Physical Geography

Unit 1 - Land and Water FormsWearing Down Landforms

DENUDATIONAL PROCESSES (p. 22)• Natural forces gradually wear down and change Earth’s topographical

features:– Weathering is the breakdown of rocks and minerals by physical pressures and

chemical reactions.– Erosion includes two processes: 1) the breakdown of rocks and minerals, but

also 2) the moving (transportation) of weathered material from its source to another location (deposition).

– Denudation refers to the laying bare or wearing down of underlying rocks by the processes of weathering and erosion.

WEATHERING (p. 22)• There are two types of weathering that affect landforms,

breaking rock down into fragments, or further disintegrating them into sand or soil:

• Physical weathering is the break down of rock through mechanical (physical) processes in the environment. Physical weathering may occur in the following five ways:– Frost fracture: the expansion of freezing water that causes rocks

to crack.– Heat expansion: rocks can expand and subsequently fracture.– Exfoliation: the breaking of rounded sheets of rock along curved

surfaces. Often caused by the release of internal pressure.– Plant growth: expansion of rock due to root growth.– Burrowing animals: tunnelling animals can increase of the size of

existing cracks in rock.

• Chemical weathering is the break down of rock through chemical change, usually involving the action of rainwater. Chemical weathering may occur in the following three ways:– Solution: occurs when rainwater absorbs carbon dioxide or

sulphur dioxide from the atmosphere along with organic acids from the soil, which then reacts with rock. Acids, such as these, often dissolve limestone, breaking it down completely, forming caves or caverns in rock.

– Hydrolysis: involves minerals reacting in solution. Carbonic acid reacts with silicates to form a new, soft clay. This clay is ultimately decayed into soil or mineral deposits.

– Oxidation: occurs when metallic minerals react with oxygen in water, producing softer minerals as a result. These softer minerals weaken rock, which begins to crumble. Oxidation with iron results in yellowish- or reddish brown stains on rock, while oxidation with copper results in pale-greenish stains.

Environmental factors and weathering. For example…

Temperature change – • Where temperature change is rapid (i.e.: deserts) there is an

increase the rate of heat expansion. In tropics the opposite is true due to more consistent temperatures.• High temperatures increase the rates of chemical reaction, such as

oxidation.

Precipitation levels – • Abundant rainfall can increase frost fracture where temperatures

are low.• A lot of rain also leads to higher rates of dissolving due to solution

and higher rates of hydrolysis.

*Complete Weathering Activity Sheet*

Rivers can be one of three types, based on characteristics determined by their location:

1. Youthful Rivers are usually found in areas of high relief (mountains).• Steep slope• Small body of water• Rapid flow• Relatively straight course

Because youthful rivers appear in mountain areas with a high slope, the water runs faster and erodes deeper into the riverbed (vertical erosion). For this reason they tend to be narrow in width.

STAGES IN A RIVER’S LIFE CYCLE (p. 25)

2. Mature Rivers are found in areas where there is a gentle slope and moderate relief.

• Broad river valley• Wide in width• Tributaries (small streams flowing into the river)• Gentle, “meandering” course (winding and curved in shape)• Late maturity rivers are characterized by broad, well developed flood

plains.

Mature rivers have slower moving water than youthful rivers due to lower slopes. They pick up sediment from a river’s banks rather than the deeper river bed (lateral erosion). Sediment tends to settle along the river’s edge, creating obstacles for the river to flow around.

3. Old Rivers usually appear in areas of nearly flat land towards the end of a river’s course.

• Very slow flowing water• Elaborate and well developed “meanders” or curves• Muddy due to suspended sediment• Swamp-like conditions due to poor drainage

Old rivers experience much more sediment deposition than mature rivers, due to a very slow movement of water. This often creates oxbow lakes, horseshoe-shaped bodies of water that that have been cut off from the main river.

Which parts of this river system would be labelled youthful, mature, or old?

*Complete “Rivers” activity sheet*

EROSIONAL AND DEPOSITIONAL PROCESSES (p.30)

• Rivers concentrate their energy to erode both vertically and laterally: – Vertical erosion: occurs when a river erodes the bottom of its

channel (primarily youthful rivers). – Lateral erosion: occurs when a river erodes the sides of its

channel (primarily old rivers).• The following processes wear down rock in rivers:– Hydraulic pressure: the physical pressure of water; the erosional

force that directly removes loose rock fragments.– Corrosion/solution: highly acidic water dissolves minerals in

solution.– Abrasion: the grinding away of bedrock by fragments of rock

incorporated in ice, water, or wind. Pieces of rock collide in water and break apart.

