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seismograph move with it, but
the pendulum remains still until long after the earthquake is over. As the instrument frame moves around, the relative movement between frame and pendulum can be recorded on a paper drum or by electronic equipment. The record produced is called a seismogram. The first pendulum seismograph was built in 1751.
Each seismograph recording station has three seismographs,
one detecting the north-south movement, another detecting east- west movement, and the third detecting up-and-down motions. From the traces it is possible to find the distance, direction, Richter scale magnitude, and type of fault of the earthquake.
By using a network of seismographs, it is possible to find the precise location of the origin of the earthquake.
(See also: First motion.)
Shadow zone
The region of the Earth that experiences no shocks after an earthquake. It lies directly opposite the focus of the earthquake on the far side of the Earth. (See also: Refraction.)
Shallow earthquakes
Earthquakes with a focus fewer than 70km below the surface. (See also: Crust.)
Shield volcano
A broad, gently sloping volcano, usually built up by many fluid lava flows of basalt. Mauna Loa, Hawaii, is an example of a shield volcano.
Shock wave
A common term for the effect of earthquake waves. (See also: Seismic waves.)
Silica, silicate
The mineral silicon dioxide. Silica is a very common mineral, occurring frequently as glassy quartz in igneous rocks. A silicate is any mineral that contains silica. (See also: Acid lava/acid rock; Augite; Feldspar; Hornblende; Mica; Olivine.)
Sill
A sheet of intrusive igneous rock (typically diorite) that has been injected between layers of rock.
The pressure in a magma chamber is fearsome and capable of causing the surrounding rock layers to crack apart. If the magma finds a weakness between two layers (beds) of rock, it may push the uppermost layer aside and inject molten material in between the two rock layers. The magma will then bake the rocks on either side, often making them very tough. Then the magma cools into a hard rock. This layer of rock is called a sill. Notice that when it is formed, a sill has a “roof” and a “floor.”
Sill rocks, like dike rocks, are often tough and stand up to erosion better than other rocks. So, as
the landscape eventually gets worn down, the sills and dikes become
exposed and stand up in the landscape as natural “ridges.”
You can tell a sill from a
dike because a dike cuts across other rocks, while a sill pries rock layers apart and lies parallel to neighboring rock layers.
You cannot tell a dike from a sill simply by whether it makes a wall or a ledge. That is because the rocks may have been turned on their sides after they were formed.
Sills often occur in the same place as dikes, and they also
make prominent ridges in the landscape. The most famous sill (called the Great Whin Sill) was followed by the Romans when they built Hadrian’s wall across northern England some 2,000 years ago.
 Sill—Sills are sheets of igneous rocks that have forced their way in between the surrounding rocks. If they are tougher than the rocks on either side, they will stand out as a sloping ridge or even a ledge horizontal to a waterfall. The Great Whin Sill in England was used by Roman Emperor Hadrian as the foundation for his famous wall that divided England from Scotland.
Sill
Sill
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