5.7B
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 Earth’s 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 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.
3.7B
Key Concept 1: Large forces can change the Earth’s surface rapidly.
Most features on Earth’s surface, such as mountains, valleys, and plains, are created over hundreds or thousands of years. However, some events, such as volcanic eruptions, earthquakes, and landslides, occur very rapidly and can completely change the surface within just hours or days. These slow and rapid changes create different landforms because of natural forces that work above and below the ground. Earth is made of three basic layers: the crust, the mantle, and the core. The crust and the rigid upper part of the mantle move together on top of soft molten rock deeper in the mantle (caused by tremendous internal heat of Earth).
Instead of one giant layer, the crust and upper mantle move very slowly together as broken puzzle pieces called plates. This slow movement is caused by the molten rock beneath sliding past the enormous plates above. Two of the three large forces that change the surface rapidly, volcanoes and earthquakes, begin below the surface of Earth along boundaries where these plates collide. The third large force, landslides, occurs above the surface.
Key Concept 2: Volcanic eruptions and earthquakes can cause rapid changes on Earth’s surface, such as creating new land, making cracks in the crust, or changing landforms.
Volcanic eruptions and earthquakes are natures pressure valves. Along plates bordering the Pacific Ocean, magma (molten rock) is forced up through cracks to the surface releasing pressure in volcanic eruptions. Numerous volcanoes and earthquakes occur along this Pacific Ring of Fire.
Volcanoes erupt when magma reaches the surface as lava. Lava flows down the sides of the volcano and hardens. Ash, cinders, and gases are also released. The largest volcanoes release slow, continuous lava flows to create wide, volcanic mountains. Some volcanoes erupt violently with pyroclastic flow (deadly avalanche of superheated fragments, ash, and gas) that destroys landscape and lives in its path. Some volcanoes form on the ocean floor over hot spots, and after repeated eruptions, builds up enough lava to form islands like the Hawaiian Islands. Volcanoes both destroy and create landscape.
When volcanic lava rock erodes after many years, soil often collects in the crevices, allowing plant growth to take root and animal life to develop. Many volcanoes are no longer active, such as those that formed the Davis Mountains in Texas, but serve as reminders of how the very forces that destroy are the same forces that can create new land and new life.
When rough blocks of rock along plate boundaries move against each other and get stuck, pressure builds. Finally, the blocks move, which suddenly releases energy in the form of shaking vibrations called earthquakes that radiate outward like ripples in a pond. These vibrations, or seismic waves, are felt many miles away in two ways. The first set stretches the rock in a back-and-forth motion. The second set produces side-to-side shaking. Together, these vibrations can create massive damage to landforms, buildings, roads, bridges, power lines, and take the lives of the people and animals caught in the collapse. If the earthquake occurs on the ocean floor, the vibrations can cause enormous waves called tsunamis that crash into coastal areas, destroying and flooding miles inland.
Landslides occur where the earth collapses suddenly. This can be caused by seismic vibrations or when the earth has become too saturated with water and slumps downward. Landslides and other inevitable rapid changes to Earth’s surface have led to a special interest in creating infrastructures (building, roads, etc.) that can withstand these strong forces of nature.
4.7B