Chapter 21: Earth's Changing Surface 

Section 1: Weathering and Soil

• Weathering: the process of physical or chemical breakdown of a material at or near Earth’s surface

  • The rocks and other materials that make up Earth’s surface are constantly changing.
  • A physical change occurs when the size or shape of an object changes.
  • A chemical change occurs when the chemical compounds that make up a material change.
  • Processes at Earth’s surface cause both chemical and physical changes to occur in rocks.
  • The corners and edges of rocks are more rapidly weathered than smooth rock faces. This often produces rounded forms.
  • Factors that influence the amount of weathering include the agent, such as water or air, the nature of the material being weathered, climate, and time.
  • Weathering varies from region to region because different weathering

  conditions are present.

• Mechanical Weathering

  • Mechanical weathering occurs when rocks are broken into smaller pieces without changing their chemical composition.
  • Physical forces exerted on rocks cause mechanical weathering to occur.
  • Rocks on Earth’s surface can be broken apart by frost wedging.
  • Frost wedging occurs when water collects in the cracks in a rock and then freezes.
  • Plant roots growing in cracks in rocks can exert forces that break the rocks apart

• Chemical Weathering

  • Chemical weathering is the breakup of rocks caused by chemical reactions that form new minerals and other materials.
  • Chemical weathering is caused mainly by chemical reactions of rocks with water, oxygen, and naturally occurring acids.
  • Some chemical compounds produced by the chemical weathering of rocks can be dissolved in water, where they form ions.
  • The chemical reaction of oxygen with other chemical compounds is called oxidation.
  • When iron-bearing rocks are weathered and eroded, they often produce reddish sediment. Iron oxidation also can give some soils a reddish color.
  • An important chemical weathering process for continental rocks is the weathering of feldspar minerals into clay minerals.
  • Chemical weathering changes the chemical composition of minerals and rocks.
  • Different rock formations tend to weather at different rates. The types of minerals, cementing agent, and the presence of defects, such as fractures, influence how a rock formation weathers.
  • Together, erosion and deposition of sediment create a variety of landforms.

• Soil: a mixture of weathered rock, organic matter, water, and air that is capable of supporting plant life.

  • The organic matter in soil comes from plants, animals, and other organisms.
  • The raw materials for soil continuously form through the weathering of bedrock below and the addition of organic matter from above.
  • If you dig a hole deep enough into the ground, you will see different soil layers, or horizons.
  • Soil horizons vary in color and thickness. Some horizons don’t appear in certain soils.
  • The horizons present depend on the composition of the parent bedrock, climate, and the kind and amount of organisms on the surface.
  • On steeper surfaces, thin layers of soil with less organic matter are likely to develop.
  • The O horizon, for organic, and the A horizon often are referred to as topsoil.
  • The E horizon, for eluviation, is a zone in which finer sediments and soluble materials are transported downward.
  • The process of dissolving soluble elements and transporting them deeper into the soil is called leaching.
  • The B horizon collects the materials from above and is usually darker than the E horizon.
  • The E horizon and the B horizon often are referred to as subsoil.
  • Collectively, horizons O, A, E, and B make up the true soil.
  • The C horizon is partially crumbled and weathered bedrock.
  • The R horizon is unweathered bedrock.
  • Soils are classified by their composition and physical properties.
  • Without organic material, a true soil cannot develop.
  • The materials from which a soil forms are called parent materials.

