GEO 101 Lecture 9: Igneous Rocks and Plate Tectonics
23 Jun 2018
School
Department
Course
Professor
Igneous Rocks and Plate Tectonics
Igneous rocks form from magmas, and most magmas are associated with plate
tectonics. Mafic (basaltic) and ultramafic magmas form along the divergent midoceanic
ridges and are major components of new oceanic crust. More felsic magmas, such as
andesites and rhyolites, are associated with the edges of continental crust at subduction
zones along converging plate boundaries. Whether a magma is intermediate or felsic
may depend on the relative amounts of oceanic crust and continental crust in the
subduction zone that melt to form the magma. The great abundance of granitic
intrusions in continental crust is thought to be related to the partial melting of the lower
continental crust.
Intraplateigneous activity occurs in the interior of a single continental plate and is
thought to be related to mantle plumes (such as the eruptions at Yellowstone National
Park) or flood basalts. Intraplate activity is not associated with moving plate boundaries
such as subduction zones.
Magmatic Differentiation
The defining characteristic of igneous rocks is that at one time they were molten and
part of magmas or lavas. A magma is a body of molten rock that occurs below the
surface of the earth. When magma rises along a deep fault and pours out on the earth's
surface, it is termed lava. This material then cooled to form a variety of intrusive and
extrusive igneous rocks. Extrusive rocks crystallized from liquid magmas that reached
the surface and were generally vented as volcanic lavas. Intrusive rocks crystallized
from magmas that did not reach the surface but moved upward into cracks and voids
deep in the crust.
When a magma cools, chemical reactions occur that create a series of different
minerals. This process of differentiation occurs along two branches: discontinuous and
continuous.
The discontinuous branch. The minerals that form in the discontinuous branch are
all ferromagnesian—that is, they contain high percentages of iron and magnesium,
which impart a dark green to black color. The branch is called discontinuous because
the minerals form at discrete temperatures and not continuously during cooling. The first
mineral to crystallize is olivine, followed by pyroxene, amphibole, and biotite.
The continuous branch. The continuous branch is made up of the plagioclase feldspars.
The calcium/sodium ratio in this mineral type changes continuously as the magma
cools. The first feldspars to form contain the highest amounts of calcium; subsequent
feldspars have progressively less calcium and more sodium. These minerals tend to be
pink, tan, brown, or whitish.
Any magma left over after all these reactions have been completed crystallizes at the
lowest temperature as quartz.
These theories were first proven in the laboratory by N. L. Bowen in the early 1900s and
are also known as Bowen's reaction series. The progression in the series explains why
the first lavas from a volcanic vent are rich in iron, magnesium, and calcium, are low in
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Igneous rocks form from magmas, and most magmas are associated with plate tectonics. Mafic (basaltic) and ultramafic magmas form along the divergent midoceanic ridges and are major components of new oceanic crust. More felsic magmas, such as andesites and rhyolites, are associated with the edges of continental crust at subduction zones along converging plate boundaries. Whether a magma is intermediate or felsic may depend on the relative amounts of oceanic crust and continental crust in the subduction zone that melt to form the magma. The great abundance of granitic intrusions in continental crust is thought to be related to the partial melting of the lower continental crust. Intraplateigneous activity occurs in the interior of a single continental plate and is thought to be related to mantle plumes (such as the eruptions at yellowstone national. Intraplate activity is not associated with moving plate boundaries such as subduction zones.
