Chapter 4- Volcanoes and Plutons
                       Physical Geology Illustrated Vocabulary

Cochise College                                                     
Geology Home Page  
Volcanoes and Igneous Rocks 
Roger Weller, geology instructor   

copyright 2018-Roger Weller   3/21/18

cinder cone, San Bernardino Valley, Cochise County, Arizona


aa lava flow

-is a lava flow with a very blocky, sharp surface.
Aa is a word used by bare-footed Hawaiians to describe this sharp, skin-ripping type of
basaltic lava.

-aa lava flow near Flagstaff, Arizona 

-aa lava flow in Craters of the Moon National Park, Idaho 
-Aa lava-Opal's Pals


Aleutian Islands

-consist of a prominent curved chain of volcanic islands extending from Alaska westward. 
Structurally, these islands are an island arc adjacent to a subduction zone.

-map of Alaska Aleutian islands

-are atoms or groups of atoms with excess negative electrical charge, such as:

Cl-1, SO4-2, CO3-2, S-2, etc



-is the process by which magma melts surrounding rocks (country rock) and adds these
new elements to its composition.
-assimilation-Opal's Pals

basaltic volcanism
a quick guide to basaltic volcanism   (a photo tour)
-basaltic volcanism, Baja California, Mexico



-is a very large igneous intrusion.



-is an igneous intrusion (pluton) that has many cracks radiating from the main magma
body and filled with magma from the intrusion; so named because of a fanciful
comparison to a cactus.



-is a large collapse depression on top of a volcano.

-Halemaumau in Hawaii-overview 

-Halemaumau in Hawaii-crater wall 


carbon dioxide

-is a gas denser than air; each molecule consists of one carbon atom and two oxygen
atoms.  It is used to produce the bubbles in soda pop.


carbon monoxide

-is a deadly gas produced by incomplete combustion of fossil fuels. 
Each molecule of this compound consists of one carbon atom and one oxygen atom.



-consist of a chain of andesitic volcanoes extending from California to British
Columbia. Located within the Cascades are Mt. Rainier, Mt. St. Helens, Lassen Peak,
Crater Lake, and others.

-Lassen Peak, California 
-Crater Lake, Oregon 

-are atoms or groups of atoms with excess positive electrical charge, such as:

Ca+2, Na+1, K+1, Al+3, Mg+2, Fe+2, Fe+3, etc.


chill zone

-is the outer part of a magma body that cools quicker because it is in contact with
the cooler country rock; consequently grain sizes in the chill zone are smaller than
for the rest of the magma
-chill zone-Opal's Pals



-are small pyroclastic fragments of vesicular basalt ejected from a volcano;
commonly about one quarter inch to one half inch in diameter.

-cinders near Flagstaff. Arizona-1 

-cinders near Flagstaff. Arizona-2   
-cinders, Haleakala, Maui, Hawaii

cinder cone

-is a small volcano made of loose cinders at the angle of repose (about a 35
degree angle slope).

-cinder cone near Flagstaff, Arizona 

-cinder cone in Craters of the Moon National Park, Idaho
-aerial photograph of cinder cones 
-cinder cone in Haleakala crater, Maui, Hawaii
-cinder cone-Opal's Pals


Columbia River basalts

-represent huge outpourings of fluid basaltic lava which inundated a large
area of the Pacific Northwest of the United States.


columnar jointing

-Upon cooling, igneous bodies shrink and crack in a hexagonal pattern on
a horizontal plane; these cracks continue to migrate downwards through
the cooling igneous body to create a structure built of six-sided columns.

