Chapter 18-Plate Tectonics  

               Physical Geology Illustrated Vocabulary    
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Roger Weller, geology instructor
wellerr@cochise.edu     


trail of the Yellowstone hot spot

Chapter 18
copyright 2015-Roger Weller
last edited: 12/7/15

accretion
-is the process of slowly adding on; applied to the theory of continental accretion where continents
have gotten larger with time as new land is added to the margins of continents.

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

allochthonous terrane
-is a large mass of land that has moved a substantial distance before reaching its present position
as part of a continent.

Alps
-are a prominent mountain range in southern Europe.

Andes Mountains of South America
-are a prominent mountain chain that extends along the western side of South America. The
mountains consist of fault blocks and volcanoes which were created by an oceanic plate being
subducted beneath the mountain chain.

Appalachian Mountains
-are a low mountain range extending from Georgia up through Maine and continuing on through
eastern Canada.

authocthonous terrane
-is a landmass that is in the location where it was formed, in contrast to an allochthonous terrane
that was moved into its present position from some place else.

Baja California
-is a prominent peninsula in Mexico that is bordered on one side by the Pacific Ocean and by the
Gulf of California on the other. It is a terrane that broke loose from the main landmass of Mexico
and is migrating northwesterly along the western side of North America assisted by the San
Andreas fault.

Benioff zone
-is the earthquake zone associated with a subduction zone.
-Benioff zone-Opal's  Pals

bouyancy
-Objects denser than the fluid in which they are placed will sink; objects less dense than the fluid
in which they are placed will float. A dense object placed within a fluid will have its apparent
weight decreased equal to the weight of an equal volume of water that it displaces.
-density-Opal's Pals

Cascades
-are a chain of andesitic volcanoes extending from California to British Columbia.  The Cascades
contain Mt. Rainier, Mt. St. Helens, Lassen Peak, Crater Lake, and other volcanoes.
-Lassen Peak
-Crater Lake

Circum Pacific belt
-is the collection of earthquake epicenters that encircle the Pacific Ocean.

Colorado Plateau
-is a large uplifted area that covers portions of Arizona, Utah, Colorado, and New Mexico.
-northwest quadrant of the Colorado Plateau

Columbia River basalts
-were formed by huge outpourings of fluid basaltic lava which inundated a large area of the
Pacific Northwest of the United States.

continental-continental plate boundary
-If this is a divergent boundary, a rift zone will form above the boundary.  If it is a convergent
boundary, a folded mountain range will be pushed up, such as the Appalachians or the Ural
mountains.

continental crust
-is a portion of the Earth's crust that is thicker and less dense than oceanic crust. Consequently, this
type of crust floats at a higher elevation and protrudes above sea level to form continents.

continental drift
-is a theory that proposes that continents were once attached together but have since split and have
moved apart; this concept preceded the modern concept of plate tectonics.

continental rifting
-occurs as a divergent plate boundary develops beneath a continent and the continent starts to
pull apart. A modern example is the East African rift valley.

convection
-is the flow of fluids due to heat.  Hot, less dense fluids rise, while cold denser fluids sink.
-convection-Opal's Pals

convergence
-is where two objects keep getting closer together.

convergent boundary
-is one of the three types of tectonic plate boundaries.  At a convergent boundary crustal plates
are coming towards each other.

crustal plates
-The crust of the Earth is brittle and is broken into large pieces.

curie point of magnetite
-is the temperature above which magnetite loses its magnetic properties. For magnetite this
temperature is 580 degrees Celsius. If originally above 580 degrees Celsius, randomly oriented
magnetite crystals will align their orientation with the magnetic field of the Earth when cooled
below this temperature.

divergent boundary
-occurs when the two crustal plates at a boundary are moving away from each other.

East African Rift Valley
-is a long rift valley marked by volcanoes and lakes and it is located in the southeastern region of
Africa.

East Pacific Rise
-is an undersea spreading ridge located in the southern and eastern portions of the Pacific basin.

Galapagos Islands
-are a group of small shield volcanoes that form islands in the eastern Pacific Ocean.  The
volcanoes are associated with a hot spot.

Gondwanaland
-is the ancient landmass consisting of the southern continents: South America, Africa, Australia,
India, and Antarctica.

