1/31/2026

 

Samuel Clifford

 

Chapter 4: Igneous Rocks and Intrusive Activity

 

4.1

 

Magma is rock that has melted completely or partly. When it cools, it becomes igneous rock, which is mostly made of silicate minerals. Scientists know that magma forms when solid rock deep in Earth’s crust or upper mantle melts a little—sometimes as deep as 250 kilometers. Because melted rock expands and becomes less dense, it slowly rises toward the surface.

 

If this molten rock reaches the surface, it is called lava. Escaping gases can sometimes blast the lava upward in fountains or cause explosive eruptions filled with steam and ash. Other times, eruptions are much calmer and simply release smooth, flowing lava.

 

Magma is made of three parts: a liquid, some solids, and dissolved gases. The liquid part, called melt, contains moving ions of the main elements in Earth’s crust—mostly silicon and oxygen, with smaller amounts of aluminum, potassium, calcium, sodium, iron, and magnesium.

 

The solid part of magma consists of crystals that form as the melt cools. As cooling continues, more and more crystals appear and grow. Near the end of cooling, the magma becomes a thick, crystal‑rich mixture that looks a bit like very dense oatmeal, with only a little liquid left.

 

The gas part of magma is made of volatiles, which are substances that turn into gas at surface pressure. The most common volatiles are water vapor, carbon dioxide, and sulfur dioxide. Deep underground, these gases stay dissolved in the magma because of the high pressure. As magma rises and pressure drops, the gases begin to separate from the melt. If enough gas builds up, it can push the magma out of a volcanic vent and drive an eruption.

 

When magma cools and solidifies deep below the surface, the leftover volatiles gather as hot, water‑rich fluids that move through surrounding rocks. These fluids are important in causing metamorphism, which you’ll learn about later.

 

Igneous rocks that harden at Earth’s surface are called extrusive igneous rocks. Because they form from lava that erupts or flows out of a volcano, they are also known as volcanic rocks, a name inspired by Vulcan, the Roman god of fire.

 

4.2

 

Igneous rocks can be grouped by their composition, which depends on the minerals they contain. At one end of the range are felsic rocks, which are made mostly of light‑colored minerals like quartz and potassium feldspar. Because these minerals are rich in silica, felsic rocks have high silica content that is around 70% or more. Felsic magma commonly cools to form granite, so geologists often call it granitic magma. Granite usually has a small amount (about 10%) of dark minerals such as biotite mica and amphibole. Felsic rocks make up much of Earth’s continental crust.

 

At the opposite end are mafic rocks, which contain at least 45% dark silicate minerals such as pyroxene and olivine. These minerals contain iron and magnesium, so mafic rocks are darker and denser than felsic rocks. Mafic magma typically forms basalt, so it is often called basaltic magma. Basalt is the main rock of the ocean floor, many volcanic islands, and large lava flows on continents.

 

Between felsic and mafic compositions are intermediate (andesitic) rocks. They contain at least 25% dark minerals, mainly amphibole, pyroxene, and biotite, along with plagioclase feldspar as the main light mineral. These rocks are commonly found in volcanic regions along the edges of continents and in volcanic island arcs like the Aleutian Islands.

 

On the far mafic end of the spectrum is peridotite, an ultramafic rock made almost entirely of olivine and pyroxene. Ultramafic rocks are rare at Earth’s surface, but peridotite is the dominant rock of the upper mantle.

 

4.3

 

Texture refers to the overall appearance of a rock from its shape, size, and arrangement of its mineral grains.

 

Three variables effect the texture of igneous rocks:

 

1. How fast, or what is the rate, that the molten rock cools

 

2. The amount of silica that is present in the magma

 

3. The amount of dissolved gases in the magma 

 

Types of Igneous Textures:

 

-Aphanitic (Fine-Grained) Texture

 

Aphanitic rocks cool quickly at or near the surface, giving crystals too little time to grow large. The minerals are so small they usually can’t be seen without magnification. This texture is typical of volcanic (extrusive) rocks like basalt.

 

-Phaneritic (Coarse Grained) Texture

 

Phaneritic rocks cool slowly deep underground, allowing large, visible crystals to form. Each mineral grain is easily distinguishable with the naked eye. This texture characterizes intrusive rocks such as granite.

