Polygons, Pingos, and Thermokarst! Oh my!

Transcription

1 Polygons, Pingos, nd Thermokrst! Oh my! Overview: Ares underlin with permfrost exhibit common fetures due to freezing nd thwing in such res. Ice wedge polygons, pingos, nd thermokrst re some of the most common lnd fetures to be seen in Alsk. In this lesson, students will crete pingo, nd explore the processes tht crete ll three forementioned fetures. Levels III-IV Grdes 5-8 Objectives: The student will: crete pingo; mke observtions; nd explin the processes involved in polygon, pingo nd/or ice wedge formtion. GLEs Addressed: Science [5-8] SA1.1 The student demonstrtes n understnding of the processes of science by sking questions, predicting, observing, describing, mesuring, clssifying, mking generliztions, inferring, nd communicting. [5] SG2.1 The student demonstrtes n understnding of the bses of the dvncement of scientific knowledge by reviewing nd recording results of investigtions into the nturl world. [8] SD1.2 The student demonstrtes n understnding of geochemicl cycles by pplying knowledge of the wter cycle to explin chnges in the Erth s surfce. Vocbulry pingo smll hill or mound consisting of core of ice ice wedge lrge, wedge-shped body of ice with its pex pointing downwrd, composed of verticlly bnded ice polygon ptterned ground feture resulting from therml contrction crcking of the ground upwelling - the process or n exmple of rising or ppering to rise to the surfce nd flowing outwrd thermokrst - the often irregulr topogrphy resulting from the melting of excess ground ice nd subsequent thw settlement. Whole Picture: Wter exhibits unique properties when it cools nd freezes unlike lmost ny other substnce, it expnds, rther thn contrcts. This mens tht when wter in the soil freezes into permfrost, it cn hve drmtic effects on the shpe nd contours of the visible terrin. In prticulr, permfrost cn result in severl common nd highly visible lnd fetures: polygons, pingos, nd ice wedges. Chnging climte conditions nd thwing permfrost collectively crete topogrphy clled thermokrst. When soils cool nd freeze in the winter, they contrct, forming contrction crcks. During the spring snowmelt seson, wter pools in these crcks. In the winter, this wter freezes into thin wedge of ice. In the summer, smll wedges of ice remin in the permfrost. In the winter, contrction crcks gin form in the cycle. This cretes solid ice wedges tht grow, yer by yer. ACMP UAF Geophysicl Institute B-1 Polygons, Pingos, nd Ice Wedges! Oh my!

2 If the ground bove the wedge is disturbed nd the ice is exposed, it my begin to melt wy, cusing the ground to sink into the void nd crete pond, which is referred to s thermokrst lke. If the lke drins wy, it leves snd tht is sturted. The very wet snd is squeezed under pressure by the surrounding freezing process nd ultimtely pushed upwrd, where it pools under the root mt nd freezes. This lrge ice mss pushes up on the soil bove it, creting the distinctive pingo hills tht dot the Arctic tundr. Pingos cn lso form when rtesin groundwter is forced up through crcks in the permfrost, pooling nd freezing in similr mnner. Mterils: Mterils for modeling, such s: pper, scissors, glue, tpe, cly, etc. Metl bucket Snd Wter OVERHEAD: Permfrost Fetures OVERHEAD: Ice Wedges OVERHEAD: Pingos STUDENT WORKSHEET: Polygons, Pingos, nd Thermokrst! Oh my! Activity Procedure: 1. Disply the OVERHEAD: Permfrost Fetures. 2. Ask students to define ice wedge polygons, thermokrst nd pingos. Write student definitions on the bord. 3. Ask students how these fetures re formed or wht processes crete them. Explin tht, unlike other substnces, ice expnds when it freezes. Soil, on the other hnd, contrcts when it is cooled. Soil in cold climtes tends to contrct nd crck, forming polygons. Ice wedges cn lso form polygons. 4. Show OVERHEAD: Ice Wedges. Ice wedges re formed when wter works its wy into the crcks during spring melt. As the wter freezes, it forms thin wedge of ice in the soil. In summer, the smll wedges of ice remin in the permfrost. In the winter, contrction crcks gin form in the soil. This cycle repets. Over time, verticlly lyers of ice re formed. 5. Show OVERHEAD: Pingos. Pingos re smll hills or mounds consisting of core of ice. They re formed in two wys nd re differentited into two ctegories, closed-system pingos nd open-system pingos. Closed system pingos re formed when lke drins, leving snd tht is sturted. The very wet snd is squeezed under pressure by the surrounding freezing process nd ultimtely pushed upwrd, where it freezes, forming pingo. 6. The upwelling of groundwter contributes to the formtion of n open-system pingo. As groundwter pools together ner the surfce of the ground nd freezes, pressure nd ice lift up the ground to mke dome-shped mounds. Techer s Note: The upwelling of groundwter occurs through process clled rtesin pressure. 7. Explin thermokrst is the often irregulr topogrphy resulting from the melting of excess ground ice nd subsequent thw settlement. Thermokrst terrin often includes fetures such s depressions in the ground (ls), lkes, nd mounds. When ice wedges in n ice wedge polygon melt, they often ACMP UAF Geophysicl Institute B-2

