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 layers of sedimentary rocks exposed on the side of a canyon.

Short answer

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

Step-by-step solution

Identifying the Dome-Shaped Structure

When erosion strips away the overlying sedimentary rock, an intrusive body known as a "laccolith" can be exposed. It often appears as a dome-shaped structure due to its original emplacement beneath the Earth's surface.

Recognizing the Vertical Wall-Like Feature

This feature describes a "dike," which is a vertical or near-vertical wall-like feature. Dikes cut across existing rock layers and often become exposed as erosion removes surrounding material.

Understanding the Huge Granitic Expanse

"Batholiths" are very large expanses of intrusive igneous rock, typically consisting of granite. These massive structures are exposed after extensive erosion and can extend tens of kilometers in width, forming mountain ranges.

Identifying the Thin Layer of Basalt

The description fits a "sill," which is a relatively thin, horizontal intrusion of igneous rock, like basalt, formed between preexisting sedimentary layers. Sills become visible in formations like canyon walls after erosion.

Key concepts

Laccolith

Laccoliths are fascinating geological formations that occur beneath the Earth's surface as intrusions of magma push between layers of sedimentary rock. As the magma flows, it creates a lens-shaped or dome-shaped bulge, which can eventually uplift the overlying rock layers.
This uplift creates a notable topographical feature often resembling a blister or dome.
Over time, natural processes such as erosion remove the overlying rock, revealing the laccolith as a majestic dome-shaped structure. This exposure can create mountain-like formations, with sedimentary rock layers flanking the igneous body in an upturned fashion. Key characteristics of laccoliths:

• Dome or mushroom-like shape
• Intruded between sedimentary rock layers
• Can form prominent landforms as they become exposed

Dike

Dikes are intriguing features of intrusive igneous activity, forming when magma forcibly inserts itself through cracks and fissures within existing rock layers.
This intrusive process results in a typically vertical or near-vertical sheet-like formation of igneous rock.
Unlike laccoliths, dikes cut across the grain of the surrounding sedimentary or metamorphic rock, acting like stone walls within the Earth. Once the surrounding softer material erodes away, dikes become visible as resistant ridges or vertical rock walls. These formations can extend for miles, but their width can be just a few meters, marking a striking contrast against the natural landscape. Attributes of dikes include:

• Vertical or steeply inclined formation
• Crosses through rock layers
• Often long and narrow

Batholith

Batholiths are some of the largest and most impressive intrusive igneous features, covering extensive areas often spanning hundreds of square kilometers.
These massive formations are composed mainly of granitic rock which forms from accumulated magma deep within the Earth over millions of years.
As erosion strips away surface layers, the sheer expanse of a batholith becomes visible, dominating the landscape. Batholiths are known for creating mountain ranges due to their sheer size and volume. Their exposure requires significant geological timeframes since they originate many kilometers below the surface. When revealed, they display vast expanses of smooth granitic rock. Characteristics include:

• Gigantic size and coverage
• Typically made of granitic material
• Form mountain ranges over long periods

Sill

A sill is an important and interesting example of an igneous intrusion, occurring when magma seeps between preexisting layers of sedimentary rock, creating a thin, horizontal formation.
This process is unlike that of dikes, as sills lie parallel to the grain of the sedimentary layers, sandwiched effortlessly between them.
They are often formed from basalt and are relatively thin compared to other igneous bodies. Erosion can expose sills along canyon walls or similar geological features, allowing them to be seen as layers resembling m asals running through the rock face. These layers are sometimes mistaken for sedimentary rocks due to their similar orientations. Key points about sills:

• Horizontally oriented rock formations
• Formed parallel to sedimentary layers
• Often composed of basalt