• Soil Conservation

  • Plants and animals continuously remove nutrients from soil.
  • Soil depletion is a serious agricultural problem in many regions.
  • The continuous recycling of plant matter returns these compounds to rain forest soils.
  • Soil depletion is most often corrected by the addition of fertilizers containing nitrogen, phosphorous, or potassium.
  • Crop rotation helps prevent erosion and reduces the risk of plant diseases and attack by pests
  • Most soil lost to erosion occurs because the vegetative cover has been removed or because the land is overly steep. Modern farmers use contour plowing, which runs the furrows around a hill instead of up and down a slope, to reduce erosion

Section 2: Shaping the Landscape

• Erosion, Transport, and Deposition
  • Forces within Earth cause the movement of Earth’s tectonic plates.
  • The interactions between plates can cause mountains to be uplifted and produce other landforms on Earth’s surface.
  • The shape of the land you see is the result of the combined effects of processes that uplift and wear down landforms.
  • Erosion: the removal of surface material through the processes of weathering.
  • Sediment Transport: the movement of eroded materials from one place to another by water, wind, and/or glaciers.
  • Deposition: When a transporting agent drops its load of eroded material
  • When valley glaciers melt they deposit sediment in characteristic landforms. Moraines consist of poorly sorted glacial till and eskers consist of layers of sorted sediment.
• Running Water
  • An important agent of erosion that exerts a downward force on slopes is running water.
  • In steep areas, running water cuts down through sediment and rock, creating V-shaped valleys.
  • Lateral cutting broadens river valleys and allows for floodplains to develop.
  • The mouth of a river is where it meets the body of water or land surface into which it ultimately flows.
  • Gradient, or slope, affects the shape of a stream and its containing valley.
  • In most river systems, small streams called tributaries flow into larger streams, which in turn flow into even larger streams.
  • Drainage Basin: All of the land area that gathers water for a major river.
  • Drainage basins are larger in area for major rivers and include all of the drainage basins of its tributaries.
  • The Mississippi River drainage basin is the largest drainage basin in the United States.
  • As surface water flows downhill under the influence of gravity, water erodes the surface, creating its own path or following existing paths called channels.
  • Steep canyons can form when downcutting is rapid.
  • When running water slows down, it drops part of the sediment load it is carrying, largest particles first.
  • Streams with many bars within their channels are called braided streams.
  • After a river floods, and as floodwaters subside, slow, and drop their sediment, floodplains form along the valley sides.
  • At the mouth of a stream that empties into a body of water, a fan-shaped sediment deposit called a delta will form.
  • Both tributaries and distributaries are sometimes compared to branches or roots of a tree.
• Glaciers
  • For a glacier to form, the amount of snow that falls in the winter must not be completely melted over the rest of the year.
  • Valley glaciers create a variety of alpine, or mountainous, features. Along their paths, they accumulate soil and surface rocks that become abrasive material used to transform the landscape.
  • Continental glaciers, or ice sheets, occupy huge land areas. Ice moves away in all directions from the highest areas of the ice sheet.
  • Glaciers become effective agents of erosion as they move either down slope as in valley glaciers or under their own weight as in continental glaciers.
  • Valley and continental glaciers leave behind deep grooves or striations, which give clues about the direction the ice was moving.
  • Valley glaciers convert V-shaped stream valleys into distinctive U-shaped valleys. The downcutting force of valley glaciers differs from those of streams.
  • Tributary glaciers with less mass cut less deeply. Larger trunk glaciers have more mass and erode more deeply.
  • Ice cannot sort material like moving water does.
  • The large ridges of till that accumulate at the edge of a glacier are called moraines.
  • An end moraine forms at the front of the ice, and lateral moraines form at the side of the ice.
  • A ground moraine forms underneath melting continental ice sheets.
  • End moraines are composed of poorly sorted soil and rocks that were scraped up by the glacier, moved ahead of it, and then deposited where it melted. Eskers are composed of finer soil and rock material that was sorted and deposited by running water within the glacier.
• Wind
  • Compared to water and ice, wind cannot pick up and carry large particles.
  • In places where sand and smaller particles are abundant and not held in place by vegetation, wind can be a very effective agent of erosion.
  • The removal of small particles by wind, leaving heavier particles behind, is called deflation.
  • Blowouts, or deflation hollows, are common landforms where wind is the dominant agent of erosion. These landforms are shallow depressions where wind has scooped out surface material, leaving low places behind.
  • As wind velocity decreases, the load of sediment is dropped.
  • The shapes and sizes of landforms that form depend on how constant the wind velocity is and on the supply of sediment.
• Wave Action
  • The action of waves on ocean shores can also cause erosion.
  • When waves break, the water crashing down on the shore exerts large forces that can change the shoreline.
  • When waves approach a shoreline at an angle, they change directions, or are refracted.
  • Longshore currents run parallel to the shoreline. They can completely remove some beaches by erosion and later deposit the sediment, forming new beaches farther down-current.
  • Longshore Current: The resulting movement of water parallel to the shoreline
  • The action of waves loaded with sediment works like sandpaper and causes abrasion.
  • Shorelines that are rock-bound with steep cliffs tend to experience differential weathering.
  • Shorelines commonly lack erosional landforms when they have a thick sediment cover.
  • A sandbar that seals off a bay from the open ocean is called a baymouth bar.
  • Spits are sand bars that project into the water from land and curve back toward land in a hook shape.
  • During severe storms, water levels can rise and create sand deposits known as barrier islands that can serve as protection for the mainland.
• Mass Wasting
  • When erosion occurs primarily as a movement of material down a slope, whether triggered by an influx of water, by earthquakes, or by human activity, mass wasting occurs.
  • Mass wasting takes place where slopes are overly steep or lacking vegetation. Events usually begin when support at the base of a slope is removed and the material above moves downhill.
  • Erosion caused by mass wasting is dependent upon the type of event and the type of materials involved.
  • Landslides or rockslides also can produce distinct scars on slopes.
  • Mudflows move rapidly and are as dense and damaging as fluid cement.
  • Mass wasting events tend to dump their material in disorganized masses.
  • When sufficient material builds up, the slope becomes stabilized and mass wasting stops.

