Volcanic rocks form when lava or ash cools at the Earth’s surface, and there are really only seven you’ll meet: basalt, andesite, dacite and rhyolite (the crystalline four, ordered by silica), plus obsidian, pumice and scoria. One number — silica content — controls their color, their weight, and how violently the lava that made them erupted. Here’s how to tell them apart, and which real volcanoes build each one.
By VolcanoDB Research Team. Data: Smithsonian Global Volcanism Program (dominant-rock classification across 1,740 volcanoes), USGS.
Common types
7
Most common dominant rock
Andesite (536)
Basalt-dominant volcanoes
408
Only one floats
Pumice
What Makes a Rock "Volcanic"?
Volcanic rocks are extrusive igneous rocks — they crystallize from magma that reached the surface as lava or ash. That single fact, cooling at the surface instead of deep underground, is what separates them from their intrusive twins. Granite and basalt can share nearly identical chemistry, but granite cools slowly kilometers down and grows big, interlocking crystals you can see, while basalt cools in the open air and freezes fine-grained. Same recipe, different oven.
You’ll find volcanic rock at every one of the volcano types in our database — but not the same rock everywhere. A shield volcano like Kīlauea erupts basalt; a stratovolcano in the Andes erupts andesite; a supervolcano caldera erupts rhyolite. The rock is a receipt for how the volcano behaves.
The One Variable That Explains Everything: Silica
If you remember one thing, make it this. The amount of silica (SiO₂) in a magma controls three things at once. First, color: more silica means a paler rock, which is why basalt is black and rhyolite is cream-pink. Second, viscosity: silica makes magma sticky, so low-silica basalt flows like hot syrup while high-silica rhyolite barely moves. Third, and most important for anyone living nearby, explosivity: sticky, high-silica magma traps gas until it erupts violently, while runny basalt lets gas escape and tends to flow. Read a rock’s silica off its color, and you’ve read the volcano’s temper.
Cooling rate and gas content add the finishing texture. Fast cooling with no time to crystallize gives you glassy obsidian. Lots of trapped gas gives you the frothy, hole-riddled textures of pumice and scoria. The chemistry sets the type; the cooling sets the look.
The Seven Volcanic Rocks, One by One
Basalt
Silica 45–52%
Basalt is the workhorse of the planet. It's the rock of the entire ocean floor, the Hawaiian volcanoes, and nearly every flood-basalt province on Earth. Low silica means the lava is runny, so basaltic eruptions tend to flow rather than explode. Up close it's a dark, heavy, fine-grained rock — the crystals are too small to see without a hand lens because the lava cooled quickly at the surface. When it traps gas bubbles as it solidifies you get vesicular basalt, full of little holes.
Color: Black to dark gray
Texture: Fine-grained, dense, sometimes vesicular
Forms at: Shield volcanoes, mid-ocean ridges, hotspots, flood-basalt provinces
Examples: Kīlauea, Mauna Loa, all of Iceland, the Columbia River Basalts
408 volcanoes in our database list basalt as their dominant rock.
Andesite
Silica 52–63%
Andesite is named after the Andes, and it's the signature rock of subduction-zone stratovolcanoes. Here's a data point worth sitting with: of the 1,740 volcanoes we track, andesite (and its close cousin basaltic andesite) is the most common dominant rock — 536 volcanoes. That's a direct fingerprint of plate tectonics, because most of the world's volcanoes sit where one plate dives beneath another. Andesite lava is stickier than basalt, so andesite volcanoes are the ones that build tall cones and erupt explosively.
Color: Gray to gray-brown
Texture: Fine-grained groundmass with visible crystals (porphyritic)
Forms at: Stratovolcanoes above subduction zones
Examples: Most of the Andes, Cascades and Ring of Fire cones
536 volcanoes — the single most common dominant rock in our database.
Dacite
Silica 63–68%
Dacite sits between andesite and rhyolite. It's the rock that built the lava dome inside Mount St. Helens' crater after 1980 — thick, pasty lava that piles up instead of flowing. Because dacite magma is viscous and gas-rich, it's behind some of the most dangerous eruptions on record. Pinatubo's 1991 VEI 6 was a dacite eruption. It tends to be pale, and you can often pick out chunky crystals of quartz and feldspar in a lighter groundmass.
