Igneous Intrusions

Summary

This lecture explores igneous intrusive structures formed beneath the Earth's surface, including plutons, batholiths, stocks, sills, dikes, volcanic necks, and diatremes. Using examples like the Sierra Nevada, Camelback Mountain, Shiprock, and Agatha Peak, it explains how these subterranean magma bodies crystallize, are exposed through uplift and erosion, and vary in size, shape, and composition. The video also highlights economic significance, showing how intrusions can host valuable ore deposits, pegmatite dikes, and kimberlite pipes—the source of diamonds. Through vivid descriptions and geologic examples, viewers gain insight into the formation, structure, and importance of these ancient magmatic features.

Takeaways

• 🪨 Plutons are intrusive igneous bodies formed beneath the Earth's surface and can take many forms, from massive batholiths to small dikes and sills.
• 🌋 Batholiths are the largest plutons, covering at least 100 square kilometers, and often consist of multiple smaller plutons called stocks.
• ⏳ Large batholiths, like the Sierra Nevada, can take thousands to hundreds of thousands of years to crystallize and may span millions of years in age.
• 🗻 Sills are sheet-like intrusions that follow the layering of surrounding rocks, typically forming at shallow depths, while dikes crosscut the rock layers.
• 🧱 Columnar jointing forms in sills and dikes as magma cools, producing fractures perpendicular to the top and bottom surfaces.
• 🌌 Volcanic necks or plugs are resistant remnants of volcanic pipes left after erosion removes surrounding softer rock.
• 🕳️ Diatremes are debris-filled volcanic pipes with a conical subsurface shape, often forming volcanic necks when exposed by erosion.
• 📍 Shiprock and Agatha Peak are classic examples of diatremes in the Four Corners and Navajo volcanic fields, showing radiating dikes from central conduits.
• 💎 Certain intrusive features, like kimberlite pipes and pegmatite dikes, are economically significant as sources of diamonds, rare earth elements, and other minerals.
• 💧 Hydrothermal fluids moving through fractures around igneous bodies can form ore deposits, either disseminated throughout the rock or concentrated in vein deposits.
• 🏞️ Uplift and erosion are critical processes that expose ancient subterranean magma bodies, allowing direct study of igneous intrusions at the surface.
• 🌍 Examples of exposed plutons and batholiths include the Sierra Nevada, Peninsular Ranges, Camelback Mountain, and the Jacumba Mountains, representing magma bodies from hundreds of millions to over a billion years old.

Q & A

What is a pluton and how does it form?

A pluton is a generic term for any intrusive igneous body that forms beneath the Earth's surface. It forms when magma cools and crystallizes underground, and can take many forms, ranging from massive batholiths to small dikes and sills.

What are batholiths and how do they differ from stocks?

Batholiths are the largest intrusive igneous bodies, typically covering over 100 square kilometers. They are usually aggregates of smaller plutons called stocks, which are smaller and may not have been molten at the same time as the batholith.

Can you give examples of famous batholiths in the United States?

Yes, notable examples include the Sierra Nevada Batholith in California, the Peninsular Ranges Batholith, and the Idaho Batholith. These represent ancient magma chamber complexes for eroded volcanic mountain ranges.

What are sills and how do they differ from dikes?

Sills are sheet-like intrusions that follow the layering of the surrounding country rock and usually form at shallow depths. Dikes, on the other hand, are sheet-like intrusions that cut across the surrounding rock layers and are also typically shallow but propagate in a different orientation.

What is a volcanic neck or plug?

A volcanic neck, or plug, is the resistant remnant of a volcanic pipe. It remains after the surrounding softer volcanic and sedimentary rocks have been eroded away, exposing the deeper magma conduit of a former volcano.

What is a diatreme and how does it relate to volcanic activity?

A diatreme is a debris-filled volcanic pipe formed by explosive eruptions. It has a conical shape underground with a broad crater at the surface. Over time, erosion can expose it as a volcanic neck.

Why are intrusions economically significant?

Intrusions can concentrate valuable minerals such as gold, silver, and copper. Ore deposits can form as disseminated deposits throughout large rock volumes or as vein deposits along fractures, making them important targets for mining.

What are pegmatite dikes and why are they important?

Pegmatite dikes are intrusions rich in water and rare elements, often containing very large crystals. They are important economically because they can host rare earth elements and other valuable minerals, and are typically mined using underground methods.

What is the significance of kimberlite pipes?

Kimberlite pipes are a type of diatreme that originate from deep within the Earth and are the primary source of natural diamonds worldwide. They are economically significant and found in regions like South Africa, Canada, and Siberia.

What is the geological history of Shiprock, New Mexico?

Shiprock is a volcanic neck and set of dikes in northwestern New Mexico. It represents the deeper conduit of a volcano that has been eroded away. The dikes radiate from the central pipe, which was originally 750 to 1000 meters below the surface, and the intrusion is about 30.6 million years old.

How do sills and dikes reveal the processes of magma propagation?

Sills show magma following existing rock layers at shallow depths, while dikes demonstrate magma cutting across rock layers, often propagating along fractures. Together, they reveal how magma can move through the crust, both along and across structural weaknesses.

How old are the granitic rocks of Camelback Mountain and what do they represent?

The granitic rocks of Camelback Mountain are Mesoproterozoic in age, between 1400 and 1700 million years old. They represent remnants of ancient batholithic intrusions, with the mountain itself forming part of a faulted and eroded batholith exposed at the surface.