5.7B Changes to Earth’s Surface
Key Concept 1: Wind, water, and ice can change Earth’s surface.
Blowing wind, running water, and freezing ice cause weathering, erosion, and
deposition that transform Earth’s surface:
Weathering is the breakdown of Earths crust into smaller particles called
sediment. As wind blows against the face of a cliff, for example, it carries with it
small particles of soil and sediment, which act as a grinding agent against the
cliff to slowly break down the rock. Running water is also a weathering agent. As
water flows over rock, it carries small particles of sediment and sand that grind
the surface into sediment. When water freezes into ice, it expands to become
larger. So when water seeps into cracks in rocks and freezes, the expansion of
the water makes the cracks larger and can eventually split the rocks into smaller
pieces.
After weathering breaks down Earth’s surface into small pieces, the process
of erosion carries away the sediment to another location. Erosion can occur over
a short period of time, such as a flash flood where portions of river banks or
chunks off the sides of cliffs are washed away after several hours of torrential
rain. Most erosion, however, occurs as a slow process where rivers slowly carve
away soft sediment, thus creating steep canyon walls.
When wind stops, water slows down, or ice melts, the sediments carried are
deposited and can form, for example, deltas at the mouths of rivers or sand
dunes in desert regions. Deposition is the building up (depositing) of new land
in a location after weathering and erosion. Often, new land is created where
rivers or streams flow into a lake or ocean.
Thus, the repeated cycles of weathering (breaking down of rock to form
sediment), erosion (carrying away of sediment), and deposition (laying down of
sediment) constantly interact to change the Earth’s surface.
Key Concept 2: Sand dunes form when wind-blown sand builds up,
and deltas form when water-born sediments are deposited at the
mouths of rivers.
Sand dunes form in desert or beach regions from two processes: erosion,
where wind constantly blows the sand from the long side of the dune
facing the wind to the top of the dune; and deposition, where the wind
stops and drops the remaining sand along the other side. Constant
erosion and deposition cause the dune to slowly shift its position
windward. Small dunes are found along sandy beaches. Enormous dunes
can be found in large desert areas, such as the Sahara in Africa or the
Gobi Desert in Asia.
Deltas are formed from sediment deposited at the mouths of rivers. A
flowing stream or river carries sediment to a lake or ocean. The flowing
currents stop upon entering the larger body of water, and the sediment
sinks to the bottom and builds new land, like spread fingers of a hand.
The water velocity slows dramatically causing the sand, silt, and clay
suspended in the river to precipitate out (settle to the bottom) with larger
particles precipitating faster than smaller ones. This creates a fan-shaped
area of deposition. The land formed in this process is called a delta.
Key Concept 3: Canyons form when moving water cuts through the
Earth’s surface.
The largest canyon on Earth is the mile-deep, 277 miles long Grand
Canyon, formed over millions of years by the Colorado River cutting
through surrounding land and slowly gouging away the rock of the
riverbed. The second largest canyon in the United States is the Palo Duro
Canyon located in the Texas Panhandle. Palo Duro Canyon was formed by
water erosion from the Red River, which deepened the canyon by moving
sediments downstream. Wind and water erosion gradually widen the
canyon.