(81).Wave erosion and deposition

Vocabulary

arch

barrier island

beach

breakwater

groin

refraction

sea stack

sea wall

spit

wave-cut cliff

wave-cut platform

Introduction

Waves are important for building up and breaking down shorelines. Waves transport sand onto and off of beaches. They transport sand along beaches. Waves carve structures at the shore.

Wave Action and Erosion

All waves are energy traveling through some type of material, such as water (Figure below). Ocean waves form from wind blowing over the water.

[Figure 1]

Ocean waves are energy traveling through water.

The largest waves form when the wind is very strong, blows steadily for a long time, and blows over a long distance.

The wind could be strong, but if it gusts for just a short time, large waves won’t form. Wave energy does the work of erosion at the shore. Waves approach the shore at some angle so the inshore part of the wave reaches shallow water sooner than the part that is further out. The shallow part of the wave ‘feels’ the bottom first. This slows down the inshore part of the wave and makes the wave ‘bend.’ This bending is called refraction.

Wave refraction either concentrates wave energy or disperses it. In quiet water areas, such as bays, wave energy is dispersed, so sand is deposited. Areas that stick out into the water are eroded by the strong wave energy that concentrates its power on the wave-cut cliff (Figure below).

[Figure 2]

The wave erodes the bottom of the cliff, eventually causing the cliff to collapse.

Other features of wave erosion are pictured and named in Figure below. A wave-cut platform is the level area formed by wave erosion as the waves undercut a cliff. An arch is produced when waves erode through a cliff. When a sea arch collapses, the isolated towers of rocks that remain are known as sea stacks.

[Figure 3]

(a) The high ground is a large wave-cut platform formed from years of wave erosion. (b) A cliff eroded from two sides produces an arch. (c) The top of an arch erodes away, leaving behind a tall sea stack.

Wave Deposition

Rivers carry sediments from the land to the sea. If wave action is high, a delta will not form. Waves will spread the sediments along the coastline to create a beach (Figure below). Waves also erode sediments from cliffs and shorelines and transport them onto beaches.

[Figure 4]

Sand deposits in quiet areas along a shoreline to form a beach.

Beaches can be made of mineral grains, like quartz, rock fragments, and also pieces of shell or coral (Figure below).

[Figure 5]

Quartz, rock fragments, and shell make up the sand along a beach.

Waves continually move sand along the shore. Waves also move sand from the beaches on shore to bars of sand offshore as the seasons change. In the summer, waves have lower energy so they bring sand up onto the beach. In the winter, higher energy waves bring the sand back offshore.

Some of the features formed by wave-deposited sand are in Figure below. These features include barrier islands and spits. A spit is sand connected to land and extending into the water. A spit may hook to form a tombolo.

[Figure 6]

Examples of features formed by wave-deposited sand.

Shores that are relatively flat and gently sloping may be lined with long narrow barrier islands (Figure below). Most barrier islands are a few kilometers wide and tens of kilometers long.

[Figure 7]

(a) Barrier islands off of Alabama. A lagoon lies on the inland side. (b) Barrier islands, such as Padre Island off the coast of Texas, are made entirely of sand. (c) Barrier islands are some of the most urbanized areas of our coastlines, such as Miami Beach.

In its natural state, a barrier island acts as the first line of defense against storms such as hurricanes. When barrier islands are urbanized (Figure above), hurricanes damage houses and businesses rather than vegetated sandy areas in which sand can move. A large hurricane brings massive problems to the urbanized area.

Protecting Shorelines

Intact shore areas protect inland areas from storms that come off the ocean (Figure below).

[Figure 8]

Dunes and mangroves along Baja California protect the villages that are found inland.

Where the natural landscape is altered or the amount of development make damage from a storm too costly to consider, people use several types of structures to attempt to slow down wave erosion. A few are pictured in the Figurebelow. A groin is a long narrow pile of rocks built perpendicular to the shoreline to keep sand at that beach. A breakwater is a structure built in the water parallel to the shore in order to protect the shore from strong incoming waves. A seawall is also parallel to the shore, but it is built onshore.

[Figure 9]

(a) Groins trap sand on the up-current side so then people down current build groins to trap sand too. (b) Breakwaters are visible in this satellite image parallel to the shoreline. (c) Seawalls are similar to breakwaters except built onshore. Extremely large storm waves may destroy the sea wall, leaving the area unprotected.

People do not always want to choose safe building practices, and instead choose to build a beach house right on the beach. Protecting development from wave erosion is difficult and expensive.

Protection does not always work. The northeastern coast of Japan was protected by anti-tsunami seawalls. Yet waves from the 2011 tsunami that resulted from the Tohoku earthquake washed over the top of some seawalls and caused others to collapse. Japan is now planning to build even higher seawalls to prepare for any future (and inevitable) tsunami.