-Devils Post Pile, California-vertical columns 

-Devils Tower, Wyoming 


composite volcano

-is a volcano made out of alternating layers of ash and lava flows; also known
as a strato-volcano.
-composite volcanoes of the Cascade Mountains, Oregon**
-Lassen Peak, California
-composite volcano-Opal's Pals


concordant intrusion

-is an intrusion of magma, such as a sill, that goes between the layers of the
rock that it is intruding.
-concordant intrusion-Opal's Pals


contact metamorphism

-is a form of metamorphism where rocks are baked by the heat from an
igneous intrusion.
-contact metamorphism-Opal's Pals


country rock

is the pre-existing rock that into which a magmatic intrusion invades.
-contact between intrusion and country rock
-country rock-Opal's Pals


-is the large depression on top of a volcano.

-Halemaumau in Hawaii-overview 

crystal settling
-occurs within a magma chamber when crystals of a mineral form that are
denser than the magma.   The denser crystals tend to sink to the bottom
of the magma chamber, removing certain elements and thereby changing
the composition of the magma.
-crystal settling-Opal's Pals


Devils Postpile, California

-Devils Postpile-vertical columns-1 

-Devils Postpile-vertical columns-2 


Devils Tower. Wyoming

-is an igneous intrusion in northwestern Wyoming that cooled and formed
prominent columnar jointing before being exposed by erosion.

-Devils Tower-1 

-Devils Tower-2 



-is a discordant igneous intrusion that cuts across the layering of the
country rock; the structure is often just igneous material filling a crack.

-dike in the Santa Catalinas near Tucson, Arizona 
-dikes exposed by weathering

-aplite dike in decomposed granite, Huachuca Mountains, Arizona
-radial dikes of Shiprock, New Mexico**
-Shiprock, New Mexico-dike A
-Shiprock, New Mexico-dike B

-granite dike in gneiss
-dike near Yuma, Arizona

discordant intrusion

-is an intrusion, such as a dike, that cuts across the grain or layering of
the pre-existing rock (country rock).
-dike near Yuma, Arizona


dissolved gases

-Within a can or bottle of soda there a significant amount of gas dissolved
within the fluid; without it the soda would be flat.  Similarly, there are many
varieties of gasses dissolved in magma which are released when lava erupts:
steam, carbon dioxide, sulfur dioxide, hydrochloric acid, chlorine,
hydrofluoric acid, etc.


Flagstaff volcanics

-are located north of Flagstaff, Arizona; the area has many cinder cones and
lava flows.

-cinder cones near Flagstaff, Arizona 

-S.P. crater and lava flow-aerial view 

flank eruption

-is an eruption from the side of a volcano


flood basalt

-is a form of basaltic eruption in which huge flows of very fluid lava issue from

-Idaho-Craters of the Moon 
-flood basalt-Opal's Pals



-is the location where volcanic gasses emerge from the ground.

-fumarole in Halemaumau, Hawaii 

-fumarole in Craters of the Moon, Idaho 
-fumarole-Opal's Pals


Galapagos Islands

-are a group of small shield volcanoes that form islands in the eastern Pacific


glowing cloud eruption

-is an explosive, violent, red hot eruption of a viscous silicic volcanic ash and
-glowing cloud eruption lecture


gravitational magmatic segregation

-Under the influence of gravity crystals that form as as a magma cools either
settle to the bottom of the magma chamber (dense compounds) or rise to the
roof of the magma chamber (low density compounds).
-gravitational magmatic segregation-Opal's Pals


Hawaiian volcanoes

-The Hawaiian islands are the tops of shield volcanoes that were formed when
the Pacific plate moved over an active hot spot.
-Lanai, a Hawaiian volcanic island
-map of the Hawaiian islands


hot spot

-is a term applied to a small region in which a prolific amount of magma is
being generated over a long interval of geologic time; since crustal plates
move over these buried sources of magma, the result is a trail of volcanoes
and/or intrusions on the crustal plate.
-trail of the Yellowstone hot spot in Wyoming

-trail of hotspot in Mexico**


hydrochloric acid

-is an acid with the formula, HCl.  It is found in increased but minor quantities
in your stomach when you take an algebra or calculus test.  It is also one of
the volcanic gases.


hydrofluoric acid

-is an extremely powerful acid with the formula, HF. Hydrofluoric acid is  capable
of dissolving glass, quartz, and the corneas of your eyes.

ignimbrite eruption
-is also known as a glowing cloud eruption.  It consists of a fast moving, dense
cloud of red-hot pyroclastics, destroying everything in its path.
-ignimbrite lecture



-is an igneous pluton, where magma has intruded country rock.

