Question

Each of the following descriptions indicates how an intrusive feature appears when exposed at Earth's surface by 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

Interpret the description for feature (a)

The feature described as a 'dome-shaped mountainous structure flanked by upturned layers of sedimentary rocks' is indicative of a laccolith. Laccoliths form when magma intrudes between rock layers close to the surface, causing the overlying layers to dome upwards.

Interpret the description for feature (b)

The description 'a vertical wall-like feature a few meters wide and hundreds of meters long' matches that of a dike. Dikes are formed when magma cuts across preexisting rock layers, solidifying into wall-like structures.

Interpret the description for feature (c)

The 'huge expanse of granitic rock forming a mountainous terrain tens of kilometers wide' corresponds to a batholith. Batholiths are large masses of intrusive igneous rock, commonly granitic, that form deep beneath the Earth's surface and are exposed after erosion removes the overlying materials.

Interpret the description for feature (d)

The 'relatively thin layer of basalt sandwiched between layers of sedimentary rocks' likely describes a sill. Sills are sheet-like intrusions that form parallel to older rock layers when magma intrudes and solidifies there.

Key concepts

Laccolith

Laccoliths are fascinating geological features that occur when molten rock, or magma, intrudes between existing rock layers. When enough magma is pushed into this space, it creates pressure that forces the overlying layers to bow upwards, forming a dome-like shape. This doming effect is similar to how a blister forms under your skin. Over time and with erosion, these structures can be exposed at the Earth's surface, revealing their distinct dome shape. Some notable examples of laccoliths are the Henry Mountains in Utah, where you can see how these massive intrusions have shaped the landscape. The term "laccolith" is derived from Greek words meaning "lake" and "rock." This name evokes the pooled shape of the intruded magma. Remember, unlike volcanoes, laccoliths do not typically erupt because they don't reach the Earth's surface.

Dike

Dikes are an interesting type of intrusive igneous feature that form when magma cuts through pre-existing rock layers. Imagine poking a pencil through a stack of papers; much like this, dikes slice vertically or steeply diagonally across geological strata. This process leaves behind a distinct wall-like formation after the magma has cooled and solidified. Dikes can vary greatly in size, being just a few inches wide in some cases to hundreds of feet wide in others, but they commonly extend for several kilometers in length. Since dikes are created under pressure from magma forcing its way through cracks, they often serve as conduits for subsequent volcanic activity. An example of impressive dike formations can be seen at the Isle of Skye in Scotland, where numerous dikes are exposed along the coastline, providing a spectacular view of these ancient geological phenomena.

Batholith

Batholiths are massive formations of intrusive igneous rock, usually composed of granitic material. They form deep beneath the Earth's surface when a large body of magma slowly cools and solidifies. Over millions of years, erosion strips away the overlying earth, revealing these vast expanses. Batholiths are among the largest plutonic bodies, often covering areas of hundreds of square kilometers. They form the core of many mountain ranges and have a crucial role in the shaping of continents. For instance, the Sierra Nevada Mountains in California are composed primarily of a large batholith, showcasing the enormous scale of these structures. Batholiths are not formed from a single intrusion but are rather an aggregate of multiple intrusions over time, contributing to their immense size. These giants of geology are windows into the deep workings of the Earth's crust, providing valuable insights into our planet's geological history.

Sill

Sills are another fascinating type of intrusive feature that form parallel to existing rock layers. When magma finds its way into a horizontal or slightly inclined plane of weakness within older sedimentary layers, it spreads out and solidifies, forming a thin, flat layer of rock known as a sill. This process often creates a hardened layer of igneous rock, typically basalt, sandwiched between sedimentary layers. While sills and dikes are similar in that they both represent pathways where magma has intruded, the key difference lies in their orientation: sills are horizontal, while dikes are vertical. You might encounter sills in areas like the Palisades along the Hudson River in New York, where an extensive sill formation creates dramatic cliffs. Sills are essential for understanding the processes of subsurface magma flow and can provide crucial information about the conditions under which the Earth's crust was formed.