UNIT 2 LESSON 12 NATURAL EROSION

OBJECTIVES

Explain how water causes erosion and deposition. Explain how waves cause erosion and deposition. Explain how wind causes erosion and deposition. Explain plate tectonics and the ocean floor features and formation.VOCABULARY

abrasion Wearing away or grinding by friction

alluvial fan A wide, sloping deposit of sediment formed where a stream leaves a mountain range

barrier beach Form when storm waves pile up large amounts of sand above sea level, forming a long, narrow island parallel to the coast

beach An area of wave-washed sediment along a coast

deflation The process by which wind removes surface materials

delta triangular deposition of fine, fertile soil at the mouth of a river

desert pavement An (desert-like) area of rock fragments caused by deflation

flood plain Flat, wide area of land along a river

glacier A large mass of ice that moves slowly over land

groundwater Underground water

headland A part of the shore that sticks out into the ocean. It’s made of harder rock that resists erosion by waves

loess large, fertile deposit of mostly silt carried long distances by wind

longshore drift The process by which waves repeatedly hit the beach and some of the beach sediment moves down the beach with the current

plucking The process by which a glacier picks up rocks as it flows over land

runoff Moving water that flows over the land

sandbar Long narrow ridges of sand parallel to the shore

sand dune A deposit of windblown sand that forms when wind meets an obstacle

sea arch A feature of wave erosion that forms when waves erode a layer of softer rock that underlies a layer of harder rock

sea stack A pillar of rock that forms when a sea arch collapses

sinkhole A depression that forms when the roof of a cave collapses because of the erosion of the underlying limestone

spit A beach that projects like a finger out into the water. Spits form as a result of deposition by longshore drift

stalactite A deposit of carbonic acid that hangs like an icicle from the roof of a cave

stalagmite A deposit that forms from the cave floor as slow dripping carbonic acid builds up in a cone-shape

tributary A stream or river that flows into a larger river

wave-cut cliff The result of waves eroding the base of a cliff so much that the rock above collapses

Topography is the shape of Earth’s surface and its physical features, such as mountains, valleys, canyons, and other landforms.

It’s these features and others that are continuously built up and broken down through processes such as weathering, erosion, deposition, and deflation.

This can be accomplished by many factors, most notably the elements, including all factors of the climate, like rain, snow, ice, and wind.

The earth is also reshaped by the actions of waves, floods, and storms. Once a rock material has been weathered (or broken down), it is ready to be

transported, or eroded (moved). Erosion refers to the transportation of rock, soil, and mineral particles from one

location to another. GRAVITY IS THE DRIVING FORCE BEHIND THE EROSION PROCESS. Without gravitational pull, major natural agents of erosion, such as wind, running

water, glaciers, waves, and precipitation wouldn’t occur. Erosion is a gradual process that usually takes years to be noticed. Most landscapes show obvious evidence of erosion such as in the creation of

hills and valleys. Erosion removes sediments from areas that were once glaciated, shapes the

shorelines of lakes and coastlines, and transports material downslope from elevated sites.

Deposition is the process of dropping the eroded material in another location.

Water Erosion and Deposition

Moving water, called runoff, is the major agent of the erosion that has shaped Earth’s land surface.

As water moves over the land, it carries particles with it. The amount of runoff in an area depends on five main factors,

1. the amount of precipitation an area gets2. the amount of vegetation such as grasses, shrubs, trees that can absorb

water3. the type of soil and its ability to hold water4. the shape of the land5. and the things people use the land for.

Due to gravity, runoff and the material it contains all flow downhill; thus steeper landscapes will have more runoff than flatter ones.

As this water moves across the land, it runs together for form rills (tiny grooves in the soil), gullies (large grooves, or channels in the soil), streams, rivers, and tributaries.

A stream, also called a creek or a brook, is a channel along which water is continually flowing down a slope.

Unlike gullies, streams rarely dry up. As streams flow together, they form larger and larger bodies of flowing water.

A large stream is often called a river. A tributary is a stream or river that flows into a larger river.

As a river flows from the mountains to the sea, it forms many features through erosion.

Waterfalls may occur where a river meets an area of rock that is very hard and erodes slowly.