 

-Porphyritic Texture

 

Porphyritic rocks contain two distinct crystal sizes: large crystals (phenocrysts) embedded in a fine-grained or coarse-grained groundmass. This indicates a two-stage cooling history—slow cooling first, then rapid cooling. It’s common in both volcanic and plutonic settings.

 

-Vesicular Texture

 

Vesicular rocks contain many small holes (vesicles) formed by gas bubbles trapped in cooling lava. The rapid solidification preserves these cavities before the gases escape. Scoria and pumice are classic examples.

 

-Glass Texture

 

Glass-textured rocks cool so rapidly that atoms don’t have time to arrange into crystals, producing a non-crystalline, glassy solid. This results in a smooth, shiny appearance with no visible mineral grains. Obsidian is the best-known example.

 

-Pyroclastic Texture

 

Pyroclastic rocks are composed of volcanic fragments, ash, pumice, bombs, ejected during explosive eruptions. These fragments are welded or compacted together after deposition. Tuff and volcanic breccia fall into this category.

 

-Pegmatitic Texture

 

Pegmatitic rocks contain extremely large crystals, often several centimeters to meters across. They form from water-rich magmas that allow rapid crystal growth in the final stages of cooling. Pegmatites often host rare minerals and gemstones.

 

4.4 

 

The mineral composition of igneous rocks depends on the chemical makeup of the magma it came from. This means that different igneous rocks may have the same chemical composition but are different do the texture stated before.

 

Felsic Igneous Rocks:

 

Granite is a widely recognized igneous rock because it’s both abundant in the continental crust and visually appealing, especially when polished for use in monuments, tombstones, and building stone. It typically contains 10–20% quartz and about 50% feldspar, with the remaining portion made up of minor dark minerals like biotite and amphibole. Its coarse-grained texture and mix of clear quartz, light-colored feldspar, and darker silicates give granite its distinctive, speckled appearance.

 

Rhyolite is the fine‑grained volcanic counterpart of granite, sharing the same light‑colored silicate composition that gives it buff, pink, or light gray tones. Because it cools quickly at or near the surface, it often contains glassy fragments and small voids. Unlike granite’s large, widespread intrusive bodies, rhyolite tends to form smaller, less common deposits—though Yellowstone’s massive rhyolite flows and ash layers stand out as major exceptions.

 

Obsidian forms when silica‑rich lava cools so quickly at Earth’s surface that its atoms cannot arrange into a crystal structure, making it a natural volcanic glass rather than a true mineral aggregate. Even though it is typically black or reddish‑brown, its chemical composition is similar to granite, not basalt; the dark color comes from tiny amounts of metallic ions dispersed through otherwise clear glass. When viewed along a thin edge, this clarity becomes obvious—obsidian appears nearly transparent.

 

Pumice is a volcanic glass full of gas‑escape bubbles, giving it a frothy, vesicular texture formed when gas rapidly escapes from silica‑rich lava. Its many voids make it so lightweight that pieces often float, and some samples show flow lines that record movement before the melt fully solidified. Pumice commonly occurs alongside obsidian, with the two forming alternating layers within the same volcanic deposit.

 

Intermediate Igneous Rocks:

 

Andesite is a fine‑grained, medium‑gray volcanic rock named after the Andes Mountains, where it is abundant in volcanic arcs. It also dominates many volcanoes of the Cascade Range and other Pacific Rim continental margins. Andesite often shows a porphyritic texture, with visible plagioclase or amphibole crystals embedded in a finer groundmass, and because it can resemble rhyolite, confirming its mineral composition often requires microscopic study.

 

Diorite is the coarse‑grained, intrusive counterpart to andesite, and while it can resemble gray granite at first glance, its mineral makeup sets it apart. It typically contains little to no visible quartz and instead has a higher proportion of dark silicate minerals. Most of its composition comes from plagioclase feldspar and amphibole, and because the light feldspar and dark amphibole occur in roughly equal amounts, diorite develops its classic salt‑and‑pepper appearance.

 

Mafic Igneous Rocks:

 

Basalt is a dark, fine‑grained volcanic rock made mostly of pyroxene and calcium‑rich plagioclase, with smaller amounts of olivine and amphibole. When it forms with a porphyritic texture, you often see light feldspar crystals or green, glassy olivine grains set in a much darker groundmass. It’s the most abundant extrusive igneous rock on Earth. Entire volcanic islands, like Hawaii and Iceland, are built largely from basalt, and the upper part of the oceanic crust is almost entirely basaltic.