Lesson Summary

Waves in the ocean are what we see as energy travels through the water.

Wave energy produces erosional formations such as cliffs, wave cut platforms, sea arches, and sea stacks.

When waves reach the shore, they can form deposits such as beaches, spits, and barrier islands.

Groins, jetties, breakwaters, and seawalls are structures that protect the shore from breaking waves.

(80).Water erosion and deposition.

Vocabulary

alluvial fan

base level

bed load

column

competence

delta

dissolved load

floodplain

gradient

groundwater

headwaters

meander

natural levee

saltation

sinkhole

stalactite

stalagmite

suspended load

travertine

Introduction

Streams – any running water from a rivulet to a raging river -- complete the hydrologic cycle by returning precipitation that falls on land to the oceans (Figure below). Some of this water moves over the surface and some moves through the ground as groundwater. Flowing water does the work of both erosion and deposition.

[Figure 1]

As streams flow towards the ocean, they carry weathered materials.

Erosion and Deposition by Streams

Erosion by Streams

Flowing streams pick up and transport weathered materials by eroding sediments from their banks. Streams also carry ions and ionic compounds that dissolve easily in the water. Sediments are carried as:

Dissolved load: Dissolved load is composed of ions in solution. These ions are usually carried in the water all the way to the ocean.

Suspended load: Sediments carried as solids as the stream flows are suspended load. The size of particles that can be carried is determined by the stream’s velocity (Figure below). Faster streams can carry larger particles. Streams that carry larger particles have greater competence. Streams with a steep gradient (slope) have a faster velocity and greater competence.

[Figure 2]

Rivers carry sand, silt and clay as suspended load. During flood stage, the suspended load greatly increases as stream velocity increases.

Bed load: Particles that are too large to be carried as suspended load are bumped and pushed along the stream bed as bed load. Bed load sediments do not move continuously. This intermittent movement is called saltation. Streams with high velocities and steep gradients do a great deal of down cutting into the stream bed, which is primarily accomplished by movement of particles that make up the bed load.

Stages of Streams

As a stream flows from higher elevations, like in the mountains, towards lower elevations, like the ocean, the work of the stream changes. At a stream’s headwaters, often high in the mountains, gradients are steep (Figure below). The stream moves fast and does lots of work eroding the stream bed.

[Figure 3]

This stream begins as snow melt from the mountains.

As a stream moves into lower areas, the gradient is not as steep. Now the stream does more work eroding the edges of its banks. Many streams develop curves in their channels called meanders (Figurebelow).

[Figure 4]

(a) At a meander, a stream actively erodes its outer banks and deposits material along the inside curves. This causes these meanders to migrate laterally over time. (b) This stream has deposited larger materials such as gravel and pebbles along the inside curve of a meander. (c) This image is a topographic map. The San Juan River eroded the land surface as the Colorado Plateau uplifted. The river’s meanders were preserved as a feature called incised meanders.

As the river moves onto flatter ground, the stream erodes the outer edges of its banks to carve a floodplain, which is a flat level area surrounding the stream channel (Figure below).

[Figure 5]

The Vistula River in Poland flows onto its floodplain.

Base level is where a stream meets a large body of standing water, usually the ocean, but sometimes a lake or pond. Streams work to down cut in their stream beds until they reach base level. The higher the elevation, the farther the stream is from where it will reach base level and the more cutting it has to do.

Stream Deposition

As a stream gets closer to base level, its gradient lowers and it deposits more material than it erodes. On flatter ground, streams deposit material on the inside of meanders. Placer mineral deposits, described in the Earth's Minerals chapter, are often deposited there. A stream’s floodplain is much broader and shallower than the stream’s channel. When a stream flows onto its floodplain, its velocity slows and it deposits much of its load. These sediments are rich in nutrients and make excellent farmland (Figure below).

[Figure 6]

The Mississippi floodplain is heavily farmed. Flooding can wipe out farms and towns, but the stream also deposits nutrient-rich sediments that enrich the floodplain.

A stream at flood stage carries lots of sediments. When its gradient decreases, the stream overflows its banks and broadens its channel. The decrease in gradient causes the stream to deposit its sediments, the largest first. These large sediments build a higher area around the edges of the stream channel, creating natural levees (Figure below).

[Figure 7]

After many floods, a stream builds natural levees along its banks.

When a river enters standing water, its velocity slows to a stop. The stream moves back and forth across the region and drops its sediments in a wide triangular-shaped deposit called a delta(Figure below).

[Figure 8]

(a) The Nile River delta has a classic triangular shape, like the capital Greek letter delta. (b) Sediment in the Yellow River delta. The main stream channel splits into many smaller distributaries.

If a stream falls down a steep slope onto a broad flat valley, an alluvial fandevelops (Figure below). Alluvial fans generally form in arid regions.

