Congealed magma, along with fragmental volcanic and wallrock materials,
can be preserved in the feeding conduits of a volcano upon cessation of
activity. These preserved rocks form crudely cylindrical masses, from
which project radiating dikes; they may be visualized as the fossil
remains of the innards of a volcano (the so-called “volcanic plumbing
system”) and are referred to as volcanic _plugs_ or _necks_. The igneous
material in a plug may have a range of composition similar to that of
associated lavas or ash, but may also include fragments and blocks of
denser, coarser grained rocks—higher in iron and magnesium, lower in
silicon—thought to be samples of the Earth’s deep crust or upper mantle
plucked and transported by the ascending magma. Many plugs and necks are
largely or wholly composed of fragmental volcanic material and of
fragments of wallrock, which can be of any type. Plugs that bear a
particularly strong imprint of explosive eruption of highly gas-charged
magma are called _diatremes_ or _tuff-breccia_.
Volcanic plugs are believed to overlie a body of magma which could be
either still largely liquid or completely solid depending on the state
of activity of the volcano. Plugs are known, or postulated, to be
commonly funnel shaped and to taper downward into bodies increasingly
elliptical in plan or elongated to dike-like forms. Typically, volcanic
plugs and necks tend to be more resistant to erosion than their
enclosing rock formations. Thus, after the volcano becomes inactive and
deeply eroded, the exhumed plug may stand up in bold relief as an
irregular, columnar structure. One of the best known and most
spectacular diatremes in the United States is Ship Rock in New Mexico,
which towers some 1,700 feet above the more deeply eroded surrounding
plain. Volcanic plugs, including diatremes, are found elsewhere in the
western United States and also in Germany, South Africa, Tanzania, and
Siberia.
[Illustration: Ship Rock, San Juan County, New Mexico.]