Section 9 - Glaciers, Shorelines

Section 9
  1. Glaciers
  2. Shorelines

Glaciers - Lecture Notes

s9-1 Penguins frequent the antarctic continental glacier in the South polar region.

What is a glacier and why are they important?

A glacier has the following characteristics:
1. It is a large mass of ice.
2. It forms in geographical region where the amount (in terms of mass) of snowfall per year exceeds the amount of ice that melts per year.
3. It is capable of flowing downhill.  The rate of flow is about 2 meters per day but this rate varies considerably depending on many factors.  Ice that is subjected to high pressure is capable of a "plastic" type of flow.  (When you bend a paper clip, the metal tends to bend in a "plastic" way.)
4. It has a huge influence on shaping the land via a special type of erosion and deposition.

There exists two types of glaciers: Valley Glaciers and Continental Glaciers.  Valley Glaciers form in mountainous regions and are sometimes called alpine glaciers.  They flow downhill following mountain valleys.  The other type is called a Continental Glacier.  Continental glaciers are usually much larger and more slowly moving than valley glaciers.  These glaciers can cover most of a continent.  Two very large continental glaciers are the Greenland and Antarctica glaciers (sometimes called ice sheets).


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MacMurdo Station, Southern most point attainable by sea.


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Ross Ice Shelf (or Sheet)


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Picture of Mt. Rainier in Washington state.
There are 25 different alpine glaciers on the side of this mountain.  It is the largest system
of glaciers outside of Alaska in the United States.  (Short PowerPoint presentation on Mt. Rainier glaciers.)

It is important because about 10% of Earth's land surface is covered with glacial ice.  Studying the gas that is trapped within glacial ice provides information about the earth's climatic past.  (The layers of ice are very analagous to tree rings in regards to what they can tell us about the earth's past.)  Studying the rate at which glaciers are melting or advancing are a good indicator of possible global climate changes such as global warming.  Glaciers are also responsible for creating much of the landforms we have Wisconsin.

Terminology Describing a Glacier

  Ablation is the general process by which a glacier loses ice mass either by melting, breaking apart, sublimation, or even wind eroson.
  Accumulation refers to the amount of snow added to the glacier (usually per year).
  The snowline separates the zones of accumulation (more snow accumulating than melting) and the zone of ablation (more snow melting than accumulating).  The balance between accumulating snow and melting snow is often refered to as the glacial budget. 
  The zone of fracture describes the top portion of the glacier and is composed of ice that is brittle or not capable of flowing.  This ice rides "piggy-back" on top of deeper ice.  Crevasses (sometimes large and deep) often form in this portion of the glacier.  (A good analogy of this behavior is a Snickers candy bar.  Unwrap a new Snickers candy bar and bend it from the ends.  The chocolate coating opens up with "crevasses" and the carmel inside closely represents the deeper, plastic flowing ice.)  
  Calving is the process by which a large portion of the glacier breaks off and drops into an ocean or lake.  Thus, creating an iceberg.

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Fig. 1

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Terminology Describing Landforms Created by Glaciers

  In moutainous regions, a "U" shaped valley can be formed in a valley by plucking (i.e. to pick up and move) and scraping the sides creating a glacial trough.  Valley glaciers commonly form cirques near their high elevation regions.  These cirques are bowl shaped depressions that are open on the downhill side.  Sharp-edged mountainous ridges and peaks formed by glacials are called aretes and horns, respectively.  Fjords are often formed at the end of glacier formed valleys

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Yosemite National Park
Yosemite was carved out by glaciers.  The valley is clearly "U" shaped.


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A water-fall in the steep sides of Yosemite Valley.
(click to enlarge)

The material that gets deposited out of a glacier as it melts and/or retreats is called glacial drift.  The subsurface below much of Western Wisconsin has a 50-60 foot thick layer of glacial drift.  If a glacier advances, stops, and then retreats, an end moraine is a landform built-up by the glacier depositing its load in the region where it stopped.  The deposition created by the furthest advance of the glacier is called the terminal moraine.  The Southwest area of Wisconsin has a unique "island" containing no glacial drift called the driftless region.