[Figure 9]

An alluvial fan in Iran. The mountains are in the lower right corner of the photograph.

 

Ground Water Erosion and Deposition

Rainwater absorbs carbon dioxide (CO₂) as it falls. The CO₂ combines with water to form carbonic acid. The slightly acidic water sinks into the ground and moves through pore spaces in soil and cracks and fractures in rock. The flow of water underground is ground water.

[Figure 10]

When water sinks into the ground, it becomes ground water.

Ground water is a strong erosional force, as it works to dissolve away solid rock (Figure above). Carbonic acid is especially good at dissolving the rock limestone.

Cave Formation

Working slowly over many years, ground water travels along small cracks. The water dissolves and carries away the solid rock gradually enlarging the cracks. Eventually a cave may form (Figurebelow).

[Figure 11]

Caverns many football fields long and as high as many meters tall form where ground water erodes away rock.

Ground water carries the dissolved minerals in solution. The minerals may then be deposited, for example, as stalagmites or stalactites (Figure below).

[Figure 12]

(a) Stalactites form as calcium carbonate drips from the ceiling of a cave, forming beautiful icicle-like formations. The word stalactite has a c and it forms from the ceiling. (b) Stalagmites form as calcium carbonate drips from the ceiling to the floor of a cave and then grow upwards. The g in stalagmite means it forms on the ground.

If a stalactite and stalagmite join together, they form a column. One of the wonders of visiting a cave is to witness the beauty of these amazing and strangely captivating structures. Caves also produce a beautiful rock, formed from calcium carbonate, travertine. Ground water saturated with calcium carbonate precipitates as the mineral calcite or aragonite. Mineral springs that produce travertine can be hot, warm or even cold (Figure belowf the roof of a cave collapses, a sinkholecould form. Some sinkholes are large enough to swallow up a home or several homes in a neighborhood (Figure below).

[Figure 14]

This sinkhole formed in Florida.

Lesson Summary

Streams erode the land as they move from higher elevations to the sea.

Eroded materials can be carried in a river as dissolved load, suspended load, or bed load.

A river erodes deeply when it is far from its base level, the place where it enters standing water.

Streams form bends, called meanders. Broad, flat areas are known as floodplains.

A delta or an alluvial fan might form where the stream drops its sediment load.

Caves form underground as ground water gradually dissolves away rock.

(79).Soil.

Vocabulary

B horizon

C horizon

humus

inorganic

laterite

loam

pedalfer

pedocal

permeable

residual soil

soil

soil horizon

soil profile

subsoil

topsoil

transported soil

Introduction

Without mechanical and chemical weathering working to break down rock, there would not be any soil on Earth. It is unlikely that humans or most other creatures would be able to live on Earth without soil. Wood, paper, cotton, medicines, and even pure water need soil. So soil is a precious resource that must be carefully managed and cared for. Although soil is a renewable resource, its renewal takes a lot of time.

Characteristics of Soil

Even though soil is only a very thin layer on Earth’s surface over the solid rocks below, it is the where the atmosphere, hydrosphere, biosphere, and lithosphere meet. Within the soil layer, important reactions between solid rock, liquid water, air, and living things take place. Soil is a complex mixture of different materials.

About half of most soils are inorganicmaterials, such as the products of weathered rock, including pebbles, sand, silt, and clay particles.

About half of all soils are organic materials, formed from the partial breakdown and decomposition of plants and animals. The organic materials are necessary for a soil to be fertile. The organic portion provides the nutrients, such as nitrogen, needed for strong plant growth.

In between the solid pieces, there are tiny spaces filled with air and water.

In some soils, the organic portion could be missing, as in desert sand. Or a soil could be completely organic, such as the materials that make up peat in a bog or swamp (Figure below).

[Figure 1]

Peat is so rich in organic material, it can be burned for energy.

Soil is an ecosystem unto itself. In the spaces of soil, there are thousands or even millions of living organisms. Those organisms could be anything from earthworms, ants, bacteria, or fungi (Figure below).

[Figure 2]

Earthworms and insects are important residents of soils.

Climate

Scientists know that climate is the most important factor determining soil type because given enough time, different rock types in a given climate will produce a similar soil. Even the same rock type in different climates will not produce the same type of soil. This is true because most rocks on Earth are made of the same eight elements and when the rock breaks down to become soil, those elements dominate.

The same factors that lead to increased weathering also lead to greater soil formation.

More rain equals more chemical reactions to weather minerals and rocks. Those reactions are most efficient in the top layers of the soil where the water is fresh and has not yet reacted with other materials.

Increased rainfall increases the amount of rock that is dissolved as well as the amount of material that is carried away by moving water. As materials are carried away, new surfaces are exposed, which also increases the rate of weathering.

Increased temperature increases the rate of chemical reactions, which also increases soil formation.

