Question

With respect to tabular intrusive bodies, what is the difference between a concordant body and a discordant body?

Short answer

The main difference between concordant and discordant tabular intrusive bodies lies in their orientation with respect to the layers of the pre-existing rock formation. Concordant bodies, such as sills, are parallel to the country rock and intrude along the bedding planes, not disrupting the overall structure. In contrast, discordant bodies, like dykes, cut through or across the bedding planes, disrupting the overall structure of the country rock.

Step-by-step solution

Define Tabular Intrusive Bodies

Tabular intrusive bodies are geological formations where one type of rock (usually igneous) is intruded into another rock formation, creating a layered appearance. These bodies are formed when magma cools and solidifies within pre-existing rock structures, resulting in distinctive shapes and orientations.

Define Concordant Bodies

Concordant tabular intrusive bodies are those where the intruded rock layer lies parallel to the pre-existing (country) rock layers. In other words, the magma intrudes along the bedding planes, not cutting or disrupting the overall structure of the country rock. An example of a concordant body is a sill, which is a horizontal or near-horizontal intrusion parallel to the bedding layers of the surrounding rock.

Define Discordant Bodies

Discordant tabular intrusive bodies, on the other hand, cut through and cross the layers of pre-existing rock. This means that the magma intrudes across the bedding planes, disrupting the overall structure of the country rock. Dykes are examples of discordant bodies, which are typically vertical or near-vertical intrusions cutting across the surrounding layers of rock.

Summarize the Differences

In summary, the main difference between concordant and discordant tabular intrusive bodies lies in their orientation with respect to the layers of the pre-existing rock formation. Concordant bodies are parallel to the country rock, while discordant bodies cut through or across the bedding planes. Examples of concordant and discordant tabular intrusive bodies are sills and dykes, respectively.

Key concepts

Concordant Bodies

In geology, concordant bodies represent a harmonious relationship between an intrusive igneous body and the surrounding pre-existing rock layers. These intrusions occur when molten rock, or magma, finds its way between rock layers and solidifies without disrupting the natural layout of these layers.

Concordant intrusions are tucked away neatly between the strata, lying parallel to the bedding planes of the country rock. A common type of concordant body is a sill, which resembles a stacked pancake-like formation within the Earth's crust. Their formation can provide valuable insights into the historical geothermal gradients and the mechanisms of magma emplacement.

Discordant Bodies

Contrary to their concordant counterparts, discordant bodies play a more intrusive role in the structure of the country rock. These geological formations occur when magma forcefully makes its way through the cracks and weaknesses in the rock, cutting across the pre-existing layers - much like a tree can split through a sidewalk.

A classic example of a discordant body is a dike, which can often be identified by its striking perpendicular or oblique angle to the hosting strata. Discordant bodies can inform geologists about tectonic stresses and the geological history of an area, acting as a record of the Earth's dynamic processes over time.

Igneous Intrusions

Igneous intrusions, often referred to as plutonic rocks, are underground formations created from magma that has cooled and solidified beneath the Earth's surface. These geological features present an intrusive igneous spectacle, varying widely in size, shape, and type - all depending on how and where the magma cools within the crust.

Their study reveals a plethora of information about the innards of our planet, including the composition of the mantle, the conditions of magma ascent, and the complexities of crustal evolution. With careful analysis, geologists can piece together a narrative of past volcanic activity, even in areas where no volcanic activity is currently evident.

Geological Formations

When we speak of geological formations, we are referring to the natural architectural structures of the Earth's crust. These structures are built over time through processes such as sedimentation, erosion, and, indeed, igneous intrusion. Each formation tells a story, detailing events that have occurred over millions of years.

Geological formations range from the grandeur of mountain ranges to the subtle stratification seen in exposed cliff faces. Studying these earthly chapters allows geologists to understand past environments and predict future geological changes. They act as a record book, spanning eons, documenting the ceaseless dance between the Earth's internal forces and surface processes.