Which of the Following Igneous Rocks Is Intrusive Formed?

Key Takeaways

• Intrusive igneous rocks form from magma trapped beneath the Earth's surface.
• Common intrusive igneous rocks include granite, diorite, gabbro, pegmatite, and peridotite.
• Not all igneous rocks are intrusive; some are extrusive and form above ground.
• Intrusive rocks cool slowly underground, allowing large crystals to form.
• Extrusive rocks cool rapidly above ground, forming small crystals.

Introduction

Igneous rocks originate from magma or lava and provide a window into the dynamic processes within the Earth's interior. A key distinction exists between intrusive and extrusive igneous rocks based on where and how they cool and solidify. Intrusive igneous rocks crystallize below the surface from trapped magma, while extrusive igneous rocks form above ground from erupted lava. This results in markedly different textures and mineral assemblages. Determining whether an igneous rock is intrusive or extrusive reveals insights into the geological conditions and events that created it. This article will comprehensively evaluate the defining characteristics, common examples, and formation processes of intrusive igneous rocks to answer the question of which types can be classified as intrusive.

Understanding intrusive versus extrusive igneous rocks enables proper identification and sheds light on the subsurface and surface environments in which they solidified. Thorough analysis of intrusive rock features and compositions facilitates accurate categorization and interpretation. With in-depth knowledge of intrusive igneous rocks, geologists can read the geological record and reconstruct the evolution of landforms and volcanic systems. This article will equip readers with the knowledge to reliably determine which igneous rocks are intrusive in origin.

By detailing the key properties and representative members of the intrusive rock family, this article will build a working knowledge of the diversity found within this pivotal igneous subgroup. Readers will discover how identification of intrusive rocks informs models of magma genesis and the construction of continental crust. Additionally, the article will highlight how certain intrusive rocks are associated with valuable mineral deposits. From granites to gabbros, the breadth of the intrusive rock class will be explored. Armed with this comprehensive overview, identifying intrusive igneous rocks in the field will become straightforward.

What Makes an Igneous Rock Intrusive?

Igneous rocks are categorized into two types based on where they solidify:

• Intrusive igneous rocks – Form beneath the Earth's surface from magma trapped within the crust.
• Extrusive igneous rocks – Form above the Earth's surface from lava erupted onto the crust.

This distinction stems from their different cooling environments, which control crystallization and texture. Slow cooling underground yields intrusive rocks while fast cooling above ground forms extrusive rocks.

What Are the Defining Features of Intrusive Rocks?

• Coarse-grained texture – Slow cooling enables large mineral crystals to grow.
• Lack of glassy material – No rapid quenching occurs to produce volcanic glass.
• Layered structure – Magma may fractionate into layers of different compositions.
• Associated with volcanic plumbing – Intrusive networks feed surface volcanism.
• Rich in silica – Differentiated magmas are felsic, like granite.
• Economic mineralization – Intrusive rocks host valuable ore deposits.

In summary, identifying an igneous rock as intrusive relies on recognizing large crystal sizes, fractional crystallization textures, silica enrichments, and associations with volcanic systems. These characteristic properties readily distinguish intrusive from extrusive rocks.

What are Some Common Examples of Intrusive Igneous Rocks?

While the intrusive rock class displays considerable diversity, certain members are frequently encountered. Some of the most notable and widely distributed intrusive igneous rocks include:

Granite

• Felsic intrusive rock with large crystals of quartz, feldspars, micas, and amphiboles.
• Forms from silica-rich magma emplaced at depth.
• Makes up large portions of continental crust.

Diorite

• Intermediate intrusive rock with plagioclase feldspar and amphibole crystals.

-Derived from magma with intermediate silica content.

• Often associated with subduction zone settings.

Gabbro

• Mafic intrusive rock composed primarily of plagioclase feldspar and pyroxene.
• Crystallizes from primitive, hot magmas.
• Forms the oceanic crust at mid-ocean ridges.

Pegmatite

• Extremely coarse-grained intrusive felsic rock with interlocking crystals.
• Represents fractionated granitic magmas.
• Source of rare element-bearing gemstones.

Peridotite

• Ultramafic intrusive rock made of olivine and pyroxene minerals.
• Derives from mantle-sourced magmas.
• Direct sample of Earth's mantle compositions.

These five examples showcase the diversity of the intrusive rock group in terms of chemistry, mineralogy, and geologic context. All share the unifying attribute of crystallizing from magma trapped below the surface.

How Does the Process of Intrusive Rock Formation Occur?

The genesis of intrusive igneous rocks hinges on the process of magma intrusion, where molten rock is emplaced into the crust and crystallizes at depth. Several key stages characterize the formation process:

Magma Generation

• Partial melting in the mantle or lower crust produces magma.
• Factors like subduction or rifting induce melting.
• Magma ascends due to buoyancy and lower density.

