Around the planet, there are approximately 1300 active volcanoes, of which 46 are erupting
right now.
These volcanoes have a wide range of shapes and sizes, in addition to a range of eruption
types which is partially owed to the lava they produce.
Lavas can cool as a dark black color, sometimes be filled with numerous green olivine crystals,
or even be a red or white color.
In fact, there are 15 types of lava with a wide range of appearances.
To help differentiate between these types of extrusive rock, I have created this video.
And as a reminder, this video will only cover types of lava that erupt onto the surface
and not molten rock which remains underground.
To understand these varieties of lava, I present the TAS or total alkali versus silica diagram.
The x axis shows the amount of silica or silicon dioxide a type of lava has, while the y axis
shows the amount of alkali compounds a lava has, which in this case is disodium oxide
and dipotassium oxide.
Generally speaking, the less silica a volcano's lava has, the less viscous its lava flows
will be, resulting in faster moving lava flows via effusive eruptions that often build broad
and gentle slopes.
In contrast, more silica rich lavas tend to build steep sided lava domes due to their
high viscosity and result in highly explosive eruptions.
The most common type of lava is known as basalt, and is distinguished by its dark black color,
typically being found at shield volcanoes such as Kilauea in Hawaii.
Basalt often contains numerous bubbles termed vesicles, occasionally contains green crystals
known as olivine, and often retains the typical shape characteristic of pahoehoe or a'a' lava
flows.
In comparison, picrobasalt or picrite basalt is a less silica rich rock which contains
more green olivine crystals, typically making up at least 20% of the rock.
Trachybasalt contains both white colored rectangular plagioclase feldspar crystals and orange-pink
alkali feldspar crystals, although the former are not always visible as large crystals.
Trachybasalt is common at volcanoes with moderately explosive eruptions such as Mount Etna.
Tephrite and basanite fill the same place in the diagram but have one difference.
Basanite has greater than 10% olivine while tephrite has less than 10% olivine by weight.
While basanite has a darker color that is lighter than its basalt cousin, tephrite has
a more gray than black coloration.
A rock type known as foidite contains 60% or more by weight compositionally of foids.
Foids, or feldspar crystals often have a generally white color in foidite, and this type of lava
is often unusually low viscosity which can on occasion move at speeds beyond 30 kilometers
per hour.
Basaltic andesite is a medium viscosity lava that erupts from both cinder cones and stratocones.
While basaltic andesite can look just like basalt with some white colored mineral patches
it can also have a grayish white color with darker patches of material.
Andesite is the most common volcanic rock associated with stratovolcanoes such as Mount
Kanlaon and Mount Rainier.
Andesite has a gray color, is often finely grained, and contains abundant white feldspar
crystals.
Dacite is a viscous high silica variety of lava often associated with lava dome complex
volcanoes such as Lassen Peak.
It has a gray to white color with abundant large feldspar crystals, but unlike andesite
has some orange-pink tinted potassium feldspar crystals.
Rhyolite is erupted at the most highly explosive volcanoes, often appearing alongside a volcanic
glass known as obsidian and often erupting during caldera forming eruptions.
Rhyolite has a tan to red color, and abundant potassium feldspar crystals.
Trachyte and trachydacite are two silica and alkali rich rock types which have one difference.
Trachyte has less than 20% by weight quartz while trachydacite has more than 20% by weight
quartz.
Both of these rock types have a light gray color and contain clusters of larger crystals
surrounded by finely grained material that looks somewhat like powder.
Trachyandesite is somewhat similar except it has close to an even distribution of white
and orange to pink tinted feldspar crystals.
Basaltic trachyandesite is difficult to initially identify, but looks like trachyandesite with
less feldspar and a slightly darker color.
Phonotephrite has a medium to dark gray color, generally has common white colored feldspar
crystals, and minimal potassium feldspar.
Tephriphonolite has a negligibly darker color than phonotephrite, contains more feldspar
overall, and at least half of its feldspar crystals are potassium rich with a pink tint.
Phonolites tend to erupt in areas associated with rift zones or low itensity hotspots that
create stratovolcanoes such as the Mount Erebus volcano in Antarctica or Tenerife in the Canary
Islands.
More than 60% of its common feldspar crystals are not plagioclase, and this rock type does
not contain any quartz.
Thanks for watching!
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