Laccolith

In Geology, laccolith is a sheet-like intrusion that has been inserted between the two layers of sedimentary rocks. Due to the intense pressure of the magma, the overlying strata are forced upward and folded, giving the laccolith a dome or mushroom-like form (or feasibly conically or wedge shape) with a substantially planar base. As time goes on, erosion can form small mountains or hills around a central peak as magma rock is likely more susceptible to weathering than the host rock. 

The laccolith growth can take as little or a few months when related to the injection of a single magma event or up to a hundred or thousands of years by multiple magnetic pulses, assembling sills on top of each other and impairing the host rock steadily. 

Laccolith Definition

A laccolith is defined as the body of igneous rock which has forced itself by the intrusion, in molten conditions between strata of sedimentary rock in such a way as to have raised the overlying strata in a dome shape arc above it. 

Laccolith Formation

Laccolith manages to form at relatively shallow depth and in few cases are formed by relatively viscous magma, such as those that crystallize to granite, diorite, and granodiorite. In such cases, underground may take steadily, giving time for larger crystals to form in cooling magma.  In other cases, less sticky magma (For example, shonkinite) may form phenocrysts of augite at the bottom, then inserted through a vertical feeder dike that ends in laccolith. The rock surface above a laccolith destroys completely, leaving the core mound of igneous rock.

The laccolith is the attribute of the region where the crust is being flattened and the direction of the least stress is vertical, whereas the region where the crust is in pressure is more likely to form dikes as the direction of the least stress is parallel. For example, in Mexico, the laccoliths of the Ortiz porphyry belt was formed during Laramide compression of the region 33 to 36 million years ago.

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Laccolith Examples

A renowned example of laccolith is found in Henry Mountain, Utah. 

The largest laccolith in the United States is Pine Valley Mountain in the Pine Valley Mountain Wilderness area near St. George Utah.

Batholith and Laccolith

Batholith (also known as a plutonic rock) is a large mass of igneous rock. It is larger than 100 square kilometres (40 sq mi) in area. The batholith forms from cooled magma deep in the Earth's crust. The rock is mainly made up of felsic or intermediate rock types such as granite, quartz monzonite, or diorite.

Laccolith

A laccolith is a rock that appears as a sheet-like intrusion and is intruded between the layers of sedimentary rock. The intrusion takes place when the pressure of magma is high enough to move the strata of sedimentary rock in an upward position or to make them fold. This pressure gives the laccolith a dome or mushroom-like appearance Generally, the base of the laccolith is planar.    

The following table shows the difference between Batholith and Laccolith

Batholith and Laccolith Differences

Did You Know?

• Laccoliths are generally formed at a relatively shallow depth and in few cases are formed by relatively viscous magma such as those crystallized to granite, diorite, and granodiorite.

• The surface rock above laccolith often completely erodes, leaving the core mound of igneous rock.

• The laccolith of the Ortiz Porphyry Belt in New Mexico has probably formed during Laramide compression of the region 33 to 36 million years ago.

• Henry Mountain in Utah is a classic example of a laccolith.

• The geology of the Henry Mountain, Utah was first studied by Grove Karl Gilbert in 1875-1876.