Igneous rocks, formed from magma as it rises and cools, are classified into two main types: extrusive and intrusive. Extrusive rocks like rhyolite cool quickly on the surface and have high silica content, while intrusive rocks like peridotite cool slowly within the crust and contain less silica. The composition and characteristics of igneous rocks vary significantly based on the type of magma and cooling conditions.

1. The Basic Types of Igneous
Rock Formed from Magma

2. Based in Westbury, NY,
Valerie Varnuska is a nature
and outdoors enthusiast
who enjoys exploring
unique geologic formations
in areas she visits. Among
Valerie Varnuska’s interests
is understanding what
minerals, gems, and rocks
are made of.

3. Alongside metamorphic and
sedimentary rocks, igneous rocks
are one of the planet’s major types
of rock. They are formed when
magma (molten minerals, rocks,
and gases) that develops
underground in the upper mantle
or lower crust, rises toward the
earth’s surface, ultimately cooling
and crystallizing. There are two
major forms of igneous rock:
extrusive and intrusive. The latter
forms and cools within the crust,
while the former is released on the
earth’s surface in the form of
magma.

4. Igneous rocks can have a wide range of compositions, depending on what type of magma
they cooled from. Cooling conditions also have a major role in their makeup: as an example,
rocks from the same type of magma can transform into granite or rhyolite, depending on
how slow or rapid the cooling process is. Rhyolite possesses the most silica of any volcanic
rock, with this high silica content meaning that the original magma was thick and viscous,
and likely expelled violently from a fissure in the earth. Cooling on the earth’s surface is
relatively quick, and this makes extrusive rhyolite contrast with intrusive, slow-cooling
peridotite, which has very little silica content.