Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 28

FIGURE 2.6 Distribution of major plates, spreading centers (diverging oceanic ridges), and subduc-
tion zones of convergence. Heavy, dark, offset lines represent ridge axes; connecting single lines
with parallel but opposing arrows indicate displacement along transform faults. Lighter toothed
lines with triangular symbols represent subduction zones. Dashed lines show uncertain plate
boundaries and areas of extension within continents. A dot with a dashed arrow indicates a hot
spot and its trace. Divergence direction and spreading rate (cm/yr) are shown with diverging
arrows and convergence direction and rate with head-to-head converging arrows. High rates of
convergence on land represent places where the mountains are growing most actively. (Adapted
from various sources.)
As our understanding of sea-floor spreading developed, it became apparent that a
characteristic sequence of igneous and sedimentary rocks formed at the spreading cen-
ters on the bottom of the sea. These ophiolites (Greek for “snakeskin,” after their char-
acteristic formations of shiny green metamorphic serpentine rock) are characterized by
layers of the dense, heavy ultramafic peridotite that seems to represent parts of the up-
per mantle. Ophiolites include gabbro batholith intrusions, multiple sheeted dike com-
plexes that probably served as feeder dikes to the upper pillow lavas of basalt that form
in the erupting submarine spreading centers. They are overlain by fine-grained deep-
sea muddy oozes that harden into cherts and limestones. Such ophiolite suites are com-
monly incorporated into landmasses and represent bits of ancient sea floor now caught
up in mountain ranges.