Color: Light gray to tan
Texture: Porphyritic, often with visible quartz and feldspar
Forms at: Lava domes and explosive stratovolcanoes
Examples: Mount St. Helens, Mount Pinatubo, many lava domes
81 volcanoes have dacite as their dominant rock.
Rhyolite
Silica 68–77%
Rhyolite is the volcanic equivalent of granite — same chemistry, just cooled fast at the surface instead of slowly underground. It's the rock of supervolcanoes. Yellowstone's caldera-forming eruptions were rhyolitic, and so was Long Valley's Bishop Tuff. High silica makes rhyolite magma extraordinarily viscous — it can barely move — which is exactly why rhyolitic systems store enormous volumes of gas-charged magma and, occasionally, blow catastrophically. When rhyolite lava does ooze out, it forms steep domes and stubby, glassy flows.
Color: Light gray, pink, cream
Texture: Very fine-grained, sometimes glassy or banded
Forms at: Calderas and supervolcanoes
Examples: Yellowstone, Long Valley, Lipari (Italy)
56 volcanoes are rhyolite-dominant.
Obsidian
Silica ~70%+ (glass)
Obsidian isn't a mineral at all — it's natural glass, formed when high-silica lava cools so fast the atoms never organize into crystals. That's why it breaks with the same razor-sharp conchoidal fracture as a broken bottle. Humans have knapped it into blades for at least 700,000 years, and surgeons still use obsidian scalpels because the edge is finer than surgical steel. It's harder than glass but softer than quartz, roughly 5–5.5 on the Mohs scale. Look for it around the edges of rhyolite flows — the Glass Mountain flow at Medicine Lake is a classic.
Color: Jet black, sometimes mahogany or sheen
Texture: Smooth glass, conchoidal fracture, no crystals
Forms at: Margins and surfaces of rhyolitic lava flows
Forms at high-silica volcanoes like Medicine Lake and Lipari.
Pumice
Silica High (rhyolitic–dacitic)
Pumice is the only rock most people will ever see float. When gas-rich, high-silica magma is blasted into the air, it foams and freezes mid-flight into a solid full of frozen bubbles — so full of holes that the trapped air keeps it buoyant. After the 1883 Krakatau eruption, rafts of floating pumice drifted across the Indian Ocean for months, some big enough to strand ships. On the practical side, it's the grit in your exfoliating scrub, the drainage layer in potting soil, and the aggregate in lightweight concrete.
Color: Off-white, cream, pale gray
Texture: Frothy, full of tiny bubbles, light enough to float
Forms at: Explosive Plinian eruptions of viscous magma
Examples: Krakatau 1883, Pinatubo 1991, Santorini
Produced by explosive VEI 4+ eruptions in our database.
Scoria
Silica Low–intermediate (basaltic–andesitic)
Scoria is pumice's heavier, darker cousin. Same idea — a rock full of gas holes — but it forms from low-silica basaltic to andesitic magma, so it's denser and it sinks. The classic red and black 'lava rock' sold for landscaping and gas-grill beds is scoria. It's what builds cinder cones: each Strombolian burst throws out molten blobs that cool into scoria and pile into the neat conical hills you see across volcanic fields.
Color: Dark red, brown, black
Texture: Vesicular but denser than pumice — it sinks
Three tests — color, texture, and weight — separate all seven. Keep a bucket of water handy for the pumice test; it’s the fastest positive ID in geology.
Rock
Color
Texture
Weight test
Dead giveaway
Basalt
Black / dark gray
Fine-grained, dense
Heavy
Runny-lava rock; may have small gas holes
Andesite
Medium gray
Crystals in fine groundmass
Medium
Speckled look from feldspar crystals
Dacite
Light gray / tan
Chunky quartz + feldspar
Medium
Pale, visible glassy quartz grains
Rhyolite
Pink / cream
Very fine, sometimes banded
Medium
Palest of the crystalline rocks; flow bands
Obsidian
Jet black
Smooth glass
Medium
Sharp curved fractures, no crystals, shiny
Pumice
Off-white
Frothy holes
Floats
So light it floats on water
Scoria
Red / black
Vesicular
Light but sinks
Gas holes but darker and denser than pumice
Which Volcanoes Make Which Rocks
The rock and the volcano are two views of the same thing. If you know the volcano type, you can predict the rock — and vice versa:
Stratovolcanoes → andesite and dacite. The steep, explosive cones of the Ring of Fire run on stickier, higher-silica magma.