 

Gabbro is the coarse‑grained, intrusive counterpart to basalt. It shares the same general mineral composition—mainly pyroxene and calcium‑rich plagioclase feldspar, with possible minor olivine or amphibole—but because it cools slowly deep within Earth’s crust, its crystals grow much larger than those in basalt. Like basalt, gabbro is typically dark green to black.

 

Pyroclastic Rocks:

 

Pyroclastic rocks form from volcanic debris that is blasted into the air during an eruption, then settles and becomes solid rock. Tuff is the most common example. It is made mostly of ash‑sized particles that have been compacted and cemented together after they fall back to the ground.

If the ash is still hot when it accumulates, the particles can fuse together, creating welded tuff. Even though welded tuff is dominated by tiny glass shards, it can also include larger fragments such as walnut‑sized pieces of pumice or bits of other rocks that were caught up in the eruption.

 

4.5

 

The increase is temperature as the depth increases in the crust is known as the geothermal gradient. Magma forms in three main ways. 

 

First, decompression melting occurs when hot solid rock rises toward the surface and the pressure drops. Lower pressure makes it easier for the rock to melt, even without adding heat. This process is common at divergent plate boundaries, where tectonic plates pull apart and mantle rock ascends. 

 

Second, water-induced melting happens when water lowers the melting point of rock, similar to how salt melts ice faster. At subduction zones, oceanic crust sinks into the mantle and releases water from hydrated minerals. These fluids rise into the hot mantle above, triggering melting and forming magma rich in water and carbon dioxide, which contributes to explosive volcanic eruptions. 

 

Third, melting from heat occurs when hot basaltic magma from the mantle heats the crust above it. In continental settings, this magma often gets trapped beneath low-density crustal rocks, which melt more easily and form felsic (granitic) magma. These felsic magmas can also cause explosive eruptions. Additionally, crustal rocks can melt during continental collisions that form mountain belts. As the crust thickens and some rocks are buried deep, elevated temperatures cause partial melting. However, the resulting magma usually solidifies underground, so volcanism is not typically associated with these collision zones.

 

4.6

 

Volcanoes have been found to extrude lava that varies in composition over time. Therefore, scientists reasoned that magma may change and can be the parent to differing igneous rocks. 

 

Bowen’s reaction series describes how magma cools and forms minerals in a specific sequence based on temperature. Unlike water, which freezes at a single point, mafic magma crystallizes gradually over a range of about 200°C, starting around 1200°C. As it cools, minerals form in a predictable order: olivine appears first, followed by pyroxene, amphibole, biotite, and then minerals like quartz, potassium feldspar, and muscovite at lower temperatures. Early-forming minerals such as olivine remove iron, magnesium, and calcium from the melt, causing the remaining liquid to become richer in silica, sodium, and potassium. If these minerals stay in contact with the melt, they can chemically react and evolve into the next mineral in the series. Although Bowen’s model is based on ideal lab conditions, real-world igneous rocks often show similar patterns, with minerals that crystallize at similar temperatures found together.

 

Assimilation is when rising magma melts and incorporates pieces of the surrounding rock, changing the magma’s composition. It’s basically magma “absorbing” nearby material as it moves through the crust.

 

4.7

 

Partial Melting- “the process by which most igneous rocks melt. Since individual minerals have different melting points, most igneous rocks melt over a temperature range of a few hundred degrees.”

(Tarbuck, Lutgens, and Linneman 123).

 

4.8

 

When magma moves upward through the crust, it pushes aside the older rocks already there, which are called host rocks. As the magma squeezes into these rocks and then cools underground, it forms solid bodies known as intrusions, or plutons. We usually see these only after long periods of uplift and erosion bring them to the surface. Intrusions come in many shapes as some are long and sheet‑like, while others are large and blob‑shaped, and they can either cut across existing rock layers or slide between them. When an intrusion cuts across the structure of the surrounding rock, it’s called discordant; when it forms parallel to the layers, it’s called concordant.

 

Tabular intrusive bodies form when magma forces its way into cracks or weak zones in existing rock. When this magma cuts across the rock layers, the resulting intrusion is called a dike, making it a discordant feature. When the magma instead spreads out between sedimentary layers and stays parallel to them, it forms a sill, which is concordant. In simple terms, dikes act like vertical or steep conduits that move magma upward, while sills spread sideways and thicken as magma collects between layers.