The river flows over this rock and then flows over softer rock downstream, eroding the softer rock away faster than harder rock.

Eventually a waterfall develops where the softer rock was removed, leaving behind the harder rock.

Areas of rough water called rapids also occur where a river tumbles over hard rock.

Over gently sloping land, the river spreads out and forms a wide river valley. The flat, wide area of land along a river is a flood plain. A river often covers its flood plain when it overflows its banks during a flood. When the flood water finally recedes sediment deposition occurs. This provides a

river valley with fertile soil.

Any time moving water slows down it drops off the sediment it’s carrying.

In this way, soil can be added to a river’s flood plain. Deposition creates landforms such as deltas and alluvial fans.

Sediment deposited where a river flows into an ocean or lake builds up a landform called a delta. Deltas can be a variety of shapes.

When a stream flows out of a steep, narrow mountain valley, the stream suddenly becomes wider and shallower as the water slows down. It’s here that sediment is deposited in an alluvial fan-shape.

Not all water evaporates or becomes runoff. Some water gets absorbed into the ground, where it fills the openings in the soil

and trickles into cracks and spaces in layers of rock.  Groundwater is the term geologists use for underground water. Like running water on the surface, groundwater affects the shape of the land

causing erosion through a process of chemical weathering. Rainwater is naturally acidic and can break down limestone.

Some of the limestone dissolves and begins to hollow out pockets inside the rock.

Over time these pockets can develop into larger holes underground, called caves or caverns.

The action of carbonic acid on limestone can also result in deposition. Inside limestone caves, deposits called stalactites and stalagmites often form. Water containing carbonic acid and calcium from limestone drips from a cave’s

roof, and carbon dioxide escapes from the solution, leaving behind a deposit of calcite.

Stalactites are deposits that hang like icicles from the roof of a cave. Slow dripping from stalactites build up a cone-shaped stalagmite on the cave floor.

In rainy regions where there is a layer of limestone near the surface, groundwater erosion can significantly change the shape of the land due to the fact the limestone is porous (contains many holes).

If the roof of a cave collapses because of the erosion of the underlying limestone, the result is a depression called a sinkhole.

GLACIERS ARE ANOTHER MAJOR FORCE OF EROSION AS THEY CHANGE THE LAND BENEATH THEM.

The two processes by which glaciers erode the land are plucking and abrasion.

As a glacier flows over of the land, it picks up rocks in a process called plucking.

Beneath a glacier, the weight of the ice can break rocks apart and these broken rock fragments freeze to the bottom of the glacier. When the glacier moves, it carries the rocks with it.

This process can even move huge boulders.

Many rocks end up remaining on the bottom of the glacier and the glacier drags them across the land in a process called abrasion.

The abrasion process can cause gouges and scratches in the bedrock. When a glacier melts, the sediment it eroded from the land is then deposited, creating a variety of landforms. These landforms can remain for thousands of years after the glacier has melted.

Wave Erosion and Deposition

Wave Erosion and Protecting Shorelines

Have you ever surfed or body surfed? Have you ever felt a wave crash onto your body and then try to drag you offshore?

Surfers use the power of waves for a wild ride. But that power can damage our coastlines. Wave energy does the work of erosion at the shore.

Waves approach the shore at some angle so the inshore part of the waves reaches shallow water sooner than the part that is further out.

The shallower 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 shoreline features such as a wave-cut cliff.

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. Intact shore areas protect inland areas from storms that come off the ocean.

Where the natural landscape is altered or the amount of development makes damage from a storm too costly to consider, people use several types of structures to attempt to slow down wave erosion.

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. Waves shape the coast through erosion by breaking down rock and moving sand

and other sediment. Large waves can strike rocks along the shore with enormous force and can break

apart rocks as they crash ashore. Over time, waves can make small cracks much larger, and eventually, the waves

cause pieces of rock along the shoreline to break off. Waves can also erode by abrasion. As a wave approaches shallower water, it picks up sediment, including sand and

gravel. This sediment is carried toward the shoreline by the wave. When the wave strikes land, the sediment wears away (erodes) rock like

sandpaper rubbing up against wood. Waves coming to shore gradually change direction due to ocean currents and

wind patterns. The change in direction occurs as different parts of a wave begin to drag on the

bottom. A headland is a part of the shore that sticks out into the ocean. It is made of

harder rock that resists erosion by the waves. But, over time, waves eventually erode these headlands and even out the

shoreline.

