Igneous
Processes
Extrusive Igneous Formations
Volcanoes
• Openings in the Earth that erupt gases,
ash, & lava.
• Lava flows destroy everything in their
path.
• Pyroclastic flows (ash & debris) rush
down the sides of the volcano.
• Acid rain is produced when sulfurous
gases mix with water vapor in the
atmosphere.
Parts of a Volcano
–Magma collects in a magma chamber
inside the Earth’s crust.
–The opening where magma is forced
up and flows onto the Earth’s surface
is called a vent.
–The steep-walled depression around
a volcano’s vent is called the crater.
Tephra
Vent
Magma Chamber
Formation of Volcanoes
–Form at Divergent & Convergent
plate boundaries.
–Hot Spots :
»An unusually hot area between
the mantle & core that forces
melted rock upward to form
volcanoes in the middle of
tectonic plates.
»Plate moves as the hot spot is
stationary.
Hawaiian Islands
Ring of Fire
3. Volcanic Eruptions
• The explosiveness of a volcano depends on:
» Amount of water vapor & other gases
trapped in the magma.
» Amount of silica present in the magma.
- Basaltic lava is low in silica & produces
quiet, non-explosive eruptions:
•Pāhoehoe (meaning "smooth,
unbroken lava” =smooth, billowy,
undulating, or ropy surface)
•A’a (meaning "stony rough
lava” = rough or rubbly surface
composed of broken lava blocks)
Pāhoehoe lava flowing over a’a
Pāhoehoe lava flowing over a’a
- Granitic & andesitic
magma are high to
intermediate in silica &
produce violent eruptions.
pyroclastic flow
Types of Volcanoes
Shield
» Broad volcanoes with gently sloping
sides.
» Quiet eruptions of basaltic lava forming
flat layers.
Cinder Cone
»Steep-sided volcanoes made of loosely
packed tephra.
»Tephra- bits of rock or solidified lava
(pyroclastic material) like ash, cinders,
bombs, & blocks.
Composite
• Volcanoes with alternating layers of tephra
& lava flows.
• Long dormancy periods
• Violent eruptions
• Also known as a stratovolcano
Mount St. Helens
Mount St. Helens after May 1980 eruption.
Intrusive Igneous Formations
Batholiths
• Largest intrusive formations
• Forms where magma has been forced
up into chambers within the crust &
cool slowly.
Dikes
• Magma that is forced into vertical
cracks that cuts across rock layers and
hardens.
Sills
• Magma that has been forced into cracks
that are parallel to rock layers and then
hardens.
Volcanic Necks
• The solid igneous rock core of an
extinct volcano.
• Weathering & erosion wear away the
cone exposing the neck.
Calderas
• Large depression caused by the
collapse of a crater during an eruption.