In warmer regions, plants and bacteria grow faster, which helps to weather material and produce soils. In tropical regions, where temperature and precipitation are consistently high, thick soils form. Arid regions have thin soils.

Soil type also influences the type of vegetation that can grow in the region. We can identify climate types by the types of plants that grow there.

Rock Type

The original rock is the source of the inorganic portion of the soil. The minerals that are present in the rock determine the composition of the material that is available to make soil. Soils may form in place or from material that has been moved.

Residual soils form in place. The underlying rock breaks down to form the layers of soil that reside above it. Only about one-third of the soils in the United States are residual.

Transported soils have been transported in from somewhere else. Sediments can be transported into an area by glaciers, wind, water, or gravity. Soils form from the loose particles that have been transported to a new location and deposited.

Slope

The steeper the slope, the less likely material will be able to stay in place to form soil. Material on a steep slope is likely to go downhill. Materials will accumulate and soil will form where land areas are flat or gently undulating.

Time

Soils thicken as the amount of time available for weathering increases. The longer the amount of time that soil remains in a particular area, the greater the degree of alteration.

Biological Activity

The partial decay of plant material and animal remains produces the organic material and nutrients in soil. In soil, decomposing organisms breakdown the complex organic molecules of plant matter and animal remains to form simpler inorganic molecules that are soluble in water. Decomposing organisms also create organic acids that increase the rate of weathering and soil formation. Bacteria in the soil change atmospheric nitrogen into nitrates.

The decayed remains of plant and animal life are called humus, which is an extremely important part of the soil. Humus coats the mineral grains. It binds them together into clumps that then hold the soil together, creating its structure. Humus increases the soil’s porosity and water holding capacity and helps to buffer rapid changes in soil acidity. Humus also helps the soil to hold its nutrients, increasing its fertility. Fertile soils are rich in nitrogen, contain a high percentage of organic materials, and are usually black or dark brown in color. Soils that are nitrogen poor and low in organic material might be gray or yellow or even red in color. Fertile soils are more easily cultivated.

Soil Texture and Composition

The inorganic portion of soil is made of many different size particles, and these different size particles are present in different proportions. The combination of these two factors determines some of the properties of the soil.

A permeable soil allows water to flow through it easily because the spaces between the inorganic particles are large and well connected. Sandy or silty soils are considered ‘light’ soils because they are permeable, water-draining types of soils.

Soils that have lots of very small spaces are water-holding soils. For example, when clay is present in a soil, the soil is heavier, holds together more tightly, and holds water.

When a soil contains a mixture of grain sizes, the soil is called a loam(Figure below).

[Figure 3]

A loam field.

When soil scientists want to precisely determine soil type, they measure the percentage of sand, silt, and clay. They plot this information on a triangular diagram, with each size particle at one corner (Figure below). The soil type can then be determined from the location on the diagram. At the top, a soil would be clay; at the left corner, it would be sand, and at the right corner it would be silt. Soils in the lower middle with less than 50% clay are loams.

Soil Horizons and Profiles

A residual soil forms over many years, as mechanical and chemical weathering slowly change solid rock into soil. The development of a residual soil may go something like this.

The bedrock fractures because of weathering from ice wedging or another physical process.

Water, oxygen, and carbon dioxide seep into the cracks to cause chemical weathering.

Plants, such as lichens or grasses, become established and produce biological weathering.

Weathered material collects until there is soil.

The soil develops soil horizons, as each layer becomes progressively altered. The greatest degree of weathering is in the top layer. Each successive, lower layer is altered just a little bit less. This is because the first place where water and air come in contact with the soil is at the top.

A cut in the side of a hillside shows each of the different layers of soil. All together, these are called a soil profile (Figurebelow).

[Figure 5]

Soil is an important resource. Each soil horizon is distinctly visible in this photograph.

The simplest soils have three horizons.

Topsoil

Called the A horizon, the topsoil is usually the darkest layer of the soil because it has the highest proportion of organic material. The topsoil is the region of most intense biological activity: insects, worms, and other animals burrow through it and plants stretch their roots down into it. Plant roots help to hold this layer of soil in place. In the topsoil, minerals may dissolve in the fresh water that moves through it to be carried to lower layers of the soil. Very small particles, such as clay, may also get carried to lower layers as water seeps down into the ground.

Subsoil

The B horizon or subsoil is where soluble minerals and clays accumulate. This layer is lighter brown and holds more water than the topsoil because of the presence of iron and clay minerals. There is less organic material. Figurebelow.

[Figure 6]

A soil profile is the complete set of soil layers. Each layer is called a horizon.

C horizon

The C horizon is a layer of partially altered bedrock. There is some evidence of weathering in this layer, but pieces of the original rock are seen and can be identified.

Not all climate regions develop soils, and not all regions develop the same horizons. Some areas develop as many as five or six distinct layers, while others develop only very thin soils or perhaps no soils at all.

