Other Volcanic Landforms

The most obvious volcanic structure is a cone, but other important landforms are also associated with volcanic activity.

Calderas (caldaria = a cooking pot) are large depressions with diameters that exceed 1 km and have a somewhat circular form. (Those less than 1 km across are called collapse pits or craters.) Most calderas are formed by one of the following processes: (1) the collapse of the summit of a large composite volcano following an explosive eruption of silica-rich pumice and ash fragments (Crater Laketype calderas); (2) the collapse of the top of a shield volcano caused by subterranean drainage from a central magma chamber (Hawaiian-type calderas); and (3) the collapse of a large area, caused by the discharge of colossal volumes of silica-rich pumice and ash along ring fractures (Yellowstone-type calderas).

The greatest volume of volcanic material is extruded from fractures in the crust called fissures (fissura = to split). These long, narrow cracks tend to emit low-viscosity basaltic lavas that blanket a wide area.

In contrast to mafic lavas, silica-rich felsic lavas are so viscous they hardly flow at all. As the thick lava is “squeezed” out of the vent, it often produces a dome-shaped mass called a lava dome. Most lava domes are only a few tens of meters high, but some are more than 1 km high. Lava domes come in a variety of shapes that range from pancake-like flows to steep-sided plugs that were pushed upward like pistons. Most develop over a period of several years following an explosive eruption of gas-rich magma. A recent example is the dome that continues to grow in the crater of Mount St. Helens. A second dome building event began in October 2004. Although lava domes often form on the summit of a composite cone they can also develop on the flanks of volcanoes. In addition, some domes occur as isolated features, whereas others form linear chains.

Most volcanoes are fed magma through short conduits, called pipes that connect a magma chamber to the surface. One rare type of pipe, called a diatreme, extends to depths that exceed 200 km. Magmas that migrate upward through diatremes travel rapidly enough that they undergo very little alteration during their ascent. Geologists consider these unusually deep pipes to be “windows” into Earth that allow us to view rock normally found only at great depths.

Volcanoes on land are continually being lowered by weathering and erosion. Cinder cones are easily eroded because they are composed of unconsolidated materials. However, all volcanoes will eventually succumb to erosion. As erosion progresses, the rock occupying a volcanic pipe is often more resistant and may remain standing above the surrounding terrain long after most of the cone has vanished. This structure is called a volcanic neck.

Task 3. Try to define the most important terms dealing with volcanic structures (in English) and translate them into Ukrainian. Be sure to include the following ones:

• caldera;

• collapse pit;

• lava dome;

• diatreme;

• volcanic pipe;

• volcanic neck.

Task 4. Answer the following questions, using the vocabulary from Task 1.

1. What do we call large depressions with diameters that exceed 1 km and have a somewhat circular form?

2. What is the name of depressions with diameters less than 1 km across?

3. What is the name of the greatest volume of volcanic material which is extruded from fractures in the crust?

4. What is the name of a dome-shaped mass?

5. What shape can lava domes have?

6. Where do lava domes often form?

7. What connects a magma chamber to the surface?

8. Why do geologists sometimes consider deep pipes to be “windows” into Earth?

9. Are volcanoes on land continually being lowered by weathering and erosion?

10. What structure is called a volcanic neck?

Task 5. Look at Figures 8.1 A, B and C. Define the landforms associated with volcanic activity: A, B and C.

Task 6. Find English equivalents for the following (see the text). Try to build up your own sentences with them.

Найбільш очевидна вулканічна структура; асоціюватись з вулканічною діяльністю; перевищувати 1км; кільцеподібна (кругла) форма; обвал вершини великого композиційного вулкана; вибухове виверження; спричинений підземним дренажем; виштовхуватись з тріщин земної кори; лінійні гірські хребти; рідкий тип трубкоподібної порожнини (вулкану); піддаватись незначній або малій зміні; знайти на великих глибинах; не витримувати ерозії; збільшення ерозії.

Task 7. Give a short summary of the text from Task 2.