volcanic arc of the Mariana Islands of the western Pacific. (4) Other convergences
are continental-continental collisions. In this case, there is no subduction because
of the relative buoyancy of both plates and a mountain range is built. The Himalayas
are the result of 35 million years of the Indo-Australian plate colliding with the
Eurasian plate. (5) Finally, some boundaries are transform in that they slide past
each other. There tend to be some major faults and earthquakes created as the jagged
pieces snag going by, but transform boundaries are conservative with no trenches or
major mountain building.
Terranes are chunks of Earth’s crust quite different in rocks and history from the
plates around them. Sometimes they have been moved thousands of km by a drift-
ing plate and geologically pasted to the edge of another plate. The complicated
mountainous landscapes of the Canadian West (frequently known as the Canadian
Cordillera) are due to subsequent accretion of terranes over hundreds of millions
of years to the North American Plate.
Some places well within plate boundaries are associated with landforms result-
ing from plate motions. Worldwide, “hot spots” are located over more than
50 mantle plumes that are rising currents that remain stationary for very long
amounts of time. Material is brought to the surface as the plate slides along over-
head. There are all sorts of volcanics associated with hot spots and the progres-
sively younger ages of the Hawaiian Islands from northwest to southeast can be
pegged to the Pacific Plate passing to the northwest over a mantle plume. A won-
derful aspect of knowledge of plate tectonics is found in understanding geogra-
phies much older than the current sequence of crustal movements. The study of
large areas of rock deformation and age indicate the existence of several supercon-
tinent formations and breakups duringEarth history with time orders of (very
approximately) 250,000,000 years. Quite a bit of climate change and history,
including some evolutionary developments, can be explained through examining
the changing locations of continents.
Population
A group of individuals sharing some characteristic or set of characteristics, and
occupying a defined, common space. Geographers are primarily concerned with
human populations at a variety ofscales: global, regional, national, and local.
However, biogeographers may study other populations, including various classes
or species of wildlife. Demographers are scholars who study the dynamics of pop-
ulations, including all the factors that direct the growth or decline of the number of
humans. Population geographers are interested primarily in the spatial qualities of
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