Types of Soils

Although soil scientists recognize thousands of types of soil – each with its own specific characteristics and name - let’s consider just three soil types. This will help you to understand some of the basic ideas about how climate produces a certain type of soil, but there are many exceptions to what we will learn right now (Figure below).

[Figure 7]

Just some of the thousands of soil types.

Pedalfer

Deciduous trees, the trees that lose their leaves each winter, need at least 65 cm of rain per year. These forests produce soils called pedalfers, which are common in many areas of the temperate, eastern part of the United States (Figure below). The word pedalfer comes from some of the elements that are commonly found in the soil. The Al in pedalfer is the chemical symbol of the element aluminum, and the Fe in pedalfer is the chemical symbol for iron. Pedalfers are usually a very fertile, dark brown or black soil. Not surprising, they are rich in aluminum clays and iron oxides. Because a great deal of rainfall is common in this climate, most of the soluble minerals dissolve and are carried away, leaving the less soluble clays and iron oxides behind.

[Figure 8]

A pedalfer is the dark, fertile type of soil that will form in a forested region.

Pedocal

Pedocal soils form in drier, temperate areas where grasslands and brush are the usual types of vegetation (Figurebelow). The climates that form pedocals have less than 65 cm rainfall per year, so compared to pedalfers, there is less chemical weathering and less water to dissolve away soluble minerals so more soluble minerals are present and fewer clay minerals are produced. It is a drier region with less vegetation, so the soils have lower amounts of organic material and are less fertile.

A pedocal is named for the calcite enriched layer that forms. Water begins to move down through the soil layers, but before it gets very far, it begins to evaporate. Soluble minerals, like calcium carbonate, concentrate in a layer that marks the lowest place that water was able to reach. This layer is called caliche.

[Figure 9]

A pedocal is the alkaline type of soil that forms in grassland regions.

Laterite

In tropical rainforests where it rains literally every day, laterite soils form (Figure below). In these hot, wet, tropical regions, intense chemical weathering strips the soils of their nutrients. There is practically no humus. All soluble minerals are removed from the soil and all plant nutrients are carried away. All that is left behind are the least soluble materials, like aluminum and iron oxides. These soils are often red in color from the iron oxides. Laterite soils bake as hard as a brick if they are exposed to the sun.

[Figure 10]

A laterite is the type of thick, nutrient poor soil that forms in the rainforest.

Many climates types have not been mentioned here. Each produces a distinctive soil type that forms in the particular circumstances found there. Where there is less weathering, soils are thinner but soluble minerals may be present. Where there is intense weathering, soils may be thick but nutrient poor. Soil development takes a very long time, it may take hundreds or even thousands of years for a good fertile topsoil to form. Soil scientists estimate that in the very best soil-forming conditions, soil forms at a rate of about 1mm/year. In poor conditions, soil formation may take thousands of years!

Soil Conservation

Soil is only a renewable resource if it is carefully managed. Drought, insect plagues, or outbreaks of disease are natural cycles of events that can negatively impact ecosystems and the soil, but there are also many ways in which humans neglect or abuse this important resource.

One harmful practice is removing the vegetation that helps to hold soil in place. Sometimes just walking or riding your bike over the same place will kill the grass that normally grows there. Land is also deliberately cleared or deforested for wood. The loose soils then may be carried away by wind or running water. In many areas of the world, the rate of soil erosion is many times greater than the rate at which it is forming. Soils can also be contaminated if too much salt accumulates in the soil or where pollutants sink into the ground. There are many practices that can protect and preserve soil resources. Adding organic material to the soil in the form of plant or animal waste, such as compost or manure, increases the fertility of the soil and improves its ability to hold onto water and nutrients (Figure below). Inorganic fertilizer can also temporarily increase the fertility of a soil and may be less expensive or time consuming, but it does not provide the same long-term improvements as organic materials.

[Figure 11]

Organic material can be added to soil to help increase its fertility.

Agricultural practices such as rotating crops, alternating the types of crops planted in each row, and planting nutrient rich cover crops all help to keep soil more fertile as it is used season after season. Planting trees as windbreaks, plowing along contours of the field, or building terraces into steeper slopes will all help to hold soil in place (Figurebelow). No-till or low-tillage farming helps to keep soil in place by disturbing the ground as little as possible when planting.

[Figure 12]

Steep slopes can be terraced to make level planting areas and decrease surface water runoff and erosion.

Lesson Summary

Soil is an important resource. Life on Earth could not exist as it does today without soil.

The type of soil that forms depends mostly on climate and, to a lesser extent, on the original parent rock material and other factors.

Soil texture and composition, plus the amount of organic material in a soil, determine a soil's qualities and fertility.

Given enough time, rock is weathered to produce a layered soil, called a soil profile.