Magma Ascent

• Magma rises through the crust in dikes and sills.
• Heat and gases allow mobility.
• Ascent stalls at levels of neutral buoyancy.

Magma Emplacement

• Magma intrudes and pools in crustal magma chambers.
• Subvolcanic networks develop.
• Differentiation and mixing may occur.

Crystallization and Cooling

• Slow cooling allows mineral grains to grow large.
• Phases crystallize sequentially depending on chemistry.
• Layered structures indicate fractional crystallization.
• Contact metamorphism alters host rocks.

In essence, magma generation, transport through the crust, emplacement at depth, and protracted cooling collectively facilitate intrusive rock formation. Contrasted with extrusive rocks, intrusions solidify slowly within the Earth.

What is the Relationship Between Intrusive Rocks and Mineralization?

Certain intrusive igneous rocks are closely associated with economically valuable mineral deposits:

• Granitic pegmatites host rare-element bearing gemstones and lithium.
• Kimberlites contain diamond deposits.
• Porphyry diorites contain concentrated ore minerals.
• Ultramafic complexes host chromite, nickel, and platinum group elements.

Several factors make intrusive rocks prime sites for mineralization:

• Differentiation concentrates incompatible elements.
• Fluids exsolved from magmas transport metals.
• Magma-country rock interactions upgrade ore elements.
• Consolidation traps and concentrates volatiles and metals.
• Large magma chambers allow extensive chemical exchanges.

In summary, the crystallization environment of intrusive rocks promotes element transport and concentration, making certain intrusions favorable sites for forming ore deposits. Many strategic metals vital to modern civilization are ultimately sourced from mineralized intrusive rock systems.

How Can You Tell Intrusive and Extrusive Rocks Apart in the Field?

Discriminating intrusive and extrusive rocks in the field relies on observing key textural and contextual clues:

• Crystal size – Intrusive rocks have larger, coarser crystals while extrusive rocks have small, fine-grained crystals.
• Surrounding rocks – Intrusions cut across and metamorphose country rock while flows overlay existing strata.
• Landforms – Extrusions shape surface topography; intrusions have no direct surface expression.
• Vertical relationships – Intrusions underlie related volcanic extrusive rocks.
• Associated features – Intrusions link to subvolcanic feeder networks.
• Fragmentation – Extrusive rocks exhibit pyroclastic and auto-brecciated textures.

With practice, interpreting the field context and textures of igneous rocks allows confident assessment of intrusive or extrusive origins.

Frequently Asked Questions

What is the key difference between intrusive and extrusive igneous rocks?

The key difference is that intrusive igneous rocks form beneath the Earth's surface from magma slowly cooling in the crust, while extrusive igneous rocks form above the surface from lava erupting and rapidly cooling. This major environmental contrast controls their distinct textures.

Are mafic igneous rocks generally intrusive or extrusive?

Mafic rocks like basalt and gabbro can form both intrusively and extrusively depending on eruptive conditions. Underwater eruptions favor intrusion while subaerial, high-volume eruptions produce floods of mafic lava. Overall, mafic magmas have a greater propensity for extrusive eruption than felsic magmas.

What setting are peridotites found in?

Peridotites are ultramafic intrusive rocks derived from the upper mantle. They intrude into the base of continental crust in certain tectonic environments associated with crustal thinning and mantle upwelling. Peridotite intrusions provide direct samples of mantle compositions.

Can pegmatites be extremely large intrusive bodies?

Yes, some pegmatite intrusions are massive, kilometers in scale, formed by exceptionally differentiated granitic magmas. The zoned mineralogy and coarse crystals of pegmatites reflect their fractional crystallization origin. Giant pegmatites provide rare element and lithium ore.

Why do intrusive rocks have economic importance?

Intrusive igneous rocks are associated with major ore deposits because their crystallization environment allows concentration of metals and incompatible elements, transport by exsolved fluids, and magma-country rock interactions that upgrade metals. The large volumes of felsic intrusions also provide sources of quarry stone.

Conclusion

In summary, intrusive igneous rocks constitute an abundant and compositionally diverse subgroup of igneous rocks defined by their mode of formation within the Earth's crust. Their characteristic coarse-grained texture, fractional crystallization features, silica-enrichment trends, and associations with volcanic plumbing identify them as intrusive. Common intrusive rock types include granite, diorite, gabbro, pegmatite, and peridotite, which solidify from a range of magma compositions at depth. Their formation through lengthy crystal growth allows development of economic mineral concentrations, establishing intrusive rocks as fundamental to metal endowments. Identifying intrusive versus extrusive rocks relies on contextual clues like cooling textures and field relationships. This comprehensive overview highlights the key attributes of the intrusive rock family, empowering accurate identification and interpretation of these widespread rocks and the stories they tell about the evolution of magmatic systems

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