Classify igneous rocks as mafic or felsic and by texture, use Bowen's Reaction Series to explain their minerals, and distinguish intrusive structures from extrusive volcanoes

Learning goal: Classify igneous rocks and minerals as felsic, intermediate, mafic or ultramafic; explain how cooling rate controls crystal size and texture; use Bowen's Reaction Series, with its continuous and discontinuous branches, to explain which minerals crystallize together and to identify the origin of a rock; distinguish intrusive igneous structures (dikes, sills, batholiths, pegmatites) from extrusive volcanic activity; relate magma composition to the shape and behavior of cinder cones, composite volcanoes and shield volcanoes; and judge whether rock types make a site suitable for a tunnel or road.

Where this lesson sits. Ohio's PG.IMS.1 (Igneous) lists seven topics, and this site teaches them in two lessons meant to be studied together. This lesson is part 1 of 2: mafic and felsic rocks and minerals, Bowen's Reaction Series with its continuous and discontinuous branches, intrusive structures (dikes, sills, batholiths, pegmatites) and extrusive volcanic activity (cinder cones, composite and shield volcanoes). Part 2, the lesson on Earth's interior, thermal energy within Earth and magnetic reversals, teaches the other three topics. Neither part alone covers PG.IMS.1.

What an igneous rock is. An igneous rock forms when molten rock cools and solidifies. Below ground the melt is called magma; once it reaches the surface it is lava. Magma exists because of thermal energy within the Earth, left from the planet's formation and released by radioactive decay; the companion lesson explains how that heat, together with falling pressure or added water, melts solid rock. Magma is not pure liquid: it is a silicate melt carrying dissolved gases (mostly water vapor and carbon dioxide, with some sulfur gases) and, usually, crystals that have already begun to grow. Two independent questions classify every igneous rock. What was the melt made of? That sets the minerals and the color. How fast did it cool? That sets the size of the crystals, the texture. Mixing these two questions up is the most common error in this unit.

Composition: mafic and felsic. Geologists sort igneous rocks mainly by their silica (SiO₂) content. Felsic (from feldspar and silica) rocks contain more than about 65% silica and are rich in sodium, potassium and aluminum; their minerals are quartz, potassium feldspar, sodium-rich plagioclase and muscovite or biotite mica, and they are usually light in color and relatively low in density (about 2.7 g/cm³). Mafic (from magnesium and ferric, iron) rocks contain about 45 to 52% silica and are rich in magnesium, iron and calcium; their minerals are pyroxene, calcium-rich plagioclase and sometimes olivine, and they are dark and denser (about 3.0 g/cm³). Intermediate rocks lie between, with amphibole, biotite and plagioclase of middle composition. Ultramafic rocks, below about 45% silica, are made mostly of olivine and pyroxene; peridotite, the rock of the upper mantle, is the example. The exact boundaries differ by a few percent between textbooks, so a sample near a boundary is described as "near the felsic–intermediate boundary" rather than forced into one box. Color is a useful first guess but not proof: obsidian is felsic yet black, because a trace of iron-oxide dust darkens glass, so composition is confirmed by minerals or chemical analysis.

Silica content also controls how a magma behaves. Silicon-oxygen tetrahedra link into chains and networks in the melt, so a silica-rich magma is far more viscous, stiffer, than a silica-poor one. Felsic magmas are also cooler (roughly 650 to 800 °C) than basaltic magmas (roughly 1,000 to 1,200 °C), which makes them stiffer still. Gas escapes easily from runny basaltic lava but is trapped in sticky felsic magma until the pressure is released violently. That single chain of cause and effect explains why eruptions differ.

Texture: cooling rate. Crystals need time to grow, because ions must travel through the melt to the crystal surface. Magma that cools slowly, kilometers underground and insulated by surrounding rock, grows a few large crystals: a coarse-grained (phaneritic) texture, crystals visible to the eye. Lava that cools in days or years at the surface grows many tiny crystals: a fine-grained (aphanitic) texture. Thin dikes and sills close to the surface also cool quickly and can be fine-grained, so a fine grain shows fast cooling, most often but not always at the surface; the shape of the body and its contacts decide whether it was extrusive. Lava quenched in seconds or minutes, often at the edge of a flow or in water, has no time to crystallize at all and forms volcanic glass; felsic glass is obsidian, and basaltic glass forms thin rinds on pillows and flows. Gas bubbles frozen into the rock make a vesicular texture, which records escaping gas rather than a particular cooling rate: pumice (felsic, so light it can float) and scoria (mafic, darker and heavier). A porphyritic texture, large crystals called phenocrysts set in a fine groundmass, usually records two stages: slow cooling at depth, then rapid cooling after the magma rose. (Less often the large crystals were picked up from older rock, so the two-stage reading is the usual one, not the only one.) A pegmatitic texture, crystals centimeters to meters long, grows from the last, water-rich fraction of a magma, where water lets ions move quickly. Combining the two questions names the rock: felsic coarse is granite, felsic fine is rhyolite; intermediate coarse is diorite, fine is andesite; mafic coarse is gabbro, fine is basalt. Granite and rhyolite have the same composition; only their cooling histories differ.

