Vocab

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 Copilot:

How do I tell intrusive from extrusive igneous rocks?

Intrusive vs. extrusive igneous rocks are all about where the magma cooled and how fast it happened. You can reliably tell them apart by looking at texture, crystal size, and sometimes special features.

🪨 Quick takeaway

Intrusive (plutonic) rocks cool slowly underground → large, visible crystals, coarse texture.

Extrusive (volcanic) rocks cool quickly at or near the surface → tiny or no visible crystals, fine or glassy texture.

🧭 How to tell them apart in hand samples

🧱 1. Crystal size is the #1 clue

Intrusive:

Crystals are big enough to see clearly (≥1–2 mm).

Texture is phaneritic (granite, diorite, gabbro).

Looks “speckled” with interlocking mineral grains.

Extrusive:

Crystals are microscopic or absent.

Texture is aphanitic or glassy (basalt, andesite, rhyolite, obsidian).

Surface may look smooth, dull, or glassy.

🧱 2. Porphyritic texture tells you BOTH happened

Since you just looked up porphyritic rocks:

Large crystals (phenocrysts) + fine groundmass = magma started cooling intrusively, then erupted and finished cooling extrusively.

So porphyritic rocks are extrusive overall, but with an intrusive early history.

🧱 3. Vesicles (gas bubbles) = extrusive

If you see:

holes,

tubular voids,

frothy textures (pumice, scoria)

→ These form when gas escapes during eruption.

Intrusive rocks never have vesicles.

🧱 4. Glassy textures = extrusive

If the rock is:

shiny,

black,

conchoidal fracture (like broken glass)

→ That’s obsidian, cooled so fast crystals couldn’t form.

🧱 5. Columnar joints = extrusive

Basalt flows often form hexagonal columns (think Giant’s Causeway).

Intrusive rocks don’t form these.

🧱 6. Overall appearance

Intrusive: chunky, speckled, “granite-like,” minerals easy to pick out.

Extrusive: uniform color, fine-grained, sometimes with scattered phenocrysts.

🧪 Examples you can match to your YouTube volcanic-rock tab

Since you have a tab open on andesite, dacite, rhyolite (all volcanic):

These are extrusive rocks.

They often show porphyritic textures because they start cooling at depth.

Andesite especially tends to have plagioclase phenocrysts in a fine matrix.

If you want, I can walk you through identifying intrusive vs. extrusive using a rock you have in hand, or help you interpret what you’re seeing in that YouTube video.

                                                     Giant's Causeway (basalt... Ireland)       

source: Wikipedia

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                                            source: Shawn Willsey #4

Geology; why are some rocks labelled 'porphyritic'?

Porphyritic describes a texture in igneous rocks — not a rock type — and it tells you something important about how that rock cooled.

🪨 Concise takeaway

A rock is called porphyritic when it contains large, well‑formed crystals (phenocrysts) embedded in a much finer-grained background. This mixed texture means the magma cooled in two stages.

🔍 What “porphyritic” actually means

Phenocrysts — big, easily visible crystals (often feldspar, quartz, amphibole, olivine).

Groundmass / matrix — tiny crystals or even glassy material surrounding them.

The contrast in crystal sizes is the defining feature.

🌡️ Why this texture forms

Porphyritic texture records a two‑step cooling history:

Slow cooling deep underground  

Large crystals begin to grow because atoms have time to arrange themselves into well‑formed crystal structures.

Sudden faster cooling near or at the surface  

The remaining melt cools quickly, forming tiny crystals around the earlier big ones.

This can happen when magma:

starts cooling in a magma chamber, then erupts,

or gets moved upward rapidly by tectonic processes.

🧭 Where you see porphyritic rocks

Porphyritic texture can occur in many igneous rocks, both volcanic and intrusive:

Porphyritic basalt

Porphyritic andesite

Porphyritic rhyolite

Porphyritic granite

It’s a texture, not a composition — so the mineral mix can vary widely.

🧪 Why geologists care

Porphyritic texture is a clue about:

magma movement (it didn’t stay in one place)

cooling rate changes

eruption timing

depth of formation

It’s basically a geological “timeline” preserved in stone.

If you want, I can help you identify porphyritic textures in the volcanic rocks from your YouTube tab or explain how to spot phenocrysts in hand samples.