Each type of climate can ultimately produce a unique type of soil.

(77).Volcanic landforms.

Vocabulary

geyser

hot spring

lava dome

lava plateau

Introduction

Volcanoes are associated with many types of landforms. The landforms vary with the composition of the magma that created them. Hot springs and geysers are also examples of surface features related to volcanic activity.

Landforms from Lava

Volcanoes and Vents

The most obvious landforms created by lava are volcanoes, most commonly as cinder cones, composite volcanoes, and shield volcanoes. Eruptions also take place through fissures (Figure below). The eruptions that created the entire ocean floor are essentially fissure eruptions.

[Figure 1]

A fissure eruption on Mauna Loa in Hawaii travels toward Mauna Kea on the Big Island.

Lava Domes

When lava is viscous, it is flows slowly. If there is not enough magma or enough pressure to create an explosive eruption, the magma may form a lava dome.Because it is so thick, the lava does not flow far from the vent. (Figure below).

[Figure 2]

Lava domes are large, round landforms created by thick lava that does not travel far from the vent.

Lava flows often make mounds right in the middle of craters at the top of volcanoes, as seen in the Figure below.

[Figure 3]

Lava domes may form in the crater of composite volcanoes as at Mount St. Helens

Lava Plateaus

A lava plateau forms when large amounts of fluid lava flows over an extensive area (Figure below). When the lava solidifies, it creates a large, flat surface of igneous rock.

[Figure 4]

Layer upon layer of basalt have created the Columbia Plateau, which covers more than 161,000 square kilometers (63,000 square miles) in Washington, Oregon, and Idaho.

Land

Lava creates new land as it solidifies on the coast or emerges from beneath the water (Figure below).

[Figure 5]

Lava hitting seawater creates new land.

Over time the eruptions can create whole islands. The Hawaiian Islands are formed from shield volcano eruptions that have grown over the last 5 million years (Figure below).

[Figure 6]

A compilation of satellite images of the Big Island of Hawaii with its five volcanoes.

Landforms from Magma

Magma intrusions can create landforms. Shiprock in New Mexico is the neck of an old volcano that has eroded away (Figure below).

[Figure 7]

The aptly named Shiprock in New Mexico.

Hot Springs and Geysers

Water sometimes comes into contact with hot rock. The water may emerge at the surface as either a hot spring or a geyser.

Hot Springs

Water heated below ground that rises through a crack to the surface creates a hot spring (Figure below). The water in hot springs may reach temperatures in the hundreds of degrees Celsius beneath the surface, although most hot springs are much cooler.

[Figure 8]

Even some animals enjoy relaxing in nature's hot tubs.

Geysers

Geysers are also created by water that is heated beneath the Earth’s surface, but geysers do not bubble to the surface -- they erupt. When water is both superheated by magma and flows through a narrow passageway underground, the environment is ideal for a geyser. The passageway traps the heated water underground, so that heat and pressure can build. Eventually, the pressure grows so great that the superheated water bursts out onto the surface to create a geyser (Figurebelow).

Conditions are right for the formation of geysers in only a few places on Earth. Of the roughly 1,000 geysers worldwide and about half are found in the United States.

[Figure 9]

Castle Geyser is one of the many geysers at Yellowstone National Park. Castle erupts regularly, but not as frequently or predictably as Old Faithful.

Lesson Summary

Viscous lava can produce lava domes along a fissure or within a volcano.

Lava plateaus form from large lava flows that spread out over large areas.

Many islands are built by or are volcanoes.

Igneous intrusions associated with volcanoes may create volcanic landforms.

When magma heats groundwater, it can reach the surface as hot springs or geysers.

(76).Where volcanoes are located.

 Where Volcanoes Are Located ?

Vocabulary

fissure

Introduction

Volcanoes are a vibrant manifestation of plate tectonics processes. Volcanoes are common along convergent and divergent plate boundaries. Volcanoes are also found within lithospheric plates away from plate boundaries. Wherever mantle is able to melt, volcanoes may be the result.

[Figure 1]

World map of active volcanoes.

See if you can give a geological explanation for the locations of all the volcanoes in Figure above. What is the Pacific Ring of Fire? Why are the Hawaiian volcanoes located away from any plate boundaries? What is the cause of the volcanoes along the mid-Atlantic ridge?

Volcanoes erupt because mantle rock melts. This is the first stage in creating a volcano. Remember from the chapter “Rocks” that mantle may melt if temperature rises, pressure lowers, or water is added. Be sure to think about how melting occurs in each of the following volcanic settings.

Convergent Plate Boundaries

Why does melting occur at convergent plate boundaries? The subducting plate heats up as it sinks into the mantle. Also, water is mixed in with the sediments lying on top of the subducting plate. This water lowers the melting point of the mantle material, which increases melting. Volcanoes at convergent plate boundaries are found all along the Pacific Ocean basin, primarily at the edges of the Pacific, Cocos, and Nazca plates. Trenches mark subduction zones, although only the Aleutian Trench and the Java Trench appear on the map in Figure above.