Section 3: Groundwater

• Freshwater on Earth
  • About 97 percent of all water on Earth is salt water in Earth’s oceans.
  • Two percent of Earth’s water is frozen as ice in glaciers and ice sheets.
  • The remaining 1 percent is freshwater that is found on land in lakes and rivers or underground as groundwater.
  • Freshwater moves between the atmosphere, land, and oceans in the water cycle.
  • The water cycle continually redistributes Earth’s water.
  • The water cycle is the result of repeating cycles of evaporation and condensation and is driven by energy from the Sun.
  • Plants also release some water into the atmosphere through their leaves. This process is called transpiration.
  • Most precipitation falls into the oceans.
  • Infiltration: the process by which water enters Earth and becomes groundwater below the surface.
• Groundwater
  • All the water that is naturally stored underground is called groundwater,
  • Groundwater is stored in porous rock called aquifers. It is often recharged, \n or replenished, by infiltration in wetlands.
  • The region near the surface where water can infiltrate freely is the unsaturated zone.
  • The region where water completely fills the pore space, is the saturated zone.
  • Water Table: the boundary separating the saturated and unsaturated boundaries.
  • For infiltration to occur, the rock and sediments beneath the surface must be permeable.
  • The slope of the land and the vegetation on the surface also affect the rate of infiltration.
  • Aquifer: A rock unit that can transmit water through its pore space
  • A rock that slows or stops infiltration is an aquitard.
  • Small, local water tables can occur where an aquitard prevents downward infiltration.
  • Porosity: the percentage of the material’s total volume that is pore space.
  • Water can move through the material only through the interconnected pore spaces. The more interconnected pore spaces there are in a material, the more quickly water moves through the material. A material that is highly permeable has a high porosity and contains many interconnected pore spaces.
• Water Resources
  • About 50 percent of the population in the United States obtains the freshwater they use from groundwater.
  • Some groundwater comes naturally from springs, which occur where the water table intersects the ground surface.
  • As water is removed from the zone of saturation, the water table is lowered.
  • The natural flow path of groundwater has both horizontal and vertical components.
  • As areas become more populated, more water wells often are drilled. This lowers the water table, causing some existing water wells to go dry.
  • Wells drilled into aquifers that are under sufficient natural pressure to force water up into a well are called artesian wells.
  • Pollutants that are spilled or dumped on the ground and even air pollutants, washed from the air by precipitation, enter groundwater through infiltration
  • Pollution can enter groundwater through infiltration. Cleaning polluted groundwater can be difficult and costly.

