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Examples of this include India, which is pushing against Asia to form the Himalayan Mountains, and Africa, which is pushing against Europe to form the Alps. Earthquakes and volcanoes are common in such places, but volcanoes are made of sticky, acid magma and are more explosive and violent than volcanoes at spreading boundaries.
In some places plates do not spread apart or collide, but simply scrape past one another. This
is happening along the coast of California. This does not create volcanoes, but instead causes many earthquakes. The earthquakes in California mainly occur along the San Andreas Fault.
(See also: Benioff zone; Colliding boundary; Creep; Destructive plate boundary; Oceanic spreading ridge; Ocean trench; Pacific Ring of Fire; Plate tectonics; Rift, rift valley; Rifting; Seismic zone; Subduction; Subduction zone.)
Plate tectonics
The theory that the Earth’s crust and upper mantle (the lithosphere) are broken into a number of
more or less rigid, but constantly moving, slabs or plates.
Plinian-type eruption
An explosive eruption that typically sends a column of ash high into the air. The ash may reach as high as 30km above the volcano.
Plug
The solidified core of an extinct volcano. Sometimes the phrase volcanic neck is used instead of volcanic plug.
All volcanoes erupt through the pipes, or vents, that connect them to the surface. Toward the end of the eruption the magma chamber that supplies the liquid rock loses pressure and is no longer able to
 Plug—Ship Rock, New Mexico, is a plug with radiating dikes.
push liquid rock out of the vent. As a result, the magma and any broken remains of the vent walls that were torn free during the eruption stay in the vent, slowly cooling until they become solid rock.
A volcanic cone is mainly composed of layers of ash and lava. Ash, in particular, is easily worn away. By contrast, the material that plugs the vent of the volcano is often very tough and better able to stand up to the action of rivers and the weather (erosion). The plug of material is therefore often the last part of a volcano to remain.
Volcanic plugs can make striking pillars of rock in the landscape. Some of them have flat tops. Some plugs have been used as fortresses and others as places for religious buildings such as churches.
The most famous church sited on a plug is at Le Puy in the Massif Central Mountains of France. The best-preserved plug is probably Ship Rock, New Mexico.
 Plug—A volcanic plug at Le Puy, France. A fortification used this high point to command the area in the Middle Ages.
Ship Rock plug
Dike
Dike
Plug
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