Question

Each statement 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

Understand Intrusive Features

Intrusive igneous features form when magma cools and solidifies below Earth's surface. These features are later exposed to the surface due to erosion.

Identify a Dome-Shaped Feature

The statement describes a dome-shaped structure surrounded by upturned sedimentary layers. This is typically a laccolith, which forms when viscous magma inflates the overlying rock in a dome shape.

Recognize Vertical Wall-like Features

A vertical, wall-like feature that is a few meters wide and extends for hundreds of meters is characteristic of a dike. Dikes form when magma cuts across preexisting rock layers.

Describe a Massive Terrain

A huge expanse of granitic rock that forms mountainous terrain tens of kilometers wide is described as a batholith. Batholiths are vast collections of magma that solidified deep below the surface and are exposed due to extensive erosion.

Identify the Thin Layer of Basalt

A relatively thin layer of basalt between horizontal sedimentary rocks suggests the presence of a sill. Sills form when magma intrudes parallel to the existing layers and solidifies.

Key concepts

Laccolith

A laccolith is an intriguing formation that results when magma pushes its way between layers of rock, creating a dome-shaped mass. This structure can be visualized as a blister, where the pressure from below causes the rock layers above to arch upward. This results in the classic dome shape often associated with laccoliths.

Laccoliths are characterized by their relatively shallow depth compared to other intrusive features. They form from viscous magma which is thick and flows slowly, allowing it to cool more quickly than in deeper formations. This cooling process creates the unique dome shape. Once erosion strips away the overlying material, the laccolith is revealed, often resulting in a small mountain or hill.

• Characteristics: Dome-shaped, shallow formation, upturned surrounding layers
• Formation: Magma intrusion between rock layers
• Location: Often in mountainous regions as small hills or bulges

Dike

A dike is an impressive geological feature made when magma slices its way through preexisting rock layers, solidifying as it cools. Unlike the horizontal arrangement of sills, dikes cut vertically or steeply across rock layers. This process creates a striking, wall-like appearance.

Dikes can vary greatly in size, but they typically appear as narrow, linear intrusions that extend for considerable distances, often standing out against the host rock. When exposed on the surface, they resemble walls of stone, a few meters wide but possibly stretching for hundreds of meters.

• Characteristics: Vertical or steeply angled, narrow but long features
• Formation: Magma intrusion cutting across rock layers
• Visibility: Often exposed as narrow walls

Batholith

Batholiths are colossal, deep-seated formations of igneous rock, usually composed predominantly of granite. They form from enormous amounts of magma that have slowly crystallized deep below Earth's surface, sometimes over millions of years.

Once extensive erosion occurs, these vast bodies are uncovered. They can extend over tens of kilometers and create entire mountain ranges due to their massive size. On Earth's surface, they present as large, rounded expanses of granite rock, marking them as significant geological landmarks.

• Characteristics: Vast, granitic, mountainous ranges
• Formation: Large magma chambers that cool and harden deep underground
• Location: Typically in mountainous areas, forming the crux of mountain chains

Sill

Sills are thin, sheetlike intrusions that occur when magma slips its way horizontally between preexisting sedimentary layers. Unlike dikes that cut across layers, sills run parallel to the formations they intrude.

When the conditions are right, the intrusive magma cools and hardens, leading to the characteristic flat formation of sills. These structures often become visible as distinct layers exposed on cliff faces or valley walls once erosion reveals them.

• Characteristics: Horizontal, parallel to sedimentary layers
• Formation: Magma intrusion along bedding planes
• Visibility: Manifests as layers between rock strata, often along river valleys or cliffs