-intrusion beneath the Escabrosa Limestone cliffs


island arc

-is a curved chain of volcanic islands adjacent to an oceanic trench and
overlying a subduction zone.
-map of the Alaskan Aleutian island arc



-is a large, mushroom-shaped concordant intrusion.  The most famous
laccoliths are found in the Henry Mountains of southern Utah.
-laccolith-Opal's Pals



-a mud flow coming off of a volcano.


Lassen Peak, California

-is an andesitic cone, part of the Cascades volcanics.

-Lassen Peak-overview 

-Lassen Peak-summit 



-is molten rock that has erupted at the earth's surface.
-lava flows in the Pinacates, Mexico**
-lava flow burning trees
-lava flow in Hawaii
-lava-Opal's  Pals


lava dome

-is a hill-like structure made of viscous magma, formed on top of a
composite volcano often following a violent explosive event. 
The present summit of Mt. St. Helens has one of these structures starting
to fill the gap where the top of the volcano was blown off.

-Lassen Peak-summit 

lava flow
-is a single outpouring of a fluid lava.
-lava flows in the Pinacates, Mexico**
-lava in the crater of Haleakala, Maui, Hawaii
-multiple lava flows, Maui, Hawaii


lava lake

-is a volcanic caldera fills with yellow-hot fluid lava; the surface of a lava
rapidly cools to a dark, non-luminescent material, but beneath the thin
crust the lava is still molten.

-Hawaii-lava lake-1 

-Hawaii-lava lake-2 


lava tube

-is a cylindrical cavity that forms as basaltic lava drains out beneath a
hardened surface crust. Some lava tubes are caves as large as a subway

-tour of lava tube under Mt. St. Helens
-tour of Thurston lava tube, Hawaii  (2012)
-lava tube in Maui, Hawaii
-exiting a lava tube 
-collapsed lava tube-Rainbow Falls, Hawaii

-Idaho-Craters of the Moon-collapsed top of small lava tube 


lunar maria

-are the large areas on the Moon covered by dark, basaltic lava flows.



-is an explosive basaltic eruption that produces a large crater.
-maars in the Pinacates, Mexico* 

-Cerro Colorado, Sonora, Mexico  

-Crater Elegante, Sonora Mexico 

-Paramore Crater, San Bernardino Valley, Arizona 
-maar-Opal's Pals



-is molten rock.
-magma-Opal's Pals


magma mixing

-occurs when magma from two or more different sources come together
 and mix underground.


magmatic differentiation

-is the process whereby the composition of a magma changes as it cools
and newly crystallized minerals separate from the magma.


magmatic stoping

-occurs when a magma body moves upwards by cracking the ceiling rocks
by pressure and allowing fragments of the country rock to fall into the
magma chamber.  Stoping is a mining term for mining upwards.
-magmatic stoping-Opal's Pals


mantle plume

-hot, low density molten (or semi-molten) material rises from the mantle
and comes up underneath the crust.


Mauna Loa

-is an active shield volcano on the island of Hawaii.
-Mauna Loa and Mauna Keas as seen from Maui


moat deposits

-are the volcanic materials deposited in the moat lake within a resurgent

-map view of the Valles caldera, New Mexico


Mt. Capulin

-is a cinder cone located in northern New Mexico.


Mt. Fuji

-is a composite volcano located in Japan
Mt. Fuji-You Tube video


Mt. Rainier

-a composite volcano, part of the Cascades, located in the state of


Mt. Shasta

-is a composite volcano, located in northern California.


