Beyond this margin a shift occurs to a commodity that has lower transportation
costs to make production profitable. The model is not confined to the crops that
von Thunen considered, of course; in theory, any group of various agricultural
products will experience different prices per unit and varying transport costs based
on the nature of the product, and these differences will result in a similar spatial
arrangement around the central market location.
There are several weaknesses to von Thunen’s model that detract from its use-
fulness. As he himself acknowledged, the assumptions he makes in the model
are not reflected in reality. Even over a fairly limited areasoilsoften tend to vary
significantly in productivity, and this fact alone would undermine the formation
of the neat, precise zones of production the model predicts. Of course, in the real
world transport is not made cross-country, but via roadways or by rail, and some
producers will have an advantage in transport costs simply by being located closer
to these lines of transportation. Furthermore, it is unlikely that even on a plain
unbroken by topographic barriers, only a single marketplace would service a
region of any significant size. Rather, a hierarchy of market nodes would evolve
based on the type of goods offered at each location (seeCentral Place Theory).
Modern transport technology has greatly reduced some of the challenges of von
Thunen’s era, and this has altered the dynamic of transport costs between some
types of goods. But despite its shortcomings, von Thunen’s model made an enor-
mous contribution to economic geography, and continues to influence the intellec-
tual approach of modern scholars.
Vulcanism
Vulcanism is the term for the motion and landforming processes associated with
molten rock. The term subsumes the more specific “volcanism” referring to
shallow and external features and processes and “plutonic activity” when it is
deep-seated.
As covered in the article aboutEarth, the interior of our planet is ferociously
hot. This interior energy reaches all the way out to the surface, builds someland-
scapesand destroys others. Earth’s crustal plates float on an upper mantle that is
quite plastic and capable of deformation and flow. Concentrated flows far under-
ground sometimes influence the surface on which we live. Other flows reach and
spill out onto the surface. When this material is in Earth’s interior it is known as
magma, and when it flows over the surface it is called lava.
When magma solidifies under the surface the spate of forms created is known as
intrusive vulcanism. Large magma intrusions under the surface are known as
Vulcanism 357