DELTA FORMATION (p.31)• Material eroded by a river may be in the form of dissolved material,

suspended particles, or heavier particles which may be rolled and bounced along the river bottom.

• As a rivers gradient (slope) decreases, particles carried along by the river begin to be deposited.

• Once the river encounters the still waters of an ocean, lake, sea or bay deposition occurs quickly:– Large particles are deposited close to the shore (heavy and cannot be

transported far), forming a thick layer of sediment.– Smaller particles are deposited out in deep water (they are lighter and travel

further than large particles), forming a thin sediment layer.– This sediment deposited at the mouth of a river as it drains into a lake or ocean

is called a delta.– The river continues to flow over the delta, continuing to deposit sediments.

Three types of deltas may develop:• Arcuate Delta: a fan-shaped delta like that of the Nile.

• Digitate Delta: a delta with long fingers of sediment extending into the sea.

• Estuarine Delta: occurs when a river empties into a bay or estuary that juts into the land:– In estuaries fresh water mixes with sea water.– Sediment is deposited both from river outflow and from inflowing sea currents

or flood tides.– At low tide you can see the estuarine delta in the form of tidal mud flats.– This delta’s shape is confined to the shape of the estuary.

GLACIERS AS AGENTS OF EROSION (p.32)• Glaciers are masses of ice that are moving or have moved over land.

They flow like rivers under the influence of gravity.• A tenth of Earth’s surface is covered by ice today, and up to a third

was glaciated over 10,000 years ago. How do we know this?• Glaciers, as agents of erosion, change the features of the land as

they move and these can be analyzed to the time period of bygone ice sheets and the direction in which these glaciers moved.

• Glaciers erode the land in three ways:1. Glacial ice pushes loose material along the sides and front of the ice sheet in

“bulldozing” action. This material is often picked up or embedded into the glacier and is transported.

2. Material picked up by the glacier will scape and gouge out new material as the ice continues to move. Smooth surfaces that have been glaciated are referred to as glacial polish, while gouged and scratched surfaces are called glacial striations.

3. Glacial ice may freeze to underlying rock and, as the glacier moves, pluck or pull blocks and fragments.

Continental Glaciation• Continental glaciers are large sheets

of ice that cover major portions of entire landmasses.

• Today they are found covering large parts of Greenland and Antarctica.

• Evidence of erosional and depositional effects of glaciers are found throughout continental areas in the northern hemisphere and help glaciologists determine former movement of glaciers.

• As continental glaciers erode the land, they create specific landform features.

See also Figure 2.13 on page 34.

A number of features result from Continental glaciation:

Feature CharacteristicsOutwash Plain •meltwater flowing from a glacier deposits silt much like a river

delta; •silt is deposited in layers; •small particles are carried further away; •larger particles drop closer to the glacier.

Terminal Moraine •heap or ridge of bulldozed gravel•marks the end of forward motion of a glacier; •as a glacier retreats it deposits debris/gravel.

Erratic •large boulders that were transported long distances and dropped; •they now sit in a region where they look very much out-of-place.

Drumlin •egg shaped hill formed under glaciers; •formation: ice melts under glacier, which deposits gravel. As glacier moves forward, these deposits are bulldozed along and where they catch on rough areas material piles up forming drumlins.

Eskers •long, ridge-like deposits of eroded glacial material; •formed by sub-glacial streams that deposit material like all rivers.

Evidence for direction of movement of a glacier• The gently-sloped end of drumlins point in the direction of glacier

movement.

• The terminal moraine marks the furthest extent of the glacier (the glacier bulldozed to a point and then reversed direction as it melted).

• The layers of silt in an outwash plain can indicate direction of glacier movement. Fine particles would be at the leading edge while larger particles would have been closer to the glacier.

Alpine Glaciation• Alpine glaciers are large sheets of ice

that form in high mountain valleys above the snowline (the altitude above which snow is permanently maintained on a mountain top).

• The slopes of mountains cause the glaciers to move downhill under the influence of gravity.

• These “rivers of ice” gouge and cut deep into rock and carve out the breathtaking landscapes we associate with many mountain regions.