Cinder cones → scoria. Each Strombolian burst tosses out molten blobs that cool into the scoria that builds the cone.
Calderas and supervolcanoes → rhyolite and pumice.Yellowstone and Italy’s Lipari are rhyolitic systems; explosive rhyolite eruptions carpet the ground in pumice and tephra.
Want to see where each type clusters geographically? The interactive volcano map plots all 1,740 volcanoes — the basalt shields trace the ocean hotspots and rifts, while the andesite-dacite cones line up along subduction margins.
Volcanic Rocks vs. Everything Else
Volcanic rocks are one branch of igneous rocks — the ones that cooled at the surface. Their underground counterparts (granite, gabbro, diorite) are intrusive igneous rocks with the same chemistry but big visible crystals. Both differ entirely from sedimentary rocks (sandstone, limestone — built from deposited grains) and metamorphic rocks (marble, slate — cooked and squeezed under heat and pressure). The quickest tell for a volcanic rock: fine grains or glass or gas holes, and often a dark color — the signature of fast cooling at the surface.
What Volcanic Rocks Are Used For
These rocks aren’t just scenery. Crushed basalt is one of the most common construction aggregates on Earth and is spun into basalt fiber for rebar and insulation. Scoria is the red and black landscaping "lava rock" and the bed in your gas grill. Pumice is the grit in exfoliating scrubs, a drainage additive in horticulture, and an aggregate in lightweight concrete. Obsidian goes into jewelry and precision blades. If you’re building a collection, a basic rock specimen kit and a geology field guide will get you identifying most of these in the field — start with the color-texture-weight table above.
Frequently Asked Questions
What are the main types of volcanic rocks?
The core volcanic (extrusive igneous) rocks are basalt, andesite, dacite and rhyolite — arranged by increasing silica content — plus the glassy and frothy varieties: obsidian (volcanic glass), pumice (floats) and scoria (dark and vesicular). Silica content is the master control: it sets a rock's color, the stickiness of the lava it came from, and how explosively that lava erupts.
What is the most common volcanic rock?
It depends on how you count. By sheer volume, basalt is the most common volcanic rock on Earth because it paves the entire ocean floor — roughly 70% of the planet's surface. But among the world's individual volcanoes, andesite is the most common dominant rock: 536 of the 1,740 volcanoes in our database are andesite-dominant, because most volcanoes sit above subduction zones.
Can pumice really float on water?
Yes. Pumice is so full of frozen gas bubbles that trapped air makes it less dense than water. After the 1883 Krakatau eruption, floating pumice rafts drifted across the Indian Ocean for months. Pumice eventually waterlogs and sinks once the bubbles fill, but fresh pieces float readily — it's the easiest volcanic rock to identify with a bucket of water.
What is the difference between pumice and scoria?
Both are vesicular (full of gas holes), but pumice forms from high-silica, light-colored magma and is light enough to float, while scoria forms from low-silica basaltic to andesitic magma and is darker, denser, and sinks. The red and black 'lava rock' sold for landscaping and gas grills is scoria.
Is obsidian a rock or a mineral?
Obsidian is a rock — specifically a natural glass. It has no crystal structure because high-silica lava cooled too fast for minerals to form, so it isn't a mineral in the strict sense. That glassy structure is why it breaks with a razor-sharp conchoidal fracture and why it's been used for cutting tools for hundreds of thousands of years.
How can you identify a volcanic rock?
Start with three quick checks: color (dark = low silica like basalt; pale = high silica like rhyolite), texture (glassy obsidian, frothy pumice/scoria, or fine-grained crystalline), and weight (only pumice floats; scoria is light but sinks; basalt is notably heavy). Those three tests separate the seven common volcanic rocks in seconds.