Ocean waves that hit a steep, rocky coast erode the base of the land there. Waves erode land that contains softer rock faster. Over time the waves may erode a hollow area in the rock called a sea cave. Eventually, waves may erode the base of a cliff so much that the rock above

collapses. The result is a land feature called a wave-cut cliff. A sea arch is another feature of wave erosion that forms when waves erode a

layer of softer rock that underlies a layer of harder rock. If an arch collapses, a pillar of rock called a sea stack may be left behind. A sea stack is nothing more than a piece of rock projecting out of the water.

A beach is an area of wave-washed sediment along a coast. The sediment deposited on beaches is usually sand. Most sand comes from rivers (the tributaries of the oceans) that carry eroded

particles of rock to the ocean. Waves usually hit the beach at an angle, creating a longshore current that runs

parallel (the same direction) to the coastline. As waves repeatedly hit the beach, some of the beach sediment moves down the

beach with the current in a process called longshore drift. Incoming waves carrying sand may build up sandbars, long ridges on sand

parallel to the shore. A barrier beach is similar to a sandbar. A barrier beach forms when storm waves pile up large amounts of sand above

sea level, forming a long, narrow island parallel to the coast. One result of longshore drift is the formation of a spit. A spit is a beach that projects like a finger out into the water. Spits form as a result of deposition by longshore drift. Spits can alter or extend the shoreline.

Wind Erosion and Deposition

Wind can be a powerful force in shaping the land. Where there are few plants to hold the soil in place, wind erosion can be even

more pronounced. Strong winds blow over loose soil which can severely reduce visibility and erode

topsoil. Wind is caused by the unequal heating of Earth’s atmosphere by the sun which

causes temperature and pressure differences that cause air to move. Wind causes erosion mainly by deflation. Deflation is the process by which wind blows over the land, picking up the

smallest particles of sediment, such as clay and silt. The stronger the wind, the larger the particles it can pick up. Slightly heavier particles, such as sand, might skip or bounce for a short

distance, but sand soon falls back to the ground. Strong winds can roll heavier sediment particles over the ground. In deserts, deflation can sometimes create an area of rock fragments

called desert pavement . Wind erosion and deposition may form sand dunes and loess deposits.  Loess is fine-grained wind deposited sediment even smaller than sand, such as

particles of clay and silt.

Plate Tectonics and Ocean Floor Features

The shape of the ocean floor is primarily the result of plate movement. The outer layer of Earth contains about 12 large pieces called plates. These plates are situated like pieces of a jig-saw puzzle with each piece floating

on top of Earth’s heated and liquefied mantle. A majority of our ocean’s floor features are a result of the interactions that take

place at these plates' boundaries when they can diverge (move away), converge (collide), or transform (slide past).

These individual pieces (plates) at one point were together to form the supercontinent called Pangaea a term coined by German scientist Alfred Wegener.

Pangaea remained intact until about 300 million years ago when plates slowly began their movement again.

This gradual breakup shifted the plates into what became the present-day continental configuration.

The movement of Earth’s plates has changed Earth’s surface as well as the size and shape of the oceans; it has also produced volcanoes, deep-ocean trenches, mountain ranges, mid-ocean ridges, and earthquakes.

Scientists have discovered that mid-ocean ridges continually add new material to our ocean floor, called sea floor spreading.

This is where molten material rises up, erupts, cools, and hardens to form a new strip of oceanic rock. However, the sea floor doesn’t keep getting wider.

It eventually sinks down into deep underwater canyons called deep-ocean trenches. It’s at these trenches the oceanic crust bends downward sinking back into the mantle.

The process in which the ocean floor sinks beneath a deep-ocean trench and back into the mantle again is called subduction. The ocean floor shifts as if it were on a giant conveyor belt, with new floor being created at mid-ocean ridges and old floor material being recycled at subduction zones.