What is a Lopolith?
A lopolith is a huge igneous intrusion that is saucer-shaped with depressed central regions that lie parallel to the strata of intruded country rock. Lopoliths are usually the concordant emplacements having an intruded stratum with a dike or funnel-shaped feeder bodies beneath the body.
Lopoliths are comparatively small plutons which customarily develop an upper surface that is concave downward. This dangling shape may be a cause of volume reduction when magmas crystallize. The weight of the blanketing strata would cause disintegration into the volume previously acquired by more voluminous liquid magma.
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Intrusive Igneous Rocks
Erosion of volcanoes will instantly reveal shallow intrusive bodies such as volcanic necks and diatremes. A volcanic neck is said to be the “throat” of a volcano and contains a pipelike conduit immersed with hypabyssal rocks. Devil’s Tower in Wyoming and Ship Rock in New Mexico are remnants of volcanic necks that were revealed after the surrounding sedimentary rocks get weathered away. Many craterlike depressions can be filled with angular pieces/particles of country rock and juvenile pyroclastic remainder.
Upon erosion, such a depression reveals a vertical funnel-shaped pipe that depicts a volcanic neck having an exception of the brecciated filling. These pipes are known as the dubbed diatremes.
Pluton
Pluton is a body of intrusive igneous rock the size. They include laccoliths, batholiths, dikes, sills, and other kinds of intrusions. Most plutons are thought to be a consequence of igneous activity which involves magma.
Formation of Lopolith
Lopolith, lenticular in shape, is igneous intrusion with a depressed central region. This mass of igneous rock developed as an attribute to magma do not find its way to the surface but spread laterally into a lenticular body forcing overlying strata to bulge upwards. They are usually lenticular in shape. They are made up of dense, mafic magma that allows depression by the overlying strata on cooling.
Bushveld Lopolith
Bushveld complex of South Africa is the world's largest and most important repository of Platinum Group Metals.
Bushveld Igneous Rock Geographics
It extends laterally over 65,000 square kilometres and is up to eight kilometres thick more than 2 billion years ago.
Formation of Bushveld Lopolith Complex
The bushveld complex was formed by volcanic eruptions that repeatedly injected molten basaltic rock known as magma or lava into some volcanic chamber inside the Earth the cooling and solidification of the molten magma resulted in the formation of igneous rock because the molten rock was beneath the surface of the Earth it cooled much more slowly than usual by slow cooling different minerals crystallized at different temperatures and accumulated as layers from the base upwards with time the weight of the cooled magma and the forces of the Earth cause the layers to Sag down in the middle giving them the shape of a pile of very thin sources time and weather combined eroding the landscape moulding the surface of the Earth the central part of the sorcerer was buried under younger rocks by the edges cropped out as two semicircular arcs of rocks.
Fun Facts
Lopoliths formed by the same process as laccoliths, but they are made of dense, mafic magma which enables depression by the overlying strata on cooling.
Many lopoliths consist of layered gabbroic rocks.
Some lopoliths are quite huge, having thicknesses of many kilometres.
The Bushveld lopolith in southern Africa is some hundred kilometres across and has the richest platinum deposits known.
FAQs on Lopolith
1. What is a lopolith in the context of physical geography?
A lopolith is a large, intrusive igneous body that has squeezed between layers of pre-existing rock. Unlike other intrusions, it is characteristically saucer-shaped or lens-shaped with a depressed or sagging central region. It is a type of concordant pluton, meaning it forms parallel to the rock layers it intrudes into.
2. How is a lopolith formed?
A lopolith forms when a large body of magma, typically mafic and dense, intrudes between sedimentary layers. Instead of spreading out flat like a sill, the sheer weight of the magma causes the underlying rock strata to sag downwards. This subsidence creates a large, basin-like or saucer-shaped magma chamber that eventually cools and solidifies to form the lopolith.
3. What is the main difference between a lopolith and a laccolith?
The primary difference between a lopolith and a laccolith is their shape and the effect on surrounding rock strata.
- A lopolith is saucer-shaped with a depressed centre, formed when the magma's weight causes the floor rocks to sag.
- A laccolith is dome-shaped with a flat floor and a convex top, formed when viscous magma pushes the overlying rock layers upwards.
4. What are some real-world examples of lopoliths?
Some of the most famous and largest lopoliths in the world are known for their vast mineral deposits. Key examples include:
- The Bushveld Igneous Complex in South Africa, which is the world's largest layered intrusion and a primary source of platinum and chromium.
- The Sudbury Basin in Ontario, Canada, which is believed to have been formed by a meteorite impact that created a basin for magma to fill.
- The Duluth Complex in Minnesota, USA, which is another massive intrusion rich in copper, nickel, and platinum-group elements.
5. Why are lopoliths often economically important?
Lopoliths are extremely important economically because the slow cooling of their massive magma chambers allows for a process called magmatic differentiation. As the magma cools, different minerals crystallise at different temperatures and settle in distinct layers. This process concentrates rare and valuable minerals like platinum, palladium, chromium, nickel, and copper into economically viable deposits, making lopoliths major targets for mining operations.
6. What types of igneous rocks are typically found in a lopolith?
Lopoliths are typically composed of layered mafic and ultramafic igneous rocks. Due to fractional crystallisation, they often show distinct layers. Common rock types include gabbro, norite, anorthosite, and peridotite. The layering reflects the sequence in which different minerals separated from the cooling magma.
7. How does a lopolith's shape relate to the type of magma it's formed from?
The saucer shape of a lopolith is directly related to the magma's properties. Lopoliths are typically formed from low-viscosity mafic magma (like basaltic magma), which is less sticky and can spread out over a large area. This mafic magma is also very dense. The combination of its ability to spread widely and its immense weight causes the underlying crust to subside, creating the characteristic basin shape rather than an upward dome.
8. How do lopoliths differ from other intrusive forms like sills and dykes?
While all are intrusive igneous forms, they differ in their shape and orientation relative to the host rock layers.
- A Lopolith is a saucer-shaped concordant intrusion that causes the underlying strata to sag.
- A Sill is a flat, sheet-like concordant intrusion that runs parallel to the host rock layers without significantly deforming them.
- A Dyke is a vertical or steeply-inclined discordant intrusion that cuts across the host rock layers.
