Igneous Intrusions
(revised 2/11/04)
Figures
Types of Magma (Lava)
- Felsic (Rhyolitic; Granitic magma)
- Intermediate (Andesitic)
- Mafic (Basaltic)
Igneous Intrusions
Discordant Plutons
Batholiths - largest intrusions; huge(>100 km2)
- Associated with convergent plate boundaries (subduction
zones)
- Found in cores of mountains
- Feed magma to form other intrusive bodies
- Examples
- Sierra Nevada Batholith
- Black Hills, SD
- Stone Mountain, GA
Stock - like batholith but smaller (<100 km
2)
Dike - tabular; < cm to > km thick; often occur
in swarms
- Dikes Swarms at Mid-Ocean Ridges (Divergent Plate
Boundaries)
- Dikes swarms (sheeted dikes) at mid-ocean ridges
feed undersea volcanism; form ocean crust
Volcanic pipe - volcanic vent, circular cross-section
- Can be deep -- conduit to mantle (e.g.,
kimberlite pipes of South Africa)
Volcanic neck - exposed volcanic vent
Concordant plutons - parallel to existing
structure
- sill - tabular
- laccolith - mushroom shaped; more viscous magma
(usually granitic); generally less than a few km wide
Features of Plutons
- Xenoliths - blocks of country rock that fall
into pluton from roof (sometimes provide the only remnant of host
rock)
- Columnar basalt - fractures associated with
shrinking (also occur in lava flows)
How do you distinguish a buried lava flow from a sill in the field?
- _________________________________________________
- _________________________________________________
- _________________________________________________