Section 4: Geologic Time

• Time
  • Earth is divided into 24 time zones.
  • Birth records often give the exact hour and minute of birth.
  • Absolute Dating: the process of assigning a precise numerical age to an organism, object, or event.
  • Relative Dating: The process of placing objects or events in their proper sequence in time.
  • The key to relative dating is that the order is controlled by logical relationships, which might or might not involve the use of numbers.
  • Uniformitarianism: states that the laws of nature operate today as they have in the past.
  • By studying how geologic events take place today, scientists can infer how events took place in the past.
• Principles of Relative Dating
  • Geologists use several rules to help determine the relative ages of rock layers.
  • Principle of Superposition: in an undisturbed sequence of rock layers, the oldest rocks will be at the bottom of the sequence and the youngest rocks will be at the top.
  • The principle of original horizontality means that sedimentary rock layers are deposited as horizontal or nearly horizontal layers.
  • According to the principle of cross-cutting relationships, an intrusion is younger than the layers it cuts across.
  • The principle of cross cutting relationships often can be used to determine the chronological order of some of Earth’s geologic events.
  • Unconformities: gaps in the rock record during which erosion occurred or deposition was absent.
• Fossils: The remains or traces of organisms found in the geologic rock record.
  • Fossils can be direct remains, such as an actual bone or shell.
  • In a process called replacement, water containing dissolved mineral material might replace original shell or bone material with different minerals such as pyrite or quartz.
  • The process of matching distinctive rock units from different regions is called correlation.
  • Sometimes the relative age of rock units can be found by matching distinctive fossil-bearing rock units from different locations.
  • Fossils that are most useful as global time markers often are from organisms that were wide- spread geographically, but lived in only a narrow, well-defined period of time. Such fossils are called index fossils.
  • The geologic time scale is based on the appearance and disappearance of certain types of fossils.
  • At these boundaries, fossils of certain life forms are no longer present and new life forms begin to appear.
• Absolute Dating
  • Absolute dating enables geologists to determine the numerical age of rocks.
  • One type of absolute dating process is based on the radioactive decay of unstable atoms.
  • The isotope that decays is the parent isotope and the isotope that forms is the daughter isotope.
  • Geologists can measure the ratio of the amount of daughter isotope to the amount of parent isotope to determine the time that has elapsed since the rock formed.
  • Every radioactive isotope has a half-life. The half-life is the time it takes for one-half of a radioactive parent sample to decay to its stable daughter.
  • Generally, absolute ages are determined from unaltered minerals in igneous rocks.
  • Relative dating and absolute dating techniques can be combined to help determine the ages of some fossils.
• Geologic Maps
  • Geologic maps show the horizontal surface distribution of various rock formations.
  • Most geologic maps also include a legend that indicates the ages of the rock formations at the surface.
  • Geologic maps are two-dimensional models of Earth’s crust for a given region.
  • Tectonic processes can cause Earth’s plates to shift, collide, and separate. Sometimes this leads to the deformation of horizontal rock layers.
  • Folding of the crust can also be recognized on geologic maps.
  • If the forces on part of Earth’s crust are mainly compression forces, then rocks subjected to the compression might wrinkle up or fold.
  • An upward wrinkle of rock formations is an anticline and a downward wrinkle is a syncline.
  • The axis of a fold is an imaginary line where Earth’s surface meets the axial plane. The axial plane divides the fold in half, and is usually perpendicular to the compression force.

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