According to the United States Geological Survey (USGS), a volcano is a vent or opening in the surface of the Earth through which magma erupts and also the landform that is constructed by the erupted material. Magma is liquid rock with dissolved gases in it including water vapor. When magma reaches the surface of the Earth, it is called lava. The liquid rock is kept so by the pressure of the rock above and by the high temperatures of the Earth’s mantle below.
Formation: The theory of plate tectonics seems to explain well the things we can observe about volcanoes. According to this theory, the Earth’s crust consists of plates atop softer, heavier rock. Because they float on the surface of moving convection currents, the plates are moving, too. The continents we inhabit ride on these plates and so are also moving. In looking at a map, we see that most of the active volcanoes of the world are situated about the Pacific Basin in a configuration known as the Ring of Fire. One of the plates the Ring of Fire surrounds is called the Pacific Northwest Plate.
Plate Tectonic Activity: Observe a thick layer of fat floating upon the surface of cooled chicken broth. Break the solid fat into plates and gently stir simulating the motion of the Earth’s plates. Notice that three things can happen: the plates meet and slide one under another, two plates slide past each other, or the plates move apart. If we did this to the Earth, magma could ooze to the planet’s surface.
Let’s look at each way plates may interact. In one case, the edges of the plates crumple and one plate begins to go under the other one. This seems to be happening on the west coast of the North American continent. When the Pacific Northwest Plate plunges under the North American Plate, Pacific Northwest Plate is forced down (subducted) into much hotter levels, melting and forming magma. The magma rises to form volcanoes which have produced the Cascade Mountain Chain. Some of the mountain-building in the area is from the crumpling effect, and some is volcanic.
When two plates slide past each other, earthquakes, displacement of the land, rivers with jogs, and other effects of a transform fault can occur. An example of this is the San Andreas Fault of California.
When plates move away from each other, magma from below flows up into the trench forming new plate material. A good example of this is the sea-floor spreading in the Mid-Atlantic Ridge where the fresh magma is forming undersea mountains.
Most of the Earth’s volcanoes occur around the plate boundaries. However, there are some interesting volcanoes which are not associated with the edges, but which are within the plates. The “Hot-Spot” hypothesis seems to explain the phenomena we see, and so the hypothesis is generally accepted. A permanently located “hot spot” in the interior of the Earth melts some of the material below a moving plate. This magma flows up through weak places and forms great shield volcanoes. The Hawaiian Island chain has been formed as the Pacific Plate moves over a “hot spot”. As the plate moves, magma comes up in new locations producing more volcanoes and, eventually, new islands in the chain.
Structure: All volcanoes are not the same. Each type of volcano and each type of magma results in special landforms which vulcanologists study, measuring them and observing eruptions. Volcanoes made out of ash and coarse solid fragments form cinder cones which have a slope of about 30°- 35°. In 1943, a Mexican farmer spotted a hole in the dirt with smoke coming out, and by evening, a small volcano had formed. Paricutin, a large cinder cone, is now 1345 feet high. Shield volcanoes are flat domes with slopes of less than 10° and are made up of basaltic materials Mauna Loa is the world’s largest active volcano and rises about 30,000 feet above the sea floor; Mons Olympus on Mars is twice as large. A spatter cone is formed when gas-charged lava is spit out of the vent into an irregularly-shaped dome. A plug dome is made from cooler, sluggish lava which just cools forming a plug over the vent. Composite volcanoes may be freeflowing one day and explosive the next because of their mixed lava, gases, ashes, and fragmented material. These materials result in large volcanoes like Mt. St. Helens, Mt. Shasta, and Mt. Fujiyama in Japan. Crater Lake is a composite volcano. Formerly, the volcano was very large, dormant, and glacier covered. Then, it blew its top in a tremendous eruption. After the eruption slowed, a new top collapsed making a caldera. Still later, the three cinder cones within the caldera formed and slowly the caldera filled with water. The top hollow of a volcano is called a caldera if it is larger than 1 mile in diameter. A smaller hollow is called a crater. All types of volcanoes are interesting to study.
Volcanic Treasure Houses: Another aspect of volcanoes is what the lava carries. New water is released during an eruption along with other gases, like sulfur compounds. The volcanic ash released into the atmosphere can cause a reduction in the total amount of sunlight the Earth receives from the sun, but also can provide nutrients for the land. Diamonds and other gem stones are often found in cooled lava. Obsidian, favored by the ancient Native Americans for arrowheads and implements, comes from lava which has cooled quickly forming small crystals. Lava which cools slowly forms larger crystals like those found in granite, another igneous rock and a wonderful building stone. There are many different and unique kinds of granite. Llanite, a granite with blue quartz, can only be found in Llano, TX. Many heavy, industrial metals are igneous rocks, as are the precious metals we use for currency: gold, silver, and copper. Volcanoes have given us many things of value.
Crystal Formation Activity: You can study the effects of different temperatures on cooling rocks by crushing and heating moth balls (paradichlorobenzene, pa-ra-di-chlor-o-ben-zene). Melt the crystals in three jars set in very hot water. After the crystals melt, place one jar into ice, one jar into warm water, and the last jar on a hot pad. The crystals formed as they cool will be different sizes just like the crystals in granites and obsidian.
The study of volcanoes is fascinating and complex. Discover new ways in which God has provided for us on this Earth by using the materials in the resource section below. Go see some volcanoes and hike their surfaces. Go into the Ice Caves of Oregon to see lava tubes. If you cannot go to them physically, visit them on the internet. The U.S. Geological survey has many great resources all available to homeschoolers. View the videos on Mt. St. Helens and read about the eruption at Pompeii. This is an important study and can lead to many other geological investigations, hobbies, and even careers. Enjoy!
the great King above all gods.
In his hand are the depths of the earth,
and the mountain peaks belong to him.
-Psalm 95: 3-4
Contact USGS by e-mail and request teachers’ packets indicating the grade level and subject of interest. Sample items to ask for: Exploring Maps, Global Change, Map Adventures, What Do Maps Show?, Natural Hazards Poster Set, CdRom (free) called Topographic Field Trip of Washington, D.C.,
Teacher’s Packet of Geologic Materials, Selected Packet of Geologic Teaching Aids, Volcano Information for Classroom Use, U.S. Geological Survey Sales Publications That Are Especially Useful to Teachers.