Short answers, pulled from the story.
Igneous rock, also called magmatic rock, is one of the three main rock types and forms through the cooling and solidification of magma or lava. Magma can be derived from partial melts of existing rocks in a planet's mantle or crust, triggered by increases in temperature, decreases in pressure, or changes in composition such as the addition of water.
Intrusive igneous rock solidifies underground, where surrounding country rock insulates the magma and allows slow cooling, producing coarse-grained (phaneritic) rocks such as granite and gabbro. Extrusive igneous rock, also called volcanic rock, forms when magma reaches the surface and cools rapidly, producing fine-grained (aphanitic) or glassy rocks such as basalt and rhyolite.
Igneous and metamorphic rocks together make up 90 to 95 percent of the top 16 kilometres of the Earth's crust by volume. Igneous rocks alone form about 15 percent of the Earth's current land surface, and most of the oceanic crust is composed of igneous rock.
In 1902, American petrologists Charles Whitman Cross, Joseph P. Iddings, Louis V. Pirsson, and Henry Stephens Washington proposed replacing all existing classifications with a quantitative system based on chemical analysis. The scheme was abandoned by the 1960s due to its impracticality in fieldwork, though its concept of normative mineralogy endured.
Feldspars, quartz or feldspathoids, olivines, pyroxenes, amphiboles, and micas are fundamental to the classification of almost all igneous rocks. Silica (SiO2) is the single most important chemical component, and the IUGS recommends classifying rocks by mineral composition using the QAPF diagram whenever possible.
Tungsten, tin, and uranium are commonly associated with granites and diorites. Ores of chromium and platinum are commonly associated with gabbros. These associations reflect the chemistry of fractional crystallization, during which incompatible elements concentrate in late-stage melts that can crystallize as pegmatite.