Unit 02 · Igneous Rocks & Volcanism
Igneous rock is the record of molten Earth cooling into stone. This unit covers the difference between magma below ground and lava at the surface, why intrusive (plutonic) rock grows coarse crystals while extrusive (volcanic) rock chills to a fine or glassy texture, how Bowen's reaction series orders minerals from ultramafic and mafic through intermediate to felsic, and how a volcano's shape and eruption style follow from the composition of its melt. Mastery means you can hold a rock, read its cooling history from the crystals, and place it in the story of the magma that made it.
| Criterion | Not yet | Approaching | Mastered |
|---|---|---|---|
| Magma, lava & cooling rate | Cannot say where a rock cooled from its appearance. | Names intrusive versus extrusive but not from the evidence in the rock. | Reads crystal size to tell slow, deep cooling from fast, surface cooling and names the setting. |
| Texture & crystal size | Describes a rock as “rough” or “smooth” with no vocabulary. | Labels coarse or glassy textures but cannot link them to cooling. | Uses texture — coarse-grained, fine-grained, glassy, or vesicular — to reconstruct the cooling rate. |
| Composition & Bowen's reaction series | Cannot order minerals or tell felsic from mafic. | Sorts felsic and mafic by color but ignores mineral content. | Uses Bowen's reaction series to rank a melt from ultramafic through mafic and intermediate to felsic. |
| Rock identification (granite, basalt, obsidian, pumice) | Confuses common igneous rocks with one another. | Names some rocks but not from texture and composition together. | Identifies granite, basalt, obsidian, and pumice from texture and composition and justifies each call. |
| Volcano type & eruption style (lab) | Treats every volcano as the same cone. | Names shield, composite, and cinder-cone forms but not why they differ. | Reads eruption style — gentle or explosive — from a rock's composition and matches it to shield, composite, or cinder-cone form. |
| Integration (cross-domain) | Treats the science as isolated facts; makes no cross-domain connection. | Names a link to history, reading, or writing but cannot defend why it matters. | Connects the unit to its anchor across History · Reading · Writing (plus chosen electives) and defends why the connection matters. |
“This is basalt — it’s dark, fine-grained, and full of tiny gas holes, so it cooled fast at the surface from a mafic, iron-rich lava. That low-silica melt is runny, which is why it built a broad shield volcano instead of an explosive cone.”
“It’s some kind of black rock. Volcanoes just explode. I think the big crystals mean it’s older.”
You demonstrate this unit at the specimen bench — classifying real igneous rocks with a hand lens, sorting them by texture and composition, and explaining each rock's cooling history and volcanic origin aloud — not a multiple-choice test. A criterion counts as mastered only when you can both identify the rock and defend the cooling story the crystals record. Mastery is demonstrated, not awarded.
A 5-page clipboard packet — unit overview, key terms, the mastery rubric, anchor examples, and a score sheet you can print and grade against.