Right. So, you want to understand igneous rocks? The ones that claw their way out of magma? Fine. It's a branch of geology, obviously, and it tangles itself up with volcanology, tectonophysics, and the whole messy business of petrology. Modern study, if you can call it that, leans on techniques pilfered from chemistry, physics, and whatever other earth sciences are willing to lend their tools. Petrography, crystallography, and isotopic studies are the usual suspects. Don't expect them to be chatty.
Methods
Determination of Chemical Composition
Figuring out what these rocks are made of is… a process. You can start by just looking at them, with your eyes or a hand lens. Primitive, I know, but it gives you a general idea of the mineralogical makeup. If you need more precision, but still don't want to break the bank, there's the petrographic microscope. It's got polarizing plates, filters, and a conoscopic lens – useful for measuring crystallographic properties.
Then there's X-ray diffraction. You powder a sample, blast it with X-rays, and compare the resulting spectrum to a library of standards. For the truly obsessive, or those with something to prove, there's the electron microprobe. It samples tiny spots, providing both bulk and trace element composition. It's precise. Almost annoyingly so.
Dating Methods
Ah, radiometric dating. The attempt to pin down when molten rock decided to freeze. It’s all about radiogenic isotopes.
Potassium–Argon Dating
This one's a classic. You measure the argon-40 trapped in the rock and compare it to the potassium-40 still hanging around. The ratio tells you how long the potassium-40 has been decaying, spewing out that argon-40. Simple, in its own way.
Rubidium–Strontium Dating
This method hinges on the decay of rubidium-87 into strontium-87. The trick is that these elements behave differently during fractional crystallization. Strontium likes to clump into plagioclase crystals, while rubidium tends to linger in the melt. Over time, rubidium-87 decays into strontium-87. Given the decay constant, and the amounts of both isotopes, you can calculate the time since the rock reached its closure temperature. Of course, you have to account for the initial strontium-87 that was already there. Sometimes, they estimate this by comparing rocks from the same magmatic body formed at different times. It’s a whole convoluted dance.
Other Methods
Sometimes, you don't need fancy isotopes. Stratigraphic principles can give you the relative age of volcanic rocks. Tephrochronology, which is essentially dating volcanic ash layers, is a common application. It’s less about when and more about before or after.
Thermobarometry Methods
In petrology, the mineral clinopyroxene is surprisingly useful. It can tell you about the temperature and pressure conditions of the magma that birthed the igneous rock. It’s one of several geothermobarometers. Why is it so useful? For one, clinopyroxene is a common phenocryst, easy to spot. For another, the crystallization of the jadeite component within it is sensitive to pressure because it involves a change in molar volume. It's a subtle indicator, but it's there.
Thermochronometry
This is another way to get at the thermal history of rocks. It’s about how long it takes for certain isotopes to diffuse out of minerals as they cool. More about the cooling story.
Publications
If you want to see the cutting edge of igneous petrology, you'll find it in American and British scientific journals like Science and Nature. They’re the usual suspects. For overviews and older research, you’ll have to delve into books. Just be aware that much of the work predating the plate tectonics paradigm shift in the late 20th century might be… optimistic, or simply wrong, about magma origins.