• As alpine glaciers erode the land, they create specific landform features.

Baltoro Glacier in Pakistan, the world’s largest alpine glacier.

Three alpine glaciers in Canada’s Yukon

A number of features result from Alpine glaciation:

Feature CharacteristicsCirque •a circular hollow cut into bedrock during glaciation;

•side and back walls are steep but front wall opens downward. Arête •steep knife edged ridge between two cirques in a mountainous

region. Terminal Moraine •mounds of deposits that mark the farthest extent of the alpine

glacier the same as with continental glaciers. Lateral Moraine •mounds of earth deposited at the side of a glacier.Medial Moraine •forms when two alpine glaciers meet and the debris on the edges

of each join and are carried on top of the enlarged glacier. Hanging Valley •a high level tributary valley from which the ground falls sharply to

the level of the lower, main valley. The depth of the lower valley is due to more severe glaciation.

Fjord •alpine Glaciers erode troughs and valleys in the mountain; •glacier valley reaches the coast; •glacier melts and sea water floods the valley, creating a fjord.

*Complete “Alpine Glaciation Worksheet*

WAVE EROSION (p.41)• Large lakes and oceans get their erosive force from wind. Wind drags

on the surface of water, transferring its energy through water as friction. The result of this energy transfer is a wave.

• When waves strike a coastline, erosion takes place through hydraulic pressure, abrasion, and corrosion (the same as river erosion).

Wave movement and directionTo understand wave action you must consider two things:1. In open water, waves develop in long ridges or fronts (roughly

perpendicular to the direction of the wind). – As wave fronts approach a shoreline they line up parallel to the shoreline.– This bending of the wave is referred to as wave refraction.

– Wave refraction results from wave fronts approaching shallow shorelines.– Energy of a wave decreases as it approaches a shoreline.– Headlands (areas of high land jutting out into sea) receive most of the wave

energy.– Beaches receive less energy then headlands.

2. The second aspect of wave movement involves the potential of the wave to move eroded sand and gravel along the shoreline, called Longshore Drift.– The largest or dominant wave hits the shoreline at an angle and moves loose

sediment along the shoreline.– Weather patterns or shoreline shape may produce secondary waves in other

directions moving the sediment in those directions.– Depositional features are created by these sediments once the wave loses its

energy

Effects of Wave Erosion and DepositionThe ways in which waves change a coastline depends on how the coastline has been formed:• Irregular submergent coastlines have headlands that protrude out

from the shore line as a result of glaciation. • Immensely heavy glaciers eroded coastline and when they melted

the land “rebounded”, causes the sea floor to rise.• These coastlines have shallow water as a result.• Valleys and low-lying land may be flooded permanently, creating a

new shoreline farther inland.

• The erosion of the headland can deposit silt in the bay which forms a bay beach. The headland is reduced due to erosion and the bay is being filled by deposition.

• Longshore drift results in some sand being deposited parallel to the shore, but connected to the headland. These silt deposits are known as spits.

• Longshore drift and deposition can continue to the point that the spit closes off the mouth of the bay. This extensive deposit is known as a bay bar.

• Continued erosion and deposition can straighten a coastline over a long period of time, reducing its irregularity.

*Read “The Curonian Spit” on pp. 45-46 and complete # 17.*

• Irregular emergent coastlines (also called cliff-and-terrace) occur when water deepens rapidly from shore and land rises steeply upwards.

• The great weight of glaciers “depressed” the land in coastal areas and as the ice sheets melted land slowly “bounced back” to earlier elevations.

• These coastlines are often fairly straight and regular in formation, with rocky shores.

Steps in the erosion of emergent coastlinesErosion creates coastal features as emergent shorelines straighten:1. Sea Cave Formation:

– waves strike the headland first – waves refract around the headland and put hydraulic pressure on both sides of

the headland – erosion of the weak portions create sea caves (hollows) and blow holes in the

sides of the head land

2. Sea Arch Formation:– Eventually sea caves on alternate sides of the headland get deeper until they

connect inside the headland forming a complete passage way or tunnel called a sea arch through the head land.

3. Sea Stack Formation:– continuous erosion of sea arches causes the collapse of the ground over the

arch (the weight of material above the arch is too great to be supported) – this leaves a pillar or column of rock called a sea stack d standing alone where

the headland once was.

*Read pp. 46-49 and complete questions 18, 19, and 20*