Question

Each statement describes 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

Analyze the Dome-Shaped Feature

The statement describes a dome-shaped mountainous structure flanked by upturned layers of sedimentary rocks. This feature usually indicates a laccolith, which forms when magma intrudes between layers of sedimentary rock and causes the overlying strata to bulge upwards.

Identify the Vertical Wall-like Feature

The vertical wall-like feature, a few meters wide and hundreds of meters long, is indicative of a dike. Dikes are formed when magma cuts across pre-existing layers of rock, solidifying as a narrow, wall-like structure.

Recognize the Huge Expanse of Granitic Rock

A significant expanse of granitic rock forming mountainous terrain over tens of kilometers is characteristic of a batholith. Batholiths are large intrusions of igneous rock that cover extensive areas and are often made of granite.

Determine the Thin Basalt Layer

The thin basalt layer sandwiched between layers of sedimentary rock is known as a sill. Sills are formed when magma intrudes horizontally between rocks, solidifying into a flat-lying igneous body.

Key concepts

Laccolith

A laccolith is an intriguing geological structure that forms when magma intrudes between layers of sedimentary rocks. Over time, this pressure causes the overlying strata to bulge upwards, creating a distinctive dome-shaped feature on the Earth's surface.
Laccoliths are generally formed by the slow cooling of viscous magma, which causes a gentle uplift of the Earth's crust. This process results in a raised, mushroom-like landform that is typically flanked by steeply tilted sedimentary rock layers.

• Laccoliths can vary greatly in size but are usually a few kilometers in diameter.
• Their thickness can range from a few meters to several kilometers.
• This feature gives us valuable insights into the subsurface geological activities.

This distinctive shape and formation process make laccoliths a fascinating subject of study for geologists and students alike.

Dike

A dike is a prominent geological structure that forms when magma squeezes through cracks and fractures in the Earth's crust, cutting across pre-existing rock layers. When this magma solidifies, it forms a narrow, wall-like feature that we recognize as a dike.
Dikes often appear as vertical or steeply-inclined walls that can extend great distances both vertically and horizontally. They are typically narrow, often being only a few meters wide, but they can stretch for hundreds of meters in length.

• Dikes are important indicators of past volcanic activity and magma movement.
• They can occasionally appear on the Earth's surface due to erosion or tectonic movements.
• As a result, dikes have a significant impact on the landscape, often forming dramatic cliff faces.

Because of their unique characteristics, dikes provide valuable information about the geological history of an area, revealing the paths that magma once traveled.

Batholith

A batholith is a massive and grandiose geological formation, primarily composed of granitic rock. It forms from the slow cooling and solidification of magma deep within the Earth's crust. Batholiths typically emerge at the surface due to significant erosion or uplifted terrain over millions of years.
These structures are characterized by their enormous size, often covering hundreds of square kilometers, and they can form mountainous terrains, making them a significant feature in the landscape.

• Batholiths are typically intrusive igneous rocks formed from felsic magma.
• They often underlie entire mountain ranges.
• Their granitic composition makes them highly resistant to weathering.

Overall, batholiths are a testament to the dynamic processes occurring below Earth's surface and are vital for understanding the geologic history of a region.

Sill

Sills provide a fascinating look at how magma can intrude and solidify parallel to pre-existing rock layers. They are formed when magma forces its way between older rock layers and then cools and solidifies without disturbing much of the surrounding material. This process creates a relatively thin, horizontal sheet of igneous rock.
Sills are often exposed in cliffs or canyon walls, giving geologists a clear view of their parallel, layered structure.

• Unlike dikes, sills run horizontally along the bedding planes of the rocks they intrude.
• They can sometimes be mistaken for lava flows on the Earth's surface, but sills are intrusive, not extrusive.
• Sills often have a fine-grained or glassy texture due to their quick cooling.

Understanding sills helps in the study of past magmatic events and offers insights into the layered nature of Earth's crust.