Many of the Earth's natural resources of energy, minerals, and soil are concentrated near past or present plate boundaries. The utilization of these readily available resources have sustained human civilizations, both now and in the past.

Volcanoes can clearly cause much damage and destruction, but in the long term they also have benefited people. Over thousands to millions of years, the physical breakdown and chemical weathering of volcanic rocks have formed some of the most fertile soils on Earth. In tropical, rainy regions, such as the windward (northeastern) side of the Island of Hawaii, the formation of fertile soil and growth of lush vegetation following an eruption can be as fast as a few hundred years. Some of the earliest civilizations (for example, Greek, Etruscan, and Roman) settled on the rich, fertile volcanic soils in the Mediterranean-Aegean region. Some of the best rice-growing regions of Indonesia are in the shadow of active volcanoes. Similarly, many prime agricultural regions in the western United States have fertile soils wholly or largely of volcanic origin.

Most of the metallic minerals mined in the world, such as copper, gold, silver, lead, and zinc, are associated with magmas found deep within the roots of extinct volcanoes located above subduction zones. Rising magma does not always reach the surface to erupt; instead it may slowly cool and harden beneath the volcano to form a wide variety of crystalline rocks (generally called plutonic or granitic rocks). Some of the best examples of such deep-seated granitic rocks, later exposed by erosion, are magnificently displayed in California's Yosemite National Park. Ore deposits commonly form around the magma bodies that feed volcanoes because there is a ready supply of heat, which convectively moves and circulates ore-bearing fluids. The metals, originally scattered in trace amounts in magma or surrounding solid rocks, become concentrated by circulating hot fluids and can be redeposited, under favorable temperature and pressure conditions, to form rich mineral veins.

The active volcanic vents along the spreading mid-ocean ridges create ideal environments for the circulation of fluids rich in minerals and for ore deposition. Water as hot as 380 °C gushes out of geothermal springs along the spreading centers. The water has been heated during circulation by contact with the hot volcanic rocks forming the ridge. Deep-sea hot springs containing an abundance of dark-colored ore minerals (sulfides) of iron, copper, zinc, nickel, and other metals are called "black smokers." On rare occasions, such deep-sea ore deposits are later exposed in remnants of ancient oceanic crust that have been scraped off and left ("beached") on top of continental crust during past subduction processes. The Troodos Massif on the Island of Cyprus is perhaps the best known example of such ancient oceanic crust. Cyprus was an important source of copper in the ancient world, and Romans called copper the "Cyprian metal"; the Latin word for copper is cyprium.

Oil and natural gas are the products of the deep burial and decomposition of accumulated organic material in geologic basins that flank mountain ranges formed by plate-tectonic processes. Heat and pressure at depth transform the decomposed organic material into tiny pockets of gas and liquid petroleum, which then migrate through the pore spaces and larger openings in the surrounding rocks and collect in reservoirs, generally within 5 km of the Earth's surface. Coal is also a product of accumulated decomposed plant debris, later buried and compacted beneath overlying sediments. Most coal originated as peat in ancient swamps created many millions of years ago, associated with the draining and flooding of landmasses caused by changes in sea level related to plate tectonics and other geologic processes. For example, the Appalachian coal deposits formed about 300 million years ago in a low-lying basin that was alternately flooded and drained.

Geothermal energy can be harnessed from the Earth's natural heat associated with active volcanoes or geologically young inactive volcanoes still giving off heat at depth. Steam from high-temperature geothermal fluids can be used to drive turbines and generate electrical power, while lower temperature fluids provide hot water for space-heating purposes, heat for greenhouses and industrial uses, and hot or warm springs at resort spas. For example, geothermal heat warms more than 70 percent of the homes in Iceland, and The Geysers geothermal field in Northern California produces enough electricity to meet the power demands of San Francisco. In addition to being an energy resource, some geo-thermal waters also contain sulfur, gold, silver, and mercury that can be recovered as a byproduct of energy production.

This question has yet to be fully resolved. Four main hypotheses have been put forward to explain this.

Convection currents

This hypothesis suggests that flow in the mantle is induced by convection currents which drag and move the lithospheric plates above the astenosphere. Convection currents rise and spread below divergent plate boundaries and converge and descend along convergent. Three sources of heat produce the convection currents:

(1) cooling of the Earth's core

(2) radioactivity within the mantle and crust

(3) cooling of the mantle

The convection hypothesis has been proposed in several different forms throughout the last 60 years. Convective models of plate evolution clearly show how important convective heat transport is to the modern Earth, over length scales as small as 100 km and times of 60 million years. Earth is a spendthrift, living on its inherited capital of primaeval heat, not on its radiogenic modern income.

Magma injection This hypothesis invokes the injection of magma at a spreading centre pushing plates apart and thereby causing plate movement.

Gravity Oceanic lithosphere thickens as it moves away from a spreading centre and cools, a configurationwhich might tend to induce plates to slide under the force of gravity, from a divergent margin towards a convergent margin.

Descending plates This hypothesis suggests that a cold dense plate descending into the mantle at a subduction zone may pull the rest of the plate with it and thus cause plate motion.

To summarize, the plate tectonic model provides a mechanism by which:

(1) continents can move across the surface of the globe

(2) patterns of volcanism can change and shift across the globe as plates and their boundaries evolve and move

(3) new oceans may grow and different sedimentary basins evolve

(4) oceans and sedimentary basins close and are deformed to produce mountains

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GLOSSARY TEST

ANSWER QUESTIONS USING Chapter 1 of Waugh

1. Where is the ‘Moho’ found ?

2. What is the difference between sial and sima ?

3. Look at the diagram on pp.18-19 which shows landforms in the Pacific Ocean. Explain the formation of the following:

i) the islands of Japan

ii) the Hawaiian Islands

iii) Easter Island

iv) the Galapagos Islands

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