Introduction to Igneous Rocks
Welcome to the study of igneous rocks! These are often called "fire rocks" because they form from the cooling and solidification of molten rock. Whether it is the dramatic eruption of a volcano or the slow cooling of magma deep beneath your feet, igneous rocks tell us the story of Earth's internal heat. In this chapter, we will learn how to identify these rocks and what their textures and minerals tell us about how and where they formed. Don't worry if the names seem a bit strange at first—once you see the patterns, it all clicks together!1. Magma vs. Lava: The Starting Point
Before a rock becomes a rock, it is molten (liquid).- Magma: Molten rock found underground.
- Lava: Molten rock that has reached the Earth's surface.
Quick Review: The Rock Cycle Connection
Igneous rocks are often considered the "parent" rocks because they form directly from cooling melt. In the rock cycle, they can later be broken down into sediment or changed by heat and pressure into metamorphic rocks.2. Classification by Grain Size (The "Cooling Rate" Rule)
One of the most important things geologists look at is the size of the crystals (grains) in the rock. The size of the crystal tells us exactly how fast the rock cooled.The Cooling Analogy
Think of crystals like people trying to find a seat in a dark cinema. If they have plenty of time (slow cooling), they can find their friends and form large groups (large crystals). If the lights suddenly come on and they have to sit down immediately (fast cooling), they sit wherever they are, and no large groups can form (small crystals). The syllabus defines three specific size categories:- Coarse-grained: Crystals are greater than 5 mm in diameter. These cooled very slowly, usually deep underground in plutonic (intrusive) settings.
- Medium-grained: Crystals are 1 mm to 5 mm in diameter. These cooled at a moderate rate, often in smaller hypabyssal (minor intrusive) bodies like dykes or sills.
- Fine-grained: Crystals are less than 1 mm in diameter. These cooled very quickly at the surface in volcanic (extrusive) settings.
3. Classification by Composition (The "Chemistry" Spectrum)
We also group igneous rocks by what they are made of. This is mostly about how much silica (\(SiO_{2}\)) they contain.- Silicic (or Felsic): High silica content. These rocks are usually light-coloured (pinks, whites, pale greys). They contain minerals like quartz and feldspar.
Example: Granite. - Intermediate: A mix between silicic and mafic. Usually medium grey or "salt and pepper" looking.
Example: Diorite. - Mafic: Low silica content, high in magnesium and iron. These are dark-coloured (black or dark grey). They contain minerals like pyroxene and olivine.
Example: Basalt. - Ultramafic: Very low silica. Often look greenish because they are almost entirely made of the mineral olivine.
Example: Peridotite.
Memory Aid: The "Lighter is Lighter" Rule
Silicic rocks are light in weight (density) and light in colour. Mafic rocks are heavy (dense) and dark in colour.4. The Igneous Rock Identification Grid
You can identify almost any igneous rock by combining its grain size and its composition. Use this mental table:
Silicic: Coarse = Granite | Medium = Microgranite | Fine = Rhyolite
Intermediate: Coarse = Diorite | Medium = Microdiorite | Fine = Andesite
Mafic: Coarse = Gabbro | Medium = Dolerite | Fine = Basalt
Ultramafic: Coarse = Peridotite
5. Igneous Textures and What They Mean
Texture isn't just how a rock feels; it's the arrangement and shape of the crystals.Common Textures to Know:
- Equicrystalline: All the crystals are roughly the same size. This means the rock cooled at a steady, constant rate.
- Porphyritic: Large crystals (called phenocrysts) are surrounded by much smaller crystals (the groundmass).
What it tells us: The rock had a two-stage cooling history. It started cooling slowly underground (forming the big ones), then erupted or moved higher and cooled quickly (forming the small ones). - Vesicular: The rock is full of holes (vesicles).
What it tells us: These are gas bubbles that were trapped as the lava solidified. This only happens in volcanic rocks. - Amygdaloidal: This happens when the holes (vesicles) in a vesicular rock get filled in later by minerals like calcite or quartz deposited by groundwater.
- Glassy: No crystals are visible at all, even under a microscope.
Example: Obsidian. This happens when lava cools instantaneously (usually in water), giving atoms no time to arrange into crystals. - Flow Banding: Visible layers or streaks produced by the movement of thick, viscous lava. Common in Rhyolite.
Did you know?
Obsidian looks black, which usually means "mafic," but it is actually silicic! It is dark because of tiny amounts of iron impurities. It's one of the few exceptions to the "dark = mafic" colour rule.6. Representing Rocks: Drawings and Diagrams
In your practical work (PAGs), you will be asked to draw igneous textures.- Crystal Shape: Note if crystals are euhedral (perfect shapes), subhedral (okay shapes), or anhedral (blobs with no clear faces).
- Annotation: Always label your phenocrysts vs. groundmass in a porphyritic rock.
- Scale: Always include a scale bar (e.g., 1 cm) so the reader knows the crystal size!
7. Measuring Temperature
Geologists need to know how hot magma was. While we can't always stick a thermometer in a volcano, we use:- Direct observation: Using thermocouples or optical pyrometers on active lava flows.
- Mineral evidence: Certain minerals only form at specific temperatures. By looking at the "mix" of minerals (like in Bowen’s Reaction Series), we can estimate the starting temperature.
Summary: Common Mistakes to Avoid
1. Don't confuse "Crystal" with "Grain": In igneous rocks, we usually say "crystals." "Grains" is more common for sedimentary rocks, though both are used.
2. Don't assume all dark rocks are the same: Basalt (fine) and Gabbro (coarse) have the same chemistry but look very different because of their cooling rates.
3. Two-stage cooling: Remember that Porphyritic texture is the "smoking gun" for a rock that changed its environment while cooling.