Ice Ages and Historical Glaciation

The earth has experienced numerous periods of global cooling and warming.  The duration and intensity (degree of cooling and warming) of each period varies.  As a period of cooling occurs, glaciers advance and the sea level falls.  When warming occurs, the glaciers retreat and the sea level increases.

There exists some evidence that most of the earth's surface was once (maybe several times) covered with ice.  This comes from studying the isotopic ratio of carbon 12 and carbon 13 found in ocean floor sediments.  This ratio can be related to the average climatic temperature.  These measurements suggests a global ice age (or a "snowball earth") happened around 570-700 million years ago and several episodes of glaciation have happened in more recent times (~1 million years ago).

The most recent period of extensive glaciation peaked about 18,000 years ago.  Surprisingly, the average global temperature does not need to change significantly for there to be a period of large-scale glaciation.  Only about 5oC change is necessary.

The Pleistocene Epoch (about 1 million years ago) of Geologic Time has been called the "Ice Age".  About 20 cycles of warming and cooling occurred during this epoch.  However, periods of glaciation have happened during other epochs (or periods) of geologic time.

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Artist's depiction of how the landscape looked during the ice ages with a glacier in the background and a Wooly Mammoth in the foreground.  Drawing courtesy of the Wisconsin Ice Age state park
.


Local Glacial Geology

Interstate State Park in Minnesota has some interesting glacial potholes that have been weathered into local rock formations.  About 10,000 years ago retreating glaciers and their outwash carved formations within igneous rock strata.  This igneous rock strata formed from lava flows about 1.1 billion years ago.


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Picture looking down upon some of the rock formations at Interstate State Park in MN.


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A glacial pothole with two small kids standing in it.  This picture is also from Interstate State Park in MN.  The St. Croix river can be seen in the upper right of the picture.

 

Chippewa Moraine Ice Age Scientific Reserve near New Auburn, WI. Concentration of glacial lakes and landforms about 60 miles Northeast of Menomonie.
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Mill Bluff and the great Wisconsin Glacial Lake

Northeast view on top of Mill Bluff.  Interstate 94 runs
North and South and can be seen in the bottom of the 
picture.

An area of about 1,825 square miles was covered with water in Wisconsin's central region due to glaciation about 70,000 years ago.  Water depths reached 150 feet.  The bluffs are composed of sandstone mounds of Cambrian-Ordovician time.  These bluffs were islands in the Wisconsin Glacial Lake.

The picture to the left was taken when my son and I went on a geological expedition to find remnants of the Great Wisconsin Glacial Lake at the end of the summer of 2001.

 


Why Does the Earth Experience Periods of Warming Or Cooling?

Theories presented to explain periods of glaciation:
1. Variations in the Earth's orbit and inclination to the Sun.
2. Plate tectonics and the changing position of continents.
3. Changes in the atmosphere.  For example, if a sufficient number of volcanoes erupted in a short period of time, the amount of sun light penetrating the atmosphere may decrease from volcanic dust and ash in the upper atmosphere.
4. Changes in sea water circulation.

Theories 1 and 2 above, seem to hold the most promise and are often cited in explaining the periods of glaciation.

A detailed calculation (using the laws of physics) shows that the Earth undergoes slight variations in its motion with respect to the Sun.  This causes differing amounts of sun light to fall on different locations at different times on the Earth.  [Credit for the first such analysis is usually given to Milutin Milankovitch, a Yugoslavian scientist.]

I.  Eccentricity - One variation is that the Earth is sometimes closer to the Sun than at other times.  This happens on cycles of about 100,000 years in duration.
II. Precession - The Earth behaves like a top that is spinning and placed at an angle onto a table.  Instead of falling over, the top rotates around a vertical line.  This happens in cycles about every 26,000 years.
III. Nutation - In addition to precessing about a vertical axis, the earth also wobbles (to a very small degree).  A wobble which is described as a change in the angle to which the spin axis makes with respect to the orbital plane. 


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fig. 2

In a related issue, the amount of CO2 in our atmosphere has been rising.  Most scientist now believe that this (together with other possible greenhouse gases) are causing the global temperature to increase in what is called the "Greenhouse Effect".  In essence, this increase in temperature causes a retreat of glaciers around the world and an increase in sea level.  Here is a good link containing information about Ice Ages and the Greenhouse Effect.

