The world began as a fiery ball of superheated space debris 4.5 billion years ago. As the ball cooled and solidified over hundreds of millions of years, massive volcanic eruptions, fed by upwelling hot material, created terra firma, forming the continents and the surrounding oceans.

Figure 1. Model for the physical evolution of the Earth After an initial stage of condensation and accretion (A), the planet was heated to the point where iron (Fe) was segregated into its core (B). Plate tectonic processes helped to form chemically distinct layers within the Earth including the crust (continental and oceanic crust have different thicknesses), the mantle, and the core (C). The lithosphere is the rigid outer layer of the Earth that forms the tectonic plates that move across the surface of the planet. The asthenosphere is an area in the upper mantle with small amounts of melted material that acts as a lubricant for the tectonic plates to move over. larger image [GIF, 11.6 kb, 588 X 254, notice]

The slow cooling of the Earth led to a gradual diminishing of the level of volcanism, but present-day volcanoes are proof that deep below our feet the Earth is still hot and churning.

Volcanoes are not unique to the our planetary home. Many other planets and satellite bodies in our solar system have either active or extinct volcanoes. For example, much of the Moon's surface is covered by ancient lava flows. These form the dark seas or 'mare' that are visible with the naked eye. Some of the ancient volcanoes on Mars are thought to have formed under a glacial ice cap or frozen sea, much like volcanoes now found in Iceland, Antarctica, and Canada. Venus appears to be one of the most volcanically active planets and in fact may still have active volcanoes. Currently, the most volcanically active body in our solar system is Io, one of the moons of Jupiter. Its large volcanoes erupt unique, sulfur-rich magmas.

On Earth, volcanoes are a vital part of life. They continue to create new land, and volcanic eruptions release water and carbon dioxide into the Earth's atmosphere in great quantities, helping replenish these crucial gases in our atmosphere. Volcanic ash is one of nature's best fertilizers, and many of the world's most productive growing regions are on the flanks of dormant volcanoes. Other important byproducts, such as hot water for heating and bathing and steam for producing electricity, are found in volcanic regions. Light-weight volcanic rocks are used for road beds and as special aggregate in cement, creating strong, yet light-weight building material. On movie sets, many of the huge boulders lifted by Hercules are really blocks of light-weight volcanic rocks. Volcanic mud is used in drilling oil and gas wells.

Volcanoes bring to the surface of the Earth and concentrate many important and valuable resources, such as diamonds, copper, and gold. Ancient deposits found in association with volcanoes are some of the most important sources of gold throughout Canada. Ancient seafloor volcanoes contributed to massive accumulations of base metals such as lead, zinc, and copper. Seafloor volcanoes are also important in controlling the chemistry of ocean waters.

Despite these positive impacts, volcanic eruptions can directly and indirectly wreak havoc on people living nearby and travelling in aircraft, and can even temporarily change the climate worldwide. Because volcanoes can so drastically change our day-to-day living, they have been studied for hundreds of years and are the focus of intense geological and volcanological research. Volcanoes spew forth not-so-welcome gases such as sulphur dioxide that can lead to acid rain and destruction of the ozone layer. Some major volcanic eruptions can inject so much ash into the upper atmosphere that it can lead to temporary global cooling. Volcanoes are also sources of potentially hazardous trace elements that can be detrimental to health. In Canada, young volcanoes occur in British Columbia and the Yukon Territory and are part of the system of volcanoes found around the margins of the Pacific Ocean that has long been referred to as the 'Pacific Ring of Fire'.

Figure 2. Pacific Ring of Fire Distribution of many active and recently active volcanoes and boundaries of major tectonic plates. Bold lines are convergent boundaries thought to have active subduction zones. The shaded area is known as the 'Pacific Rim of Fire'. Shown are the names and locations of several volcanoes mentioned in the text (modified from Krafft, 1993) larger image [GIF, 19.4 kb, 563 X 310, notice]

The seriousness of the hazard represented by a volcanic eruption depends on many things, but especially on the style of eruption. Volcanoes dominated by passive, lava-forming eruptions (like those typically seen in Hawaii) generally only threaten immobile objects such as buildings, but are not a serious threat to human life. However, poisonous gases such as sulphur dioxide and fluorine are released during some passive eruptions and can be life-threatening. On the other hand, large (and fortunately rare) eruptions such as those that formed the calderas underlying Yellowstone National Park in Wyoming (600 000 years ago) can have a devastating impact on literally hundreds of thousands of square kilometres of land downwind from the volcano. Such large-scale eruptions may also cause worldwide shifts in weather patterns, including a lowering of global average temperatures; it has been suggested that a huge eruption in Indonesia 74,000 years ago started the last ice age.

Canada has examples of almost every type of volcano found in the world, including stratovolcanoes, shield volcanoes, calderas, cinder cones, and maars. Although none of Canada's volcanoes are currently erupting, several volcanoes and volcanic regions are considered to be potentially active. In addition, volcanic eruptions in Alaska or along the west coast of the United States (Washington, Oregon, and California) can have a significant impact on agriculture and air travel across much of western Canada. Large-scale eruptions anywhere in the world have the potential to affect weather patterns in Canada.