3 leve behind smll mounds typicl of thermokrst terrin. 8. Divide students into smll groups. Distribute the STUDENT WORKSHEET: Polygons, Pingos, nd Thermokrst! Oh my! nd explin ech group will pick one of the three formtions discussed in this lesson (ice wedge polygon, pingo, or thermokrst), crete model of it, nd describe how it is formed. Groups should use the worksheet to guide their work. Groups my use ny clss mterils to mke their model; they my drw, sculpt, crve, etc. 9. Ask students to shre their model nd their explntion of how the feture is formed. Allow time to discuss the questions nd observtions from #4 of the worksheet. 10. In the winter, demonstrte the formtion of closed-system pingo by filling the bottom of metl bucket with snd. Add wter to the bucket until the snd is just sturted. Plce the bucket outside. Check the bucket every two hours for dy or two. Ask students to drw the contents of the bucket nd mke observtions ech time. By the end of the experiments, students should be ble to see smll mound form in the center of the bucket. Remind students tht the wter in the snd is under pressure; wter constntly pushes in on itself. Ultimtely the wter will move in the direction of lest resistnce (up). The temperture will cuse it to freeze, forming pingo. Extension Ides: (1) Interested students my wish to reserch nd report on rtesin pressure. (2) Introduce mthemticl polygons nd contrst them with permfrost polygons (which re not lwys closed nd do not lwys hve stright lines). ([6] G-1, [7] G-1, [8] G-1) Answers: 1. Either A or B. 2. Answers will vry, but should resemble the formtion identified in #1. 3. Answers will vry, but should be correct. Possible nswers include: A. For Ice Wedge Polygons: Ice wedges re formed when wter works its wy into the crcks during spring melt. As the wter freezes, it forms thin wedge of ice in the soil. In summer, the smll wedges of ice remin in the permfrost. In the winter, contrction crcks gin form in the soil. This cycle repets. Over time, verticlly lyers of ice re formed. B. For Pingos: Closed-system pingos re formed when n Arctic lke drins, leving snd tht is sturted. The very wet snd is squeezed under pressure by the surrounding frost lyer nd ultimtely pushed upwrd where it freezes, forming pingo. Open-system pingos re formed by the freezing of the upwelling of ground wter in the permfrost. 4. Answers will vry, but should be relevnt to the selected formtion. ACMP UAF Geophysicl Institute B-3 Polygons, Pingos, nd Ice Wedges! Oh my!

4 Nme: Polygons, Pingos, nd Thermokrst! Oh my! Student Worksheet Levels III-IV 1. Pick formtion to model nd describe: A. ice wedge polygon B. pingo 2. Mke sketch of your model. 3. Explin the process tht cuses the formtion. 4. List ny questions or observtions you hve bout this type of formtion. ACMP UAF Geophysicl Institute B-4 Polygons, Pingos, nd Ice Wedges! Oh my!

5 Pingos Overhed Closed-System Pingo Open-System Pingo 1. Lke 1. unfrozen sturted snd ground wter rises ground level 2. Lke drins wter under pressure 2. wter pools wter forced upwrd pingo ice 3. wter freezes nd expnds ACMP UAF Geophysicl Institute Polygons, Pingos, nd Thermokrst! Oh my!

6 Permfrost Fetures Overhed Ice Wedge Polygon Polygons re closed, multi-sided, roughly equi-dimensionl ptterned ground fetures, bounded by more or less stright sides; some of the sides my be irregulr. Ice wedges re formed in therml contrction crcks in which hor frost forms nd into which wter from melting snow penetrtes in the spring. Repeted nnul contrction crcking of the ice in the wedge, followed by freezing of wter in the crck, grdully increses the width of the wedge nd cuses verticl bnding of the ice. Pingo A perennil frost mound consisting of core of mssive ice, produced primrily by injection of wter, nd covered with soil nd vegettion. Pingo is n Inukitut term. Most pingos hve circulr or ovl bse nd fissured top tht my be crtered. The fissures nd crters re the result of rupturing of the soil nd vegettion cover during doming due to progressive development of the ice core. Thermokrst Terrin Thermokrst terrin is the often irregulr topogrphy tht results from the melting of excess ground ice. Thermokrst terrin often includes the presence of thermokrst lkes nd drunken forests. Drunken forests re forests of trees lening in rndom directions. Drunken forest is descriptive term for trees usully growing on ice-rich terrin nd subject to repeted frost heve. Active, forested rock glciers my lso exhibit this phenomenon due to differentil movements. Polygon nd pingo photos courtesy Vldimir Romnovsky. Drunken forest photo courtesy Kenji Yoshikw. ACMP UAF Geophysicl Institute Polygons, Pingos, nd Thermokrst! Oh my!

7 Ice Wedges Overhed Ice wedges cn grow outwrd or upwrd: ground ctive lyer b c b ground ctive lyer c b b ACMP UAF Geophysicl Institute Polygons, Pingos, nd Thermokrst! Oh my!