Glaciers - Related Web Links

Mendenhall Glacier in Alaska (PowerPoint), by Nancy Novotney (Geology student at UW-Stout in 2005.)

Svartisen Subglacial Laboratory - A scientific research laboratory located beneath 700 feet of ice.
ICE and Glacier web sites about glaciers from Rice University
National Snow and Ice Data Center (NSIDC), all about glaciers
Animated Glacier Fly-bys, a NASA web site
Regional Landscape Ecosystems of Michigan, Minnesota, and Wisconsin: A Working Map and Classification, A web site of the USGS.
Course on Glacial Geology
Pictures (1,2) of glaciers from Duke University
Slide Show on glaciers, by Sharon L. Gabel, SUNY at Oswego
ice_age_sm Ice Age Park & Trail in Wisconsin, Ice Age Trail Landscape
Discussion of Wisconsin's Glacial Landscape
Geographical Provinces of Wisconsin (a lot of geological information)
Google - Search for Glaciers
Google - Search for Wisconsin Glaciers
Google - Search for Valley Glaciers or Continental Glaciers


Shorelines - Lecture Notes

"Drifting on the whims of sand and sea, barrier islands by the hundreds rim our Atlantic and Gulf coasts, buffering the mainland from storms and offering beach lovers a glimpse of paradise. Yet these delicate strands are often asked to do more: to anchor homes and hotels, lighthouses and lifestyles -- in short, to hold still. It's against their nature."

Jennifer Ackerman, "Islands at the Edge"

Shorelines are very dynamic places.  The major influences include erosion, deposition, water waves, and tides.  What is presented below can be considered the "steady state" characteristics.  Of course, this steady state can be greatly disrupted (temporarily) by an encroaching hurricane or other more infrequent phenomena.

Waves

Waves are generated mostly by the wind.  (If one observes a lake on a windy day, the largest waves on the lake are downwind.)  The water molecules don't travel along the wave like a surfer using a surf board.  What does travel along the wave is energy.  This wave energy is mostly confined to the top portion of the water.  When waves reach the shore, the tops of the waves usually "topple" forming breakers.  The region where breakers usually form is called the surf zone.


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Surf zone along the Pacific Ocean.  This site is near Long Beach, Washington.


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Waves along the Northern shoreline of Lake Menomin on a windy day.  Click on the picture to see a more detailed description.

 


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fig. 3

The waves usually approach the coastline at a slight angle.  These waves experience refraction as they approach.  Such that, the shallower the water, the slower the waves will travel.  This process is shown in figure 3 above where the wave fronts are slowing down surrounding the light-house headland.


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Water currents and sediment transport effect one another on a beach. 
Longshore current and longshore drift - Since the waves approach the coast at an angle a current develops which moves parallel to the shore.  Beach sediments move along with this current.
Rip currents - Narrow, deep currents can form in the surf zone and are directed out to sea.  They develop in the surf where the bottom forms small channels.  These currents can also be found along side artificials structures that shoot out into the surf such as piers.  Swimmers can sometimes get caught in rip currents and pushed out to sea.
The longshore drift will form baymouth bars and spits when they encounter an estuary (a funnel shaped inlet into the sea).  The longshore drift continues to deposit sediments parallel to the shoreline even when estuaries are encountered.
Barrier Islands - Long sandbar that builds up from longshore drift.  These sandbars are usually separated from the coast by tidal flats or shallow lagoons.  Some common barrier islands include Cape Hatteras and Padre Island off the coast of Texas.

Illustrations of Rip Currents

rip_currents

Signs at Whitefish Dunes State Park in Wisconsin warn swimmers of a dangerous rip current (or sometimes called an undertow).

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The picture to the left shows a rip current area at low tide.  The water in the foreground is rushing preferentially out to sea at this spot.  The water is moving left to right.  The location and size of sand bars create this current of water.  (The picture is on the Pacific coast near Long Beach, WA.)

