Page 158 - Laboratory Manual in Physical Geology

DIVERGENT
PLATE BOUNDARY
Andesitic Linear eruption of Basaltic Basaltic
Andesitic lava flow basaltic magma submarine volcanic
volcanoes Trench at mid-ocean volcano island
ridge

Metamorphic
Rhyolitic Magma rocks
volcano chamber

Fault
Granite Basalt crust Basalt crust
Mafic magma
Mafic magma assimilates Lithosphere produced by HOT
crust, cools, and evolves partial melting of SPOT
into intermediate or mantle peridotite
felsic magma
Mafic magma Ocean plate subduction
produced by
partial melting Asthenosphere
of mantle (mantle peridotite)
peridotite
CONVERGENT
PLATE BOUNDARY

FIGURE 5.7 Tectonic settings where igneous rocks form. Different types of igneous rocks are formed in different geologic setiings:
a hot spot (such as the Hawaiian Islands), divergent plate boundary (mid-ocean ridge), convergent plate boundary (subduction zone), and
Earth’s mantle. See text for discussion.
a magma may melt some of the wall rocks surrounding magma cools along the mid-ocean ridges and ocean hot
it and assimilate its elements. This is called assimilation. spots (e.g., Hawaiian Islands), it forms gabbro ( FIGURE 5.9 )
Magma mixing may also occur. Bowen’s continuous series and seafloor basalt ( FIGURE 5.10 ).

of crystallization, fractional crystallization, assimilation,
and magma mixing are all factors that can contribute
to magmatic differentiation (any process that causes
Hand sample
magma composition to change). Magmatic differentiation (actual size)
produces more than one rock type from a single body of
magma.

Plate Tectonics and Igneous Rocks
The four compositional groups of igneous rocks occur in
specific tectonic settings ( FIGURE 5.7 ).

Ultramafic Rocks Occur in the Mantle
Ultramafic igneous rocks, like peridotite, are associ-
ated with Earth’s mantle. They are denser than rocks
of the crust, so they are not normally found at Earth’s
surface. Billions of years ago, when the body of Earth
was much hotter and the crust was thinner, ultramafic
magmas occasionally erupted to the surface. However,
no such eruptions have occurred for more than 60
million years. Xenoliths of peridotite found in some
volcanic rocks are thought to have originated in the
mantle ( FIGURE 5.8 ). 10 close-up of peridotite
FIGURE 5.8 Peridotite (ultramafic, intrusive,
Mafic Rock at Divergent Plate Boundaries phaneritic). Peridotite—an intrusive, phaneritic igneous
and Ocean Hot Spots rock having a very high MCI (>85%) and mostly made of
Partial melting of mantle peridotite beneath ocean hot ferromagnesian mineral crystals. This sample is a peridotite
spots and mid-ocean ridges produces mafic magma rather xenolith extracted from a basaltic volcanic rock. It is made mostly
of olivine mineral crystals.
than ultramafic magma ( FIGURE 5.7 ). When the mafic

Igneous Rocks and Processes ■ 139