Vulcanian Type

The term Vulcanian is from the island of Vulcano, also in the Aeolian group. The lava from Vulcano is more pasty and viscous than Strombolian lava, and it forms a thick, solid crust over the crater between the infrequent eruptions. Gases accumulate beneath the congealed crust, and in time the upper part of the magma column becomes thoroughly gas-saturated. Finally, with strong explosions, the obstructions are blown out and the broken fragments of the crater plug, together with some new lava in the form of “breadcrust bombs” and scoria, are ejected. When the plug is suddenly blown out, the gas-saturated magma is disrupted into pumice and ash by the explosively expanding gases. After the obstruction is cleared, lava flows may issue either from the crater or from fissures on the sides of the cone.

Pelean Type

Pelean is derived from Mont Pelee, on the island of Martinique, which erupted in 1902, destroying the city of St. Pierre with the loss of more than 30,000 lives. The Pelean type produces magma of the highest viscosity and is characterized by extreme explosiveness. The distinguishing feature of the Pelean eruption is the nue ardente, or “glowing cloud”. It is a highly heated gas, yet is dense enough to maintain contact with the surface as it rushes down the slopes of the mountain with hurricane force. It was such a cloud that overwhelmed St. Pierre. In Pelean eruptions, the upward escape is frequently blocked by a plug of lava in the crater, and the explosions break out as horizontal blasts from beneath the plug. The magma is expelled as a highly ash-charged gas and no lava issues except that pushed up as a viscous plug in the crater.

Fred M. Bullard

Adapted from Volcanoes of the Earth

Drafting

1). Choose one of the following subjects, or, with your teacher’s approval, another subject for an expository essay:

• classes in school

• occupations to consider

• inventions

• medical science

2). Make a list of the ideas and details you would like to include in your essay. Read over your list to see what it suggests with regard to natural groupings of information. Let the material guide you as you begin to select a principle of choice.

3). List three possible bases for classifying the material in your essay. Be sure each basis will be broad enough to cover all your material.

4). Review your list of bases-and select the most promising principle of choice. Put a check next to that basis.

5). Make an outline using the groups suggested by the basis for classification as main headings. Fill in details from your notes as the subheadings.

Composing

Write an essay of classification based on your outline. Work through your outline one section at a time, writing about the material in each group. Use as many paragraphs, as are needed to effectively write about your subject. Be sure to include transitions between the para­graphs to link the ideas and to make their relationship clear.

Revising

Ask yourself the following questions:

• How is each grouping of material introduced and developed?

• In what ways is the development of each group similar to that of other groups?

• What additional details, if any, are needed to fully explain each main thought or idea?

• What transitions, if any, would help link groups of material?

Make any changes that are needed to improve your essay.