Hawaiian Islands
-are the tops of shield volcanoes that were formed when the Pacific plate moved over an active
hot spot.  The Hawaiian islands are not associated with subduction zones.
-Haleakala volcano on Maui, Hawaii
-Hawaiian Islands

Himalaya Mountains
-are the tallest mountain range on Earth.  They were formed by India is crashing into Asia.

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.  A preferred term for hot spot is
mantle plume.

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

Laurasia
-is the ancient landmass consisting of Asia, Europe, and most of North America.

magnetic
-is a physical property in which a material is attracted to a magnet
-magnetite, lodestone
-magnetic-Opal's Pals

magnetic anomaly
-is a spot on the ocean floor where the oceanic crust is magnetized in a direction opposite to
today's orientation of the magnetic poles.

magnetic reversal
-occurs when the Earth's magnetic field changes direction.  During a magnetic reversal the North
and South magnetic poles trade positions.

magnetite
-is the most common occurring mineral that is attracted to a magnet.  Magnetite is a black mineral
made of iron oxide.
-magnetite, lodestone

magnetometer
-is a highly sensitive device that can detect changes in the Earth's magnetic field.

mantle plume
-consists of hot, low density molten (or semi-molten) material rising from the mantle and coming
up underneath the Earth's crust.

Mid-Atlantic Ridge
-is a spreading ridge where two crustal plates are moving apart.  This underwater ridge goes
straight up the middle of the Atlantic Ocean.

oceanic crust
-is the crust of the Earth beneath the oceans; it is typically only 2 to three miles thick.

oceanic-continental plate boundary
-If this is a convergent boundary, the oceanic crustal slab is subducted beneath the continental
crust at about a 30 degree angle.

oceanic-oceanic plate boundary
-If this is a divergent plate boundary, an oceanic ridge will form at the boundary.  On the other
hand, if it is a convergent boundary, the oceanic plate on one side of the boundary will be
subducted under the other at a steep angle.

oceanic ridge
-is an underwater ridge formed where two crustal plates are pulling apart. The Mid-Atlantic Ridge
is a good example of an oceanic ridge.

orogenic belt
-is a long line of mountains that were produced by a common tectonic event.

orogeny
-is a mountain building event.

Pacific Plate
-is the largest of the crustal plates, it is covered by the Pacific Ocean and is nearly surrounded by
subduction zones.

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

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

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.
-Crater Elegante
-Pinacates-satellite photo

plastic flow
-Under pressure, a material flows instead of breaking.

plate tectonics
-is a concept in which the crust of the Earth is considered to be brittle and is broken into several
large pieces (tectonic plates). Most of the seismic activity on the Earth today is located at the
edges of the plates where the plates are either diverging, converging, or sliding past each other.

regional metamorphism
-occurs where a large area of a continent has been metamorphosed due to a major tectonic event
such as collision of continents.

rift valley
-forms where crustal plates are pulling apart (divergence).  Long graben depressions created by
rifting form valleys.

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

San Andreas Fault, California
-is a prominent strike-slip fault extending from near Yuma, Arizona, running northeast of Los
Angeles up through San Francisco. This fault separates the Pacific plate on the West from the
North American plate to the East.
-San Andreas Fault-Los Angeles area
-San Andreas Fault-San Francisco area

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.
-seafloor spreading-Opal's Pals

spreading center volcanism
-occurs along an oceanic ridge where seafloor spreading is occurring.  Lava emerges where the
oceanic plates are pulling apart and opening up fissures.

subduction zone
-When two crustal plates converge, one crustal plate often dives beneath the other at an angle
between 30 degrees and 60 degrees from the horizontal.
-subduction zone-Opal's Pals

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.

thermal convection
-Within the mantle of a planet, hot fluid material rises because of its lower density, while cold
material sinks because of its higher density.

transform boundary
-is one of the three kinds of plate tectonics boundaries.  At a transform boundary crustal plates slide
past each other.

transform fault
-is a strike-slip fault where blocks on opposite sides of a fault move in opposite horizontal directions.

trench
-is a depression on the ocean floor where oceanic crust is being subducted.
-trench-Opal's Pals

triple junction
-consists of three oceanic spreading ridges radiating from a common point.

Ural Mountains
-are a fold mountain range that separates Europe from Asia.

Wegener, Alfred
-is the geologist who developed and promoted the concept of continental drift.

Wilson, J. Tuzo
-is the Canadian geologist who promoted and helped to develop the concept of plate tectonics.

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