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Spatter
Spatter
Splashes of liquid lava produced close to a vent during an eruption. They often make a spatter cone.
Spreading boundary
A line on which two plates are moving apart. Basaltic magma wells up in the fissure created by the parting plates. Most spreading boundaries are on ocean floors. They are the most common places for fissure eruptions. (See also: Oceanic spreading ridge; Rift, rift valley.)
Stock
A vertical protrusion of a batholith that reaches close to the surface. Similar to boss.
Stratovolcano
A steep-sided “classic” volcano, usually formed into a cone and made of both lava and ash. The same as composite volcano and central vent volcano.
 Spatter—The material that splashes from a vent.
Strombolian-type eruption
A type of volcanic eruption in which there are periodic bursts
of fluid lava, usually basalt,
from a crater. The volcanic eruption is explosive enough to send out volcanic bombs. The Italian volcano Stromboli, after which this eruption type is named, sends out fiery bombs and is known as “the lighthouse of the Mediterranean Sea.”
Strong motion
Earthquake motion big enough to cause damage to buildings.
Subduction
The process of one tectonic plate descending beneath another.
Subduction zone
The part of the Earth’s surface along which two tectonic plates collide, and one tectonic plate descends into the mantle. A subduction zone is often marked by a deep, narrow trench called an ocean trench. As one plate
is pushed back into the mantle, crust melts and begins to rise
to the surface, forming volcanoes. The volcanoes often form a pattern in the shape of an arc. Most volcanoes occur parallel to and inland from the boundary between the two colliding plates. Some subduction zones create wide areas
 Subduction zone—The place on the Earth’s surface where one crustal plate is forced below another. Subduction zones are places where mountains form, earthquakes are common, and explosive volcanoes occur. They are geologically the world’s most spectacularly active places. The Pacific Ring of Fire is the most active of all.
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