Mt. St. Helens

-is a composite volcano, part of the Cascades, located in the state of
Mt. St. Helens Tour- Steve Tyminski- Cochise College
Mt. St. Helens- YouTube video


nuees ardentes volcanic eruptions

-are violent silicic eruptions often known as glowing cloud eruptions
or ignimbrite eruptions.
-glowing cloud eruption lecture


pahoehoe lava flow

-is a relatively smooth surface lava flow; often described as ropey
or bulbous.
-ropey pahoehoe near Hilo, Hawaii

-surface of pahoehoe lava flow, Hawaii-1 

-surface of pahoehoe lava flow, Hawaii-2 


Paramore crater, Cochise County, Arizona

-is a maar type eruption located in the San Bernardino volcanic field,
southeastern Cochise County,

-Paramore crater 



-is a famous cinder cone that formed in a cornfield in Mexico.


parasitic cone

-is a small volcanic cone formed on top of a larger volcano.


partial melting

-occurs as a subducted slab of crust moves down a subduction zone. 
Minerals within the slab with the lowest melting temperatures melt first.
-partial melting-Opal's Pals


pillow lavas

-are underwater eruptions of basaltic lava that form pillow-like shapes.
-pillow lavas-Opal's Pals



-is a tube-like igneous intrusion, circular or elliptical in cross-section.


Pinacates volcanic field, Mexico

-is a volcanic hot spot in northern Sonora, Mexico, consisting several
large maars, numerous cinder cones, and fresh appearing basaltic lava

- Moon Crater from the air 

-cinder cone and lava flow 



-is an igneous intrusion.
-pluton-Opal's Pals

 -are the hot fiery fragments tossed out of a volcano.


resurgent caldera

-is a silicic form of volcanism in which a collapse crater is formed after
the explosive eruption of a magma chamber that has uplifted the surface
of the Earth.  Magma production after the main eruption does not cease;
at a later time new magma domes the floor of the crater; in other words
the surface surges upwards again to provide the "re" in resurgent.
-resurgent caldera and welded tuff lecture

-map view of the Valles caldera, New Mexico


rim deposits

-consist of the volcanic ash deposited on the rim of a crater from which
it was erupted.


Ring of Fire

-is the collection of volcanoes that are found encircling the Pacific Ocean.


San Bernardino Volcanic Field, Cochise County, Arizona

-is located in the extreme southeast corner of Arizona and consists of basaltic
lava flows, small cinder cones, and a maar, Paramore crater.

-San Bernardino-cinder cone 

 -are bands of melted xenoliths that have not been completely mixed into the
magma.  These bands are commonly found near the outer margins of a
granite pluton.
-schlieren in granite, Mt. Whitney, California
-schlieren-Opal's Pals

seafloor spreading

-occurs along the crest of oceanic ridges where the oceanic crust is being
pulled apart and lava is filling the fissures, thereby creating new seafloor.


shield volcano

-is a large basaltic volcano that is very wide but has a low profile.  This type
of volcano is so named because it resembles a Roman shield laid flat on the
ground.  The most famous volcanoes of this type located on Earth are the
Hawaiian and Galapagos islands.
-Haleakala, Maui, Hawaii
-Lanai, Hawaii
-shield volcano-Opal's Pals


Shiprock, New Mexico

-is an outstanding weathered volcanic neck on the northeastern portion of
New Mexico.  All that remains is a shark peak with three radiating dikes.

-is a concordant intrusion of magma between layers of country rock.
-sill-Opal's Pals



-is a medium-sized, usually discordant pluton that is between a pipe and a
batholith in size; less than forty square miles.
-Sacramento stock in Bisbee, Arizona (pre-open pit mining)



-is a composite volcano made of alternating layers of ash and lava; intermediate
in composition.  Many of the most famous and prominent volcanoes in the
world are of this type: Mt. Fuji, Mt. Rainier, Mt. St. Helens, Mt. Shasta,
Mt. Kilimanjaro, Lassen Peak, etc.

