GEO 101 Lecture Notes - Lecture 8: Partial Melting, Geothermal Gradient, Intrusive Rock
23 Jun 2018
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Intrusive Structures
Intrusions are also classified according to size, shape, depth of formation, and
geometrical relationship to the country rock. Intrusions that formed at depths of less
than 2 kilometers are considered to be shallow intrusions, which tend to be smaller and
finer grained than deeper intrusions.
Dikes. A dike is an intrusive rock that generally occupies a discordant, or cross‐
cutting, crack or fracture that crosses the trend of layering in the country rock. Dikes
are called pegmatites when they contain very coarse grained crystals—a single such ‐
crystal can range in size from a few centimeters to 10 meters in diameter.
Sills. Sills are formed from magmas that entered the country rock parallel to the
bedding (layering) and are thus concordant with the country rock. Sills can sometimes
look like volcanic flows that were interbedded with sedimentary units.
Laccoliths. A laccolith resembles a sill but formed between sedimentary layers from a
more viscous magma that created a lensshaped mass that arched the overlying strata
upward.
Volcanic necks. A volcanic neck is the rock that formed in the vent of a volcano at the
end of its eruptive life and remains “standing” after the flanks of the volcano have
eroded away.
Plutons. Plutons are discordant intrusive rocks that formed at great depths. They tend
to be large, coarse grained, and irregular in shape. If the intrusion occupies less than
100 square kilometers (60 square miles) at the earth's surface it is called a stock; if it is
larger than 100 square kilometers, it is termed a batholith. Batholiths are usually
composed of granite. They have formed over long periods through the accumulation of
smaller magma blobs called diapirs, which result from localized melting of the crust; the
diapirs then slowly move upward toward the surface and coalesce into a larger mass.
Granitic batholiths usually form the cores of mountain complexes and are a result of
plate tectonic action.
How Different Magmas Form
Both extrusive and intrusive igneous rocks are derived from magmas. The temperature
and pressure conditions in the crust and upper mantle influence the melting
temperatures of the minerals in the rocks.
Temperature and pressure increase with depth from the surface and eventually reach a
point at which rocks melt. The geothermal gradient is the rate at which temperature
increases with depth. In the upper crust, the geothermal gradient is about 2.5 degrees
centigrade for every 100 meters (330 feet). Geothermal gradients are higher in volcanic
regions. Mantle plumes are “hot spots” in the crust where mantle material has ascended
along deep penetrating cracks in the crust and contributes heat for higher level melting. ‐
Country rock can also be melted from the heat of adjacent intrusions.
Friction is a source of heat in areas where large rock masses are grinding against one
another—for example, during mountain building and plate tectonic activity. Heat is also ‐
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