Bowen's Reaction Series. In laboratory experiments in the early twentieth century, Norman Bowen cooled basaltic melts and recorded the order in which minerals crystallized. His results are summarized as two branches that join. On the discontinuous branch, a series of different minerals appears as temperature falls: olivine first, then pyroxene, then amphibole, then biotite. Each has a different silicate structure (isolated tetrahedra, single chains, double chains, sheets), and each earlier mineral reacts with the remaining melt to form the next if the crystals stay in contact with it; the changes are discontinuous because the structure jumps. On the continuous branch, plagioclase feldspar crystallizes throughout, keeping the same structure but changing its composition continuously from calcium-rich at high temperature to sodium-rich at lower temperature. The branches merge at the lowest temperatures, where potassium feldspar, muscovite and finally quartz crystallize. The course asks you to use the series, not memorize it, and its use rests on one idea: minerals that crystallize at similar temperatures are found together, and minerals far apart on the chart are not. Basalt and gabbro contain the high-temperature minerals (olivine, pyroxene, calcium plagioclase); granite contains the low-temperature ones (quartz, potassium feldspar, sodium plagioclase, micas). Finding olivine and abundant quartz in the same igneous rock would be a signal to recheck the identification. If early crystals settle out and are removed, the remaining melt is richer in silica than the original, which is how a basaltic magma can produce a small volume of more felsic melt, a process called fractional crystallization. The misconception to avoid is that every magma runs through the whole series: most basaltic magma solidifies long before it reaches quartz.

Here is a model worked through. A hand specimen has interlocking grains 3 to 8 mm across: glassy gray grains that scratch glass and show no cleavage (quartz, about a quarter of the rock), pink blocky grains with two cleavages (potassium feldspar), white grains with fine parallel striations (sodium-rich plagioclase) and small black flakes that peel (biotite). Texture first: all grains visible and interlocking, so it cooled slowly underground and is intrusive. Composition next: quartz, potassium feldspar and sodium plagioclase all sit at the low-temperature end of Bowen's series, so the melt was felsic and crystallized at comparatively low temperature. Name: granite. Origin: part of a large body of magma that cooled kilometers down and now appears at the surface only because the rock above it was eroded away.

Intrusive structures. Magma that solidifies underground forms plutons, named by shape and by their relationship to the layers they invade. A dike is a sheet-like body that cuts across existing layers (discordant). A sill is a sheet that slides between layers, parallel to them (concordant). A batholith is an enormous body of many merged plutons, by convention exposed over more than 100 km², usually granitic, forming the cores of mountain ranges such as the Sierra Nevada. A pegmatite is a dike or pod of very coarse crystals from the last water-rich melt, and it is mined for feldspar, mica, lithium minerals and gemstones. Every intrusion is younger than the rock it cuts, a fact the rock-record lesson relies on.

Extrusive activity and volcano shape. Lava flows, fragments blown out as ash, cinders and bombs, and gases are all extrusive products. Shield volcanoes are built from thousands of runny basaltic flows that travel far before stopping, so their slopes are gentle, a few degrees; Mauna Loa and Kīlauea in Hawaii are examples, and their eruptions are usually effusive. Cinder cones are small, steep cones, their sides at about 25 to 32°, the angle at which loose cinders rest, built of gas-rich fragments, often in a single eruptive episode; Parícutin in Mexico began in a cornfield in 1943. Composite volcanoes (stratovolcanoes) are tall cones of alternating lava flows and ash layers fed by intermediate to felsic magma, and they erupt explosively; Mount St. Helens (1980) and Mount Fuji are examples, and most stand above subduction zones. Ohio's bedrock at the surface contains no igneous rock at all: the granite, basalt and gabbro boulders in Ohio fields are glacial erratics carried from Canada, and igneous and metamorphic basement rock lies buried beneath the sedimentary layers.

