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Island arc
 Island arc—Where ocean and continental plates collide, volcanoes form in long arcs. The Aleutian Islands form one such arc.
Island arc
A crescent pattern of volcanic islands. Island arcs occur where an ocean plate is pushing below
a continental plate. The best examples surround the Pacific Ocean (see: Pacific Ring of Fire).
(See also: Arc; Ocean trench; Subduction zone.)
Isoseismal
A line on a map showing places of equal earthquake intensity. They can be thought of as “contours” of earthquake intensity.
L
Laccolith
A lens-shaped body of intrusive igneous rock with a dome-shaped upper surface and a flat bottom surface. It is a relatively uncommon feature.
Lahar
A flow of volcanic igneous rock fragments and water down the sides of a volcano. It can cause great destruction. Lahars are caused by melting ice on top of volcanic mountains mixing with loose debris on the sides. Eruptions will often melt an ice cap quickly, releasing huge amounts of water.
Lapilli
Rock fragments between 2mm
and 64mm in diameter that are ejected from a volcano during an explosive eruption. Lapilli means “little stones” in Italian. It is a form of tephra.
Alaska
Kamchatka Peninsula
Aleutian Islands
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Lava
Molten rock that reaches the Earth’s surface. It can be runny or sticky depending on the minerals it contains.
Lava is the liquid part of the material called magma that comes from deep within the Earth. It normally flows from a volcano at temperatures of about 1,000°C.
There are many kinds of lava, but it is easiest to think of lava
as belonging to two main groups: Acid lava and basic lava. Both types cool to form extrusive rock.
Acid lava is sticky and viscous. It flows from central vent volcanoes in the form of slowly moving tongues. Acid lava moves only a few meters a day. It only adds small amounts of material to the cones of volcanoes. Much of
the sticky lava in volcanoes cools to a brown rock called andesite, named for the Andes Mountains, where it is common.
Basic lava, which is runny, flows quickly from gently sloping volcanoes or fissures and often appears as rivers or sheets.
Basic lava can reach speeds of many kilometers an hour. The most common form of basic lava is basalt. It is also by far the most common form of lava in the world. Basalt makes huge sheets of rock both on the ocean floor and on land.
Basalt lava sheets can have a smooth surface, in which case it is called pahoehoe lava. Alternatively,
 Lava—As lava emerges from a volcano, it is so hot that its color is yellow or red. As it flows away from the vent, it cools, solidifies and turns black.