Question

Which of the following igneous features is parallel to the rock layers that it intrudes? A. batholith C. sill B. volcanic neck D. dike

Short answer

The correct answer is C. sill.

Step-by-step solution

Identify the Definitions

First, we need to define the key terms used in the question: - **Batholith:** A very large igneous intrusion extending deep into the earth's crust. - **Sill:** A tabular sheet intrusion that forms parallel to existing rock layers. - **Volcanic Neck:** A cylindrical-shaped volcanic feature formed when magma solidifies within a vent. - **Dike:** A slab of volcanic rock that forms in a fracture in the existing rock and cuts across rock layers.

Focus on Parallel Feature

From the definitions, identify which feature aligns parallel to existing rock layers. A sill is known to form parallel to the rock layers it intrudes.

Eliminate Non-parallel Features

Eliminate options that do not form parallel to rock layers: - Batholiths do not have a parallel alignment with rock layers as they are large masses that usually cut across several layers. - Volcanic necks are vertical structures not related to layer parallelism. - Dikes cut across rock layers, forming perpendicular or diagonal orientations.

Highlight the Correct Answer

Based on the analysis, the answer is clearly the feature that forms parallel to existing rock layers, which is a sill.

Key concepts

Sill

A sill is a fascinating geological feature. It forms when magma intrudes between existing horizontal rock layers and solidifies. Unlike other intrusions, a sill is unique because it aligns itself parallel to the rock layers it penetrates. This parallel alignment is what really sets it apart.

Think of a sill as a giant pancake that slides in between the layers of a sandwich. It's this non-aggressive, parallel process that makes sills significant in geological studies. Sills usually form in regions where the magma has enough pressure to push through but not enough to break through and create disruptive features. This gives them a more predictable, even appearance.

• Sills can vary in size and thickness, depending on the magma flow and rock layer characteristics.
• They are typically composed of basalt or dolerite, although the chemical composition can vary.

Understanding sills helps geologists learn more about the earth's internal processes and past geological events. Their presence often signifies stable conditions during formation, unlike more chaotic intrusions.

Rock Layer Intrusion

When magma forces itself through existing rock formations, it creates what is known as an intrusion. This is a fundamental concept in studying geology. Essentially, the intrusion process happens when magma finds a crack or weak spot between or through rock layers and moves in.

There are different types of intrusions, characterized by their orientation relative to the existing rock:

• Concordant Intrusions: These occur along the natural planes of the rock layers. Sills belong to this category since they align parallel to these layers.
• Discordant Intrusions: These cut across the rock layers. Features like dikes fall into this category.

Intrusion of magma is a key process in understanding mountain formation, continental shifts, and other large-scale geological phenomena. The study of these intrusions provides insight into the dynamic nature of geo-processes beneath the earth's surface.

Igneous Intrusion Characteristics

Igneous intrusions are diverse and influence the earth's crust in various ways. Understanding their characteristics helps in appreciating their role in geological formations and processes.

Some common features of igneous intrusions include:

• Composition: Most intrusions are composed of basalt, granite, or similar volcanic rock types. The specific mineral composition affects the resulting rock's texture and hardness.
• Structure: Intrusions can form as large, singular bodies or as smaller, branched formations. The structure often depends on the surrounding pressure and rock resistance.

Sills, for example, have a distinct sheet-like structure due to their formation process, while dikes are often more rigid and vertical.

By studying these characteristics, geologists can infer past volcanic activity and tectonic movements. Such information is vital for predicting future geological events and understanding the history of our planet's surface.