pressure in

the Earth’s crust as magma escapes from the volcano(explanation)

There is a broader line of earthquakes along the west coast of America and the Eastern Pacific associated with the subduction of the Nazca Plate beneath the South American Plate – a destructive plate margin

EARTHQUAKES

Broad belts of earthquakes are associated with : 1. Subduction (example) and

(explanation)2. Collision margins e.g in the Himalayas

(explanation)

EARTHQUAKES

Narrow belts are associated with : 1. Constructive margins (example and

explanation)2. Conservative margins e.g in California’s San

Andreas Fault line(example and explanation)

NB: Note that 100km wide can be classified as NARROW

EARTHQUAKES

There are also isolated occurrences of earthquakes that may be due to:

HUMAN ACTIVITIES

or

HOTSPOTS

HUMAN ACTIVITIES

HOTSPOTS

VOLCANOES

WHERE ARE VOLCANOES FOUND?

WHERE?Most volcanoes are found along plate boundaries although some volcanoes are found on hotspots e.g volcanoes of Hawaii

About ¾ of the Earth’s 550 historically active volcanoes lie along the Pacific Ring of Fire

This includes many of the world’s most recent volcanoes e.g Mt. St. Helens, USA and Mt. Unzen, Japan

WHERE?

Other areas of vulcanicity include:IcelandMontserrat in the CarribeanMt. Nyiragongo in the DRC

And some submarine volcanoes e.gKick ‘emJenny, off Grenada in the Carribean

WHERE?Volcanoes are found along the plate boundaries of major plates

Although the deeper levels of the Earth’s crust are much hotter than the surface, the rock is usually not molten because of high pressure

However, along plate boundaries, there is molten rock, magma, which supplies the volcanoes

WHERE?Most of the world’s volcanoes are found in the Pacific Ring of Fire

These are related to the subduction beneath either continental or oceanic crust

Subduction in the oceans produces island arcs, such as the Aleutian Islands, formed by the Pacific Plate subducting beneath the North American Plate

WHERE?

Subduction of oceanic crust beneath the continental crust, results in formation of young fold mountains e.g The Andes, have been formed where the Nazca Plate subducts beneath the North American Plate (map showing this)

HOTSPOTS

HOTSPOTSHOTSPOT: A jet of hot material

rising from deep within the mantleNot all volcanoes are found along

plate margins Those in Hawaii are found in the

middle of the oceanThe Hawaiian islands are a line of

increasingly older volcanic islands which stretch north-west across the Pacific Ocean

These volcanoes are related to the movement of a plates above the hot fluid mantle

HOTSPOTS

Hotspots can be found beneath continents e.g The East African Rift Valley

These hotspots can:Produce volcanoesBreak up continents and form

new OCEANS

CONTRASTING VOLCANOES AT DIFFERENT PLATE MARGINS

Viscous lava (composition)

Erupt explosively (gases)

Relatively fluid lava (composition)

Erupt less explosively (gases)

SUBDUCTION ZONE MID-OCEAN RIDGES

THE TYPE OF MAGMA

VISCOUS FLUID

CONTRASTING VOLCANOES AT DIFFERENT PLATE MARGINS

Slab of the oceanic plate warms up slowly as it slides down the subduction zone. Volatile compounds such as carbon dioxide and water leave the slab and rise to the mantle so that it melts. Magma is then able to rise

Hot fluid rock rises up due to convection currents and the upper mantle begins to melt causing eruption of basaltic lava

SUBDUCTION ZONE MID-OCEAN RIDGES

HOW BIG AND HOW VIOLENT WILL THE EXPLOSION BE?Huge explosions occur whenever

water meets hot rock because water vaporises increasing pressure until the rock explodes

The likelihood of a huge eruption depends largely on the viscosity of magma and hence, its composition

Gases dissolve quite easily in molten rock underground because there are very high pressures there

However, as magma rises to the surface the pressure drops and some of the gas may become insoluble and form bubbles

STYLE OF ERUPTIONProcesses occurring at different plate margins determine the style of eruption

These processes produce different, but predictable compositions

Some minerals melt before others in Partial Melting which alters the composition of the molten rock produced

PARTIAL MELTING

PARTIAL MELTINGPartial melting of the Earth’s

mantle produces BASALT.At subduction zones, the older and deeper slabs experience greater partial melting and this produces a silica-rich magma

REFERENCESGeography (Guiness and Nagle)http://earthquake.

usgs.gov/learn/topics/plate_tectonics/rift_man.php

http://geology.com/volcanoes/types-of-volcanic-eruptions/

Mauna Loa, Hawaii

DONE BY:VIMBIKAYI CHIMUKA