 

Interactive map showing parts of a beach

Artificial Structures

For many reasons, people have built structures along shorelines to influence sedimentation or erosion.  The processes that add or subtract sediments to the shore are related to the sand budget of the beach.

Sedimentation Processes

Waves eroding the backshore cliffs or rock
Longshore drift
River loads being dropped at the mouth of river

Erosional Processes

Offshore winds blowing sand in-land
Longshore drift (note: can be erosional or depositional)
Tidal currents, rip currents, waves

Structures built to change the sedimentation or erosional processes:
1. Groins - (shown in figure 3)   Barrier built perpendicular to the shore.  It is designed to capture more sediments for a particular part of the shore.  In figure 3, House A is getting more beach by building a groin which ultimately takes beach sediments away from House B.
2. Jetties - Very similar to groins but are usually designed to protect inlets (for marina boat traffic) from excessive sedimentation.


WA_jetties


3. Breakwater - Large barrier built parallel to the shore.  Designed to protect boats from large waves.  In essence, breakwater creates a semi-quiet marina along the shore.  These are sometimes physically connected to the shore for construction purposes.

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An artificial breakwall (or breakwater) built to protect a harbor and the Trump Casino boat in Gary, IN.  Photo is courtesy of Craig Wenner, former student in the Geology and Soil Mechanics course.  Craig's company Rocks and Docks (email: rocks@Lakefield.net) was involved in the breakwall construction project.  The project involved quarrying the stone, transporting it, and placement.  A miniature model of the harbor was built to examine the effectiveness of the breakwater prior to construction.

 

 

Development on Shoreline Areas

Many beach areas are popular spots on which to build homes or hotels.  But the beach is a dynamic system that is in continuous change.  Some changes can be dramatic as when a hurricane makes landfall.  The combinations of high tide and sea surge (in-coming part of the rotating hurricane), can cause the sand on the beach to shift dramatically.  Millions of dollars of taxpayer's money are spent annually to rebuild eroded shorelines and rebuild weakened structures built on the shifting sand.  Many believe it is foolish to interfere with the natural processes effecting the beach and present the notion that "The best way to save a beach is to leave it alone."  The debate of whether to develop or not is strikingly similar to the issue of building in a floodplain of a river.  How many times is one willing to put money into maintenance and repair?  It would be a non-issue if taxpayer money was not involved.

Tides

The water level along coastlines rise and fall twice each 24 hour period.  This cyclic behavior is referred to as tides.  It is the result of the gravitational influence of the Moon and the Sun on the Earth's oceans.  The magnitude of the high tides and low tides depends on your (1) geographical location and (2) the alignment of the Earth, Moon, and Sun.  This influence can be illustrated (see below) with gravitational force vectors acting upon the ocean.

tides1
Take all the black vectors and subtract the average force shown in red.  This results in the following "effective" force (shown in grey vectors below) acting upon the oceans.

tide2

I have also developed an animation of tides.

The largest amount of swing in high and low tide is called the Spring Tide.  It results when the Earth-Moon-Sun are aligned adding to the gravitational effect.  A Neap Tide occurs when the Moon-Earth-Sun form a 90o angle and the gravitational force is partially cancelled out.  Disastrous flooding can occur for a location along the coast that experiences a high tide combined with the landfall of a hurricane, simultaneously.  

The constant rise and fall of water does have a very small effect on the Earth's rotation.  It causes the Earth's rotation to slow thus increasing the time required for one rotation (such that, one day).  About 570 million years ago the Earth rotated once every 21 hours.  Gravitational tidal effects - of the Earth acting upon the Moon - have also caused the Moon to always have the same side facing the Earth. 

Shorelines - Related Web Links

Coastal Geology by the National Park Service
Waves from the Grand Valley State University, Department of Geology
Barrier Islands of the United States Atlantic and Gulf Coasts
Pictures of Beach Processes and Barrier Islands
Coastlines of Atlantic Canada
(Excellent web site with pictures of shorelines.)
Shore Protection Projects
in New Jersey
Cape Hatteras Lighthouse home page
Shoreline lecture notes from Dr. Terry Engelder, Penn State
Google - Search for Shoreline Geology
Google - Search for Tides, Geology