Remember your plate tectonics knowledge. Large earthquakes are extremely common along convergent plate boundaries. Since the Pacific Ocean is rimmed by convergent and transform boundaries, about 80% of all earthquakes strike around the Pacific Ocean basin (Figure below). Why are 75% of the world’s volcanoes found around the Pacific basin? Of course, these volcanoes are caused by the abundance of convergent plate boundaries around the Pacific.

[Figure 2]

The Pacific Ring of Fire is where the majority of the volcanic activity on the Earth occurs.

A description of the Pacific Ring of Fire along western North America is a description of the plate boundaries.

Subduction at the Middle American Trench creates volcanoes in Central America.

The San Andreas Fault is a transform boundary.

Subduction of the Juan de Fuca plate beneath the North American plate creates the Cascade volcanoes.

Subduction of the Pacific plate beneath the North American plate in the north creates the Aleutian Islands volcanoes.

The Cascades are shown on this interactive map with photos and descriptions of each of the volcanoes: http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/interactive/6.Volcanoes4Rollover.swf.

This incredible explosive eruption of Mount Vesuvius in Italy in A.D. 79 is an example of a composite volcano that forms as the result of a convergent plate boundary (3f): http://www.youtube.com/watch?v=1u1Ys4m5zY4 (1:53).

Divergent plate boundaries

Why does melting occur at divergent plate boundaries? Hot mantle rock rises where the plates are moving apart. This releases pressure on the mantle, which lowers its melting temperature. Lava erupts through long cracks in the ground, or fissures

Volcanoes erupt at mid-ocean ridges, such as the Mid-Atlantic ridge, where seafloor spreading creates new seafloor in the rift valleys. Where a hotspot is located along the ridge, such as at Iceland, volcanoes grow high enough to create islands (Figure below).

[Figure 3]

A volcanic eruption at Surtsey, a small island near Iceland.

Eruptions are found at divergent plate boundaries as continents break apart. The volcanoes in Figure below are in the East African Rift between the African and Arabian plates.

[Figure 4]

Mount Gahinga, a mountain in Uganda, located in the East African Rift valley.

Volcanic Hotspots

Although most volcanoes are found at convergent or divergent plate boundaries, intraplate volcanoes are found in the middle of a tectonic plate. Why is there melting at these locations? The Hawaiian Islands are the exposed peaks of a great chain of volcanoes that lie on the Pacific plate. These islands are in the middle of the Pacific plate. The youngest island sits directly above a column of hot rock called a mantle plume. As the plume rises through the mantle, pressure is released and mantle melts to create a hotspot (Figure below).

[Figure 5]

(a) The Society Islands formed above a hotspot that is now beneath Mehetia and two submarine volcanoes. (b) The satellite image shows how the islands become smaller and coral reefs became more developed as the volcanoes move off the hotspot and grow older.

Earth is home to about 50 known hot spots. Most of these are in the oceans because they are better able to penetrate oceanic lithosphere to create volcanoes. The hotspots that are known beneath continents are extremely large, such as Yellowstone (Figure below).

[Figure 6]

Prominent hotspots of the worl

Lesson Summary

Most volcanoes are found along convergent or divergent plate boundaries.

The Pacific Ring of Fire is the most geologically active region in the world.

Volcanoes such as those that form the islands of Hawaii form over hotspots, which are melting zones above mantle plumes.

Points to Consider

Some volcanoes are no longer active. What could cause a volcano to become extinct?

Hot spots are still poorly understood by Earth scientists. Why do you think it’s hard to understand hotspots? What clues are there regarding these geological phenomena?

Volcanoes have been found on Venus, Mars, and even Jupiter’s moon Io. What do you think this indicates to planetary geologists?

(75).Volcanos.

Vocabulary

caldera

cinder cone

composite volcano

shield volcano

supervolcano

Introduction

A volcano is a vent through which molten rock and gas escape from a magma chamber. Volcanoes differ in many features such as height, shape, and slope steepness. Some volcanoes are tall cones and others are just cracks in the ground (Figure below). As you might expect, the shape of a volcano is related to the composition of its magma.

[Figure 1]

Mount St. Helens was a beautiful, classic, cone-shaped volcano. The volcano’s 1980 eruption blew more than 400 meters (1,300 feet) off the top of the mountain.

Composite Volcanoes

Composite volcanoes are made of felsic to intermediate rock. The viscosity of the lava means that eruptions at these volcanoes are often explosive (Figurebelow).

[Figure 2]

Mt. Fuji, the highest mountain in Japan, is a dormant composite volcano.

