Chapter 4- Volcanoes and Plutons
                       Physical Geology Illustrated Vocabulary

Cochise College                                                     
Geology Home Page  
Roger Weller, geology instructor   

cinder cone, San Bernardino Valley, Cochise County, Arizona

Chapter 4
copyright 2015-Roger Weller
last edited: 12/7/15

volcano videos

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 fissures.

-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 Ocean.


glowing cloud eruption

-is an explosive, violent, red hot eruption of a viscous silicic volcanic ash and pyroclastics.
-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
-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
-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 tunnel.

-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 caldera.

-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 Washington.


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 Washington.
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 flows.

- 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 Herculaneum.



-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


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

-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