Geomorphology 33 J Bret Bennington

Igneous Intrusive Landforms

Updated 3-01

Igneous magma injected into the surrounding country rock rarely has any intitial effect on topography. However, as erosion etches the landscape the hard, crystalline igneous rock commonly stands in relief to form prominent topographic features.

Dicordant Plutons

These intrusions cut across the bedding or foliation of the host rock.

Large Scale Plutons:

Batholith - an irregular mass of igneous rock exposed over an area of greater than 100 km². Batholiths are usually granitic in overall composition and represent significant magma bodies that crystallized deep in the crust, later to be exposed by erosion and isostatic uplift.

Examples: Sierra Nevada Mts., California, Enchanted Rock Batholith, Texas, Mt. Desert Island, Maine.

Stock - similar to a batholith but smaller in scale of exposure (less than 100 km²).

Examples: Stone Mountain, Georgia

Stocks and batholiths commonly weather to form exfoliation domes - low to moderate relief, mound-like masses of nearly barren rock. Soil formation and plant growth is inhibited by the tendency for slabs of weathering granite to sheet and spall away, leaving a relatively barren rock surface.

Smaller Scale Plutons

Volcanic neck / throat - cylindrical to spire-shaped landforms composed of magma that crystallized in the interior of an extinct volcano and was later exposed by erosion.

Examples: Shiprock, New Mexico, Devil's Tower, Wyoming

Dikes - tabular plutons that are expressed as relatively vertical sheets of erosion-resistant rock. They form as magma is injected into faults, fractures and fissures in the country rock. Dikes often occur in a radial pattern surrounding a volcanic neck (feeder dikes), or in large numbers that may cross cut one-another (dike swarm).

Examples: Shiprock, New Mexico

Concordant Plutons

These intrusions parallel the bedding or foliation of the host (country) rock.

Sills - tabular plutons that follow the bedding of the surrounding sedimentary rock. Because of the ability to resist erosion of the crystalline igneous rock, sills commonly form mesas, cuestas, and hogbacks, depending on the dip of the sill and host rock.

Examples: Palisades Cliffs, New York, Salisbury Crags, Edinburgh

Laccolith - similar to sills, but mushroom-shaped and causing updoming of the overlying sedimentary layers. On erosion, laccoliths tend to form relatively steep-sided mountains with cliffs produced by the columnar jointing of the eroding igneous rock.

Examples: Bear Butte, South Dakota, Henry Mts., Utah