The viscous lava cannot travel far down the sides of the volcano before it solidifies, which creates the steep slopes of a composite volcano. Viscosity also causes some eruptions to explode as ash and small rocks. The volcano is constructed layer by layer, as ash and lava solidify, one upon the other (Figurebelow). The result is the classic cone shape of composite volcanoes.

[Figure 3]

A cross section of a composite volcano reveals alternating layers of rock and ash: (1) magma chamber, (2) bedrock, (3) pipe, (4) ash layers, (5) lava layers, (6) lava flow, (7) vent, (8) lava, (9) ash cloud. Frequently there is a large crater at the top from the last eruption.

Shield Volcanoes

Shield volcanoes get their name from their shape. Although shield volcanoes are not steep, they may be very large. Shield volcanoes are common at spreading centers or intraplate hot spots (Figure below).

[Figure 4]

Mauna Loa Volcano in Hawaii (in the background) is the largest shield volcano on Earth with a diameter of more than 112 kilometers (70 miles). The volcano forms a significant part of the island of Hawaii.

The lava that creates shield volcanoes is fluid and flows easily. The spreading lava creates the shield shape. Shield volcanoes are built by many layers over time and the layers are usually of very similar composition. The low viscosity also means that shield eruptions are non-explosive.

This Volcanoes 101 video from National Geographic discusses where volcanoes are found and what their properties come from (3e): http://www.youtube.com/watch?v=uZp1dNybgfc (3:05).

Cinder Cones

Cinder cones are the most common type of volcano. A cinder cone has a cone shape, but is much smaller than a composite volcano. Cinder cones rarely reach 300 meters in height but they have steep sides. Cinder cones grow rapidly, usually from a single eruption cycle (Figure below). Cinder cones are composed of small fragments of rock, such as pumice, piled on top of one another. The rock shoots up in the air and doesn’t fall far from the vent. The exact composition of a cinder cone depends on the composition of the lava ejected from the volcano. Cinder cones usually have a crater at the summit.

[Figure 5]

In 1943, a Mexican farmer first witnessed a cinder cone erupting in his field. In a year, Paricutín was 336 meters high. By 1952, it reached 424 meters and then stopped erupting.

Cinder cones are often found near larger volcanoes (Figure below).

[Figure 6]

This Landsat image shows the topography of San Francisco Mountain, an extinct volcano, with many cinder cones near it in northern Arizona. Sunset crater is a cinder cone that erupted about 1,000 years ago.

Supervolcanoes

Supervolcano eruptions are extremely rare in Earth history. It’s a good thing because they are unimaginably large. A supervolcano must erupt more than 1,000 cubic km (240 cubic miles) of material, compared with 1.2 km³ for Mount St. Helens or 25 km³ for Mount Pinatubo, a large eruption in the Philippines in 1991. Not surprisingly, supervolcanoes are the most dangerous type of volcano.

Supervolcanoes are a fairly new idea in volcanology. The exact cause of supervolcano eruptions is still debated. However, scientists think that a very large magma chamber erupts entirely in one catastrophic explosion. This creates a huge hole or caldera into which the surface collapses (Figure below).

[Figure 7]

The caldera at Santorini in Greece is so large that it can only be seen by satellite.

The largest supervolcano in North America is beneath Yellowstone National Park in Wyoming. Yellowstone sits above a hotspot that has erupted catastrophically three times: 2.1 million, 1.3 million, and 640,000 years ago. Yellowstone has produced many smaller (but still enormous) eruptions more recently (Figure below). Fortunately, current activity at Yellowstone is limited to the region’s famous geysers.

[Figure 8]

The Yellowstone hotspot has produced enormous felsic eruptions. The Yellowstone caldera collapsed in the most recent super eruption.

Long Valley Caldera, south of Mono Lake in California, is the second largest supervolcano in North America (Figurebelow). Long Valley had an extremely hot and explosive rhyolite about 700,000 years ago. An earthquake swarm in 1980 alerted geologists to the possibility of a future eruption, but the quakes have since calmed down.

[Figure 9]

The hot water that gives Hot Creek, California, its name is heated by hot rock below Long Valley Caldera.

An interactive image of the geological features of Long Valley Caldera is available here:

A supervolcano could change life on Earth as we know it. Ash could block sunlight so much that photosynthesis would be reduced and global temperatures would plummet. Volcanic eruptions could have contributed to some of the mass extinctions in our planet’s history.

Lesson Summary

Composite, shield, cinder cones, and supervolcanoes are the main types of volcanoes.

Composite volcanoes are tall, steep cones that produce explosive eruptions.

Shield volcanoes form very large, gently sloped mounds from effusive eruptions.

Cinder cones are the smallest volcanoes and result from accumulation of many small fragments of ejected material.

An explosive eruption may create a caldera, a large hole into which the mountain collapses.

Supervolcano eruptions are devastating but extremely rare in Earth history.