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OVERVIEW

The central idea of plate tectonics, which was discussed in chapter 18, is that Earth’s surface is made up of rigid
plates that are moving slowly across the surface. Since the plates and the continents riding on them are in constant
motion, any given map of the world is only a snapshot that shows the relative positions of the continents at a given
time. The continents occupied different positions in the distant past. They will occupy different positions in the
distant future. The surface of Earth, which seems so solid and stationary, is in fact mobile.
Plate tectonics has changed the accepted way of thinking about the solid, stationary nature of Earth’s surface and
ideas about the permanence of the surface as well. The surface of Earth is no longer viewed as having a permanent
nature but is understood to be involved in an ongoing cycle of destruction and renewal. Old crust is destroyed as it is
plowed back into the mantle through subduction, becoming mixed with the mantle. New crust is created as molten
materials move from the mantle through seafloor spreading and volcanoes. Over time, much of the crust must cycle
into and out of the mantle.
The movement of plates, the crust-mantle cycle, and the rock cycle all combine to produce a constantly changing
surface. There are basically two types of surface changes: (1) changes that originate within Earth, resulting in a
building up of the surface (Figure 19.1), and (2) changes that result from rocks being exposed to the atmosphere
and water, resulting in a sculpturing and tearing down of the surface. This chapter is about the building up of the
land. The concepts of this chapter will provide you with something far more interesting about Earth’s surface than
the scenic aspect. The existence of different features (such as mountains, folded hills, islands) and the occurrence of
certain events (such as earthquakes, volcanoes, faulting) are all related. The related features and events also have a
story to tell about Earth’s past, a story about the here and now, and yet another story about the future.

was a keen observer of rocks, rock structures, and other features
19.1 INTERPRETING EARTH’S SURFACE
of the landscape. He noted that sandstone, for example, was
Because many geologic changes take place slowly, it is diffi- made up of rock fragments that appeared to be (1) similar to the
cult for a person to see significant change occur to mountains, sand being carried by rivers and (2) similar to the sand making
canyons, and shorelines in the brief span of a lifetime. Given up the beaches next to the sea. He also noted fossil shells of sea
a mental framework based on a lack of appreciation of change animals in sandstone on the land, while the living relatives of
over geologic time, how do you suppose people interpreted the these animals were found in the shallow waters of the sea. This
existence of features such as mountains and canyons? Some and other evidence led Hutton to realize that rocks were being
believed, as they had observed in their lifetimes, that the moun- ground into fragments, then carried by rivers to the sea. He sur-
tains and canyons had “always” been there. Expressions such mised that these particles would be reformed into rocks later,
as “unchanging as the hills” or “old as the hills” illustrate this then lifted and shaped into the hills and mountains of the land.
lack of appreciation of change over geologic time. Others did He saw all this as quiet, orderly change that required only time
not believe the features had always been there, but believed and the ongoing work of the water and some forces to make
they were formed by a sudden, single catastrophic event (Fig- the sediments back into rocks. With Hutton’s logical conclusion
ure 19.2). A catastrophe created a feature of Earth’s surface all came the understanding that Earth’s history could be interpreted
at once, with little or no change occurring since that time. The by tracing it backward, from the present to the past. This tracing
Grand Canyon, for example, was not interpreted as the result of required a frame of reference of slow, uniform change, not the
incomprehensibly slow river erosion, but as the result of a giant catastrophic frame of reference of previous thinkers. The frame
crack or rip that appeared in the surface. The canyon that you of reference of uniform changes is today called the principle
see today was interpreted as forming when Earth split open and of uniformity (also called uniformitarianism). The principle of
the Colorado River fell into the split. This interpretation was used uniformity is often represented by a statement that “the pres-
to explain the formation of major geologic features based on the ent is the key to the past.” This statement means that the geo-
lack of change that could be observed during a person’s lifetime. logic processes you see changing rocks today are the very same
About 200 years ago, the idea of unchanging, catastrophically processes that changed them in the ancient past, although not
formed landscapes was challenged by James Hutton, a Scottish necessarily at the same rate. The principle of uniformity does
physician. Hutton, who is known today as the founder of mod- not exclude the happening of sudden or catastrophic events on
ern geology, traveled widely throughout the British Isles. Hutton the surface of Earth. A violent volcanic explosion, for example,

478 CHAPTER 19 Building Earth’s Surface 19-2