Question

Each of the following statements describes how an intrusive feature appears when exposed at Earth's surface due to erosion. Name the feature. a. A dome-shaped mountainous structure flanked by upturned layers of sedimentary rocks b. A vertical wall-like feature a few meters wide and hundreds of meters long c. A huge expanse of granitic rock forming a mountainous terrain tens of kilometers wide d. A relatively thin layer of basalt sandwiched between horizontal layers of sedimentary rocks exposed along the walls of a river valley

Short answer

a. Laccolith, b. Dike, c. Batholith, d. Sill

Step-by-step solution

Analyze Feature a

The first intrusive feature described is a dome-shaped mountainous structure with upturned layers of sedimentary rocks on the flanks. This characteristic is typical of a laccolith, where magma intrudes between layers of sedimentary rock and causes the overlying strata to uplift and form a dome shape.

Analyze Feature b

The second feature is described as a vertical wall-like feature that is a few meters wide and hundreds of meters long. This description fits a dike, which is a sheet-like body of intrusive rock that cuts through pre-existing structures, typically being narrow and extensive in length.

Analyze Feature c

This feature is a huge expanse of granitic rock that forms a mountainous terrain, extending tens of kilometers in width. Such a large exposure of granitic rock is indicative of a batholith, which is a massive, deep-seated intrusive body that has been exposed at the surface due to erosion.

Analyze Feature d

The final feature is a relatively thin layer of basalt situated between horizontal layers of sedimentary rocks and exposed along river valley walls. This description matches a sill, which is a concordant intrusive sheet, meaning it lies parallel to the sedimentary layers into which it intrudes.

Key concepts

Laccolith

A laccolith is a fascinating geological formation primarily associated with the intrusion of magma into existing rock layers. Picture it as a pocket of magma that pushes its way between pre-existing strata. As the magma accumulates, it uplifts the overlying layers, creating a dome-shaped structure on the Earth's surface.

Laccoliths often result in impressive, mountainous landscapes. These domes can vary extensively in size, with some covering large areas, giving rise to notable topographic landmarks.

• Formation Process: Magma intrudes between layers of sedimentary rock, lifting the superposed layers into a dome.
• Characteristics: Dome-shaped with uplifted sedimentary layers around the flanks.

Understanding laccoliths provides insight into the Earth's dynamic crust, showcasing the incredible forces at work beneath our feet.

Dike

Dikes are striking features that stand out due to their wall-like appearance. Essentially, a dike is formed when magma cuts through rock layers, solidifying into a vertical or near-vertical sheet of igneous rock.

Imagine a robust and narrow wall of rock, stretching across the landscape—this is the distinctive figure of a dike. They can be several meters wide but can extend for hundreds of meters or even kilometers in length.

• Formation: Magma forces itself vertically through existing rock layers and solidifies.
• Appearance: Narrow, vertical, and extensive, resembling a wall cutting through the ground.

Dikes often become visible on the surface once erosion removes the softer surrounding material, leaving the more resilient rock standing.

Batholith

A batholith represents one of the grandest scales of intrusive igneous features, often forming the backbone of mountain ranges. These geological giants are formed from vast masses of magma that crystallize deep within the Earth's crust and then are exposed after extensive erosion.

Batholiths are predominantly composed of granitic rock and can extend over areas of hundreds of square kilometers. They provide a glimpse into the deep-seated processes of our planet.

• Size: Huge, covering tens to hundreds of kilometers.
• Composition: Mainly granitic.

Once exposed at the surface, batholiths create expansive and rugged terrains, offering dramatic landscapes and invaluable geological records.

Sill

Sills are an intriguing type of intrusive igneous feature that lie parallel to the layering of sedimentary rocks. They are formed when molten magma squeezes between rock layers and solidifies, creating a relatively flat sheet of rock.

Unlike dikes, which cut across rock layers, sills do not disrupt the pre-existing layers as they intrude along the bedding planes, which means they maintain concordance with the original strata.

• Formation: Magma intrudes sideways between layers of existing rock, solidifying into a thin layer.
• Position: Concordant, lying parallel to rock layers.

Sills become visible through erosion, revealing themselves as layers of igneous rock sandwiched between sedimentary formations.