Worked example, naming by composition and texture: Three hypothetical samples. Sample A: 49% silica, crystals too small to see, a few gas holes. Silica 45 to 52% is mafic; invisible crystals mean fast cooling, and the gas holes point to lava at the surface; the rock is basalt (vesicular basalt, because of the holes). Sample B: 72% silica, grains 2 to 5 mm. Above 65% is felsic; visible grains mean slow cooling, normally at depth; granite. Sample C: 60% silica, white plagioclase crystals 4 mm long scattered through a gray groundmass too fine to see. Intermediate composition, most likely two-stage cooling; andesite porphyry. The reasoning always uses two separate pieces of evidence, one for composition and one for cooling.

Worked example, using Bowen's series to identify an origin: A dark, coarse rock contains pyroxene, calcium-rich plagioclase and a little olivine. All three crystallize at the high-temperature end of the series, so the melt was mafic and hot, and the coarse grains mean it cooled slowly underground: gabbro, perhaps from the lower oceanic crust or a mafic intrusion. Quartz is absent, as the series predicts; a mafic melt normally solidifies completely before reaching the quartz stage. A second rock is light-colored, fine-grained and contains quartz phenocrysts. Quartz is a lowest-temperature mineral, so the melt was felsic: rhyolite porphyry, erupted or intruded near the surface after some crystals had grown at depth.

Worked example, dike or sill or buried lava flow: In a hypothetical outcrop, a 3 m thick body of basalt cuts across sandstone and shale layers at 70°: it is discordant, so it is a dike, and it is younger than every layer it cuts. A second basalt body lies parallel between two sandstone layers. It could be a sill or a lava flow that was later buried. Check both contacts. A sill bakes the rock above and below it and may contain broken pieces of both. A lava flow bakes only the rock beneath it; its top may be weathered or bubbly, and the sandstone above may contain eroded pebbles of the basalt. Baking on both sides here would make it a sill, younger than the layer above it.

Worked example, volcano shape from slope: A hypothetical cone rises 300 m over a horizontal distance of 500 m from its base to its summit. tan θ = 300 ÷ 500 = 0.6, so θ ≈ 31°: the angle of loose cinders, consistent with a cinder cone. A second volcano rises 4 km over 80 km. tan θ = 4 ÷ 80 = 0.05, so θ ≈ 2.9°: gentle slopes built by fluid basaltic flows, a shield volcano. The slope is evidence of the magma's viscosity.

Investigation, cooling rate and crystal size: Materials: phenyl salicylate (salol, which melts at about 42 °C), two glass microscope slides, a warm-water bath at about 50 °C, a tray of ice, a hand lens or microscope, goggles and gloves. The teacher melts the salol in a test tube in the warm bath. Warm one slide in the bath and chill the other on ice; place one drop of melt on each and cover with a second slide. If no crystals appear on a slide within a minute (salol easily stays liquid below its melting point), touch the drop with a tiny seed crystal of solid salol on a toothpick. Record, every 30 seconds for five minutes, the number of crystals you can count in a 1 cm square and the length of the three largest crystals. Result that supports the claim: the chilled slide grows many small crystals quickly; the warm slide grows fewer, larger crystals. What the model cannot show: a real magma is a mixture of many compounds crystallizing in sequence, contains dissolved gas, and cools over thousands of years, not minutes. Wash hands afterwards.

Representation task, a dichotomous key: Build a key for eight igneous specimens (granite, rhyolite, diorite, andesite, gabbro, basalt, obsidian, pumice). Each step asks one yes-or-no question about a single observable property (Are the crystals visible? Is the rock mostly light-colored? Is it glassy? Does it have holes?). Test the key on each specimen and record the path it takes.

Independent performance, a tunnel or road decision: A county plans a 400 m road tunnel through a hill. Borehole logs (hypothetical) show massive granite in the western half, with fractures about 2 m apart and a laboratory compressive strength of about 150 MPa, and a zone of welded volcanic tuff cut by three basalt dikes in the eastern half, with water seeping along the dike contacts. Tuff is rock made of volcanic ash; in a welded tuff the ash landed hot enough for its glassy fragments to fuse, so it is a solid rock, but it is usually weaker and more variable than granite. Here the logs give the tuff a strength of about 40 MPa, cooling joints about 0.3 m apart and soft clay where it has weathered along the joints. A rock's name does not fix its strength, which is why the logs are given. Write a recommendation of at least 250 words. Success criteria: it names each rock and explains its texture and cooling history; it uses the borehole data to compare the strength, fracturing and ease of excavation of the granite and the tuff; it explains why dike contacts are planes of weakness and water flow; it identifies the section needing extra support or drainage; and it reaches a stated decision supported by that evidence.