-California-Lassen Peak 


subduction zone volcanics

-In a subduction zone, a slab of oceanic crust with sediments is subducted
down into the hot mantle where low temperature minerals in the slab
start to melt.  The newly formed magma rises to the surface adjacent to
the trench to produce volcanoes.
-subduction zone volcanics-Opal's Pals


sulfur dioxide

-is a poisonous gas given off by volcanoes. Upon exposure to oxygen in
the air plus sunlight, sulfur dioxide turns into sulfur trioxide that then
combines with moisture to form sulfuric acid and then goes on to become
acid rain.
-sulfur dioxide-Opal's Pals


sulfuric acid

-is a powerful acid whose formula is hydrogen sulfate.


Sunset crater, northern Arizona

-is a cinder cone north of Flagstaff, Arizona.  This small volcano is reported
to be the most recent volcanic eruption in Arizona.
-Sunset Crater



-is an igneous rock consisting of compacted volcanic fragments smaller
than one eighth of an inch.


Turkey Creek caldera, Cochise County, Arizona

-is a large, eroded resurgent caldera in the Chiricahua Mountains of
southeastern Arizona; radiometrically dated at 26 million years old.

-map view of the Chiricahuas 


Valles caldera, northern New Mexico

-is a new, hardly weathered resurgent caldera, complete with resurgent dome
and moat volcanics northwest of Albuquerque, New Mexico.
-Valles Caldera geologic maps



-is an active volcano in Italy that is responsible for destroying Pompeii and



-is the measure of how resistant a fluid material is to flowing. 
For example, corn syrup is more viscous than water.
-viscosity-Opal's Pals


volcanic ash

-consists of very fine volcanic materials, ranging from dust to particles less
than one eighth of an inch.


volcanic bomb

-is a blob of lava ejected out of an active volcano.  As the material sails
through the air, the outer surface of the projectile cools and solidifies
before it hits the ground.  Commonly, these molten blobs are spinning
as they are ejected from the volcano and may develop aerodynamic or
twisted shapes.

-Idaho-Craters of the Moon-volcanic bomb-1 

-Idaho-Craters of the Moon-volcanic bomb-2 
-volcanic bombs-Haleakala, Maui, Hawaii
-Arizona-San Bernardino volcanic field-volcanic bomb-1
-Arizona-San Bernardino volcanic field-volcanic bomb-2


volcanic dome

-is a steep-sided, hill-like mass of viscous lava extruded by a volcano; after
the explosive eruption of Mt. St. Helens, one of these features slowly formed
in the newly created gap at the top of the volcano; also called a lava dome.

-California-Lassen Peak summit 


volcanic gases

-are the most common gases emitted by volcanoes: steam, carbon dioxide,
sulfur dioxide, hydrochloric acid, chlorine, hydrofluoric acid, fluorine, carbon
monoxide, and methane.


volcanic neck

-After a volcano has undergone intense weathering, all that often remains
is the lava that cooled and hardened in the conduit that supplied lava to
the volcano.
-Shiprock, New Mexico
-Shiprock, New Mexico-2


volcanic vent

-is where volcanic materials emerge from the ground.
-volcanic vent in the Chiricahua Mountains of Arizona



-is a landform produced by the eruption of lava and ash from a single vent.

-Idaho-Craters of the Moon-cinder cone 

-San Bernardino-cinder cone 

-California-Lassen Peak 



welded tuff

-is hot, silicic ash from a violent volcanic eruption melts back together
on the ground due to its high temperature and the weight of the volcanic
ash (tuff) above it.

-welded tuff in the Chiricahuas 
-resurgent caldera and welded tuff lecture

Weller's spaghetti theory of magma
 -explains how the constituents of magma (silica, cations, and dissolved gases)
 affect the viscosity of a magma.
 -Weller's spaghetti theory of magma-Opal's Pals  *

-are fragments of country rock that have fallen into a magma chamber and
are slowly being baked, then melted, and finally added to the magma.
-xenoliths-Opal's  Pals