Laccolith in Geology
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.
FAQs on Laccolith
1. What is a laccolith in geography?
A laccolith is a type of intrusive igneous rock formation. It is created when viscous magma pushes its way between layers of pre-existing sedimentary rock and collects in a lens-shaped or mushroom-like mass. This intrusion causes the overlying rock strata to bulge upwards, forming a distinct dome-shaped landform on the surface. Unlike magma that erupts onto the surface (lava), the magma forming a laccolith cools and solidifies underground.
2. What are the main characteristics of a laccolith?
The key characteristics of a laccolith are determined by its formation process. The most important features include:
Shape: It has a flat or nearly flat floor and a convex, dome-shaped roof.
Formation: It is a concordant intrusion, meaning it forms parallel to the existing rock layers without cutting across them.
Composition: They are typically formed from highly viscous, silica-rich magma (like diorite) which doesn't flow easily, hence its tendency to bulge upwards rather than spread out.
Surface Expression: Over time, as the softer overlying rock erodes, the hard igneous rock of the laccolith can be exposed as a small hill or mountain.
3. How is a laccolith formed and what determines its classic dome shape?
A laccolith forms when a body of magma, typically rising from a deeper source, intrudes between horizontal layers of sedimentary rock. Its distinctive dome shape is primarily determined by the viscosity of the magma. If the magma is very thick and pasty (high viscosity), it cannot spread far horizontally. Instead, the pressure from the rising magma forces the overlying rock layers to bend and arch upwards, creating the characteristic dome or mushroom shape as the magma pools and solidifies.
4. How do laccoliths differ from other intrusive landforms like sills, dikes, and batholiths?
While all are intrusive igneous landforms, they differ mainly in their shape, size, and relationship to the surrounding rock layers:
Sills vs. Laccoliths: Both are concordant intrusions (parallel to rock layers). However, a sill is a relatively thin, flat sheet of magma, while a laccolith is a much thicker, dome-shaped intrusion that has actively pushed the overlying strata upwards.
Dikes vs. Laccoliths: A dike is a discordant intrusion, meaning it cuts vertically or steeply across existing rock layers. A laccolith, being concordant, is injected between them.
Batholiths vs. Laccoliths: A batholith is an enormous, deep-seated mass of igneous rock with an irregular shape and no known floor, often forming the core of major mountain ranges. A laccolith is significantly smaller, has a defined floor, and is associated with the doming of surface rock layers.
5. How does the formation of a laccolith impact the Earth's surface?
The formation of a laccolith directly impacts the Earth's surface by creating a distinct uplift. As the viscous magma pushes upward, it forces the overlying sedimentary rock layers to bulge, forming a dome or a rounded hill on the landscape. This process is a clear example of how activity beneath the crust can shape surface topography. Over millions of years, erosion may wear away the softer, overlying rock layers, eventually exposing the harder, more resistant igneous rock of the laccolith itself as a prominent geological feature.
6. Are laccoliths a type of volcano?
No, a laccolith is not a type of volcano. It is a form of intrusive volcanism, where magma cools and solidifies beneath the Earth's surface. A volcano, on the other hand, is associated with extrusive volcanism, where magma (called lava once it reaches the surface) erupts out onto the Earth's crust. While both are related to magmatic activity, the key difference is that the magma in a laccolith never breaks through to the surface.
7. Can you provide an example of a famous laccolith?
A classic example of laccoliths can be found in the Henry Mountains of Utah, USA. These mountains are where the geologist G.K. Gilbert first studied and named this type of landform in the 1870s. The Pine Valley Mountains, also in Utah, are formed by what is considered one of the largest laccoliths in the world. These formations stand out as prominent domes and hills in the landscape due to the erosion of the surrounding, less resistant rock.