                                                                 

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Etymology 'plagioclase'

Plagioclase has a wonderfully nerdy etymology that tells you exactly what makes this mineral special.

🪨 Concise takeaway

Plagioclase comes from Greek roots meaning “oblique fracture.”  

It refers to the mineral’s characteristic cleavage planes, which meet at non‑right angles.

🧬 Full etymology breakdown

Greek: plagios (πλάγιος)

Meaning “oblique,” “slanted,” “sideways.”  

This describes the angle between the two cleavage planes.

Greek: klasis (κλάσις)

Meaning “breaking” or “fracture.”  

Used in mineralogy to refer to cleavage.

Put together:

Plagioclase = “oblique‑breaking” feldspar.”

🧭 Why this matters in geology

Plagioclase feldspars have:

Two cleavage planes at about 94° and 86°,

instead of the neat 90° angles seen in orthoclase (a potassium feldspar).

This oblique cleavage is one of the easiest ways to distinguish plagioclase from other feldspars in hand samples.

🧪 Bonus: where the name fits in the mineral family

Plagioclase is the sodium–calcium feldspar series, ranging from:

Albite (Na‑rich)

to Anorthite (Ca‑rich)

All members share the same oblique cleavage that inspired the name.

If you want, I can also break down the etymology of orthoclase, feldspar, or any other mineral name you’re curious about

                                                                                   

Plagioclase is the most common and abundant mineral group in the Earth's crust. Part of the feldspar family of minerals, it is abundant in igneous and metamorphic rock, and it is also common as a detrital mineral in sedimentary rock.^[6][7] It is not a single mineral, but is a solid solution of two end members, albite or sodium feldspar (NaAlSi3O8) and anorthite or calcium feldspar (CaAl2Si2O8). These can be present in plagioclase in any proportion from pure anorthite to pure albite.

source: Wikipedia

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feldspar

Feldspar is a mineral name with a very literal origin: it comes from German and means “field‑spar.” 🪨 Concise takeaway Feldspar derives from German Feldspat — Feld meaning field and Spat meaning a mineral that cleaves easily into flakes. 🧬 Full etymology breakdown Feld (German) → “field” This may refer to: its common occurrence in rocks found in fields, or its appearance as “fields” or patches within granites. These interpretations were noted by early mineralogists in the 18th–19th centuries. Spat (German) → “spar” / “easily cleaved mineral” Spat was a general term for minerals that break into smooth flakes. English later influenced the spelling, shifting Feldspat → feldspar. 🧭 Historical notes Earliest English use: 1757, originally spelled feldspath. Modern spelling feldspar became standard after English influence on the German term. The alternative spelling felspar is now obsolete. 🧪 Why this name fits the mineral Feldspars: are rock‑forming silicate minerals, make up ~60% of Earth’s crust, and have good cleavage, matching the “spar” part of the name. If you want, I can also break down the etymology of orthoclase, microcline, or albite — all of which have equally interesting linguistic histories.

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Are feldspars plagioclase?

Short answer: Some feldspars are plagioclase, but not all. Plagioclase is one branch of the feldspar family. 🪨 Clear breakdown using your open tab From the feldspar page you’re viewing, feldspar is a group of minerals. It has two major sub‑groups: Plagioclase feldspars — the sodium–calcium series (albite → anorthite) Alkali feldspars — the potassium–sodium series (orthoclase, microcline, sanidine) So: ✔️ Plagioclase is feldspar ❌ Feldspar is not always plagioclase 🧭 What counts as plagioclase? The plagioclase series includes (with % anorthite): Albite (0–10%) Oligoclase (10–30%) Andesine (30–50%) Labradorite (50–70%) Bytownite (70–90%) Anorthite (90–100%) These are all triclinic feldspars. 🧭 What counts as non‑plagioclase feldspar? The alkali feldspars: Orthoclase Microcline Sanidine Anorthoclase These are potassium‑rich (or potassium–sodium) feldspars. 🧪 A simple way to remember it Think of feldspar as the big family. Inside it, you have two “siblings”: Plagioclase — Na–Ca Alkali feldspar — K–Na If you want, I can help you identify which type you’re looking at in a rock sample or in the volcanic‑rock video you have open.

source: Wikipedia