Lect16_Glaciers_notes.doc-MIT.doc

Glaciers I: Intro, Geology and Mass Balance I. Why study glaciers? Role in freshwater budget Fraction of earth’s water that is fresh (non-saline): 3% Fraction of earth’s freshwater that is ice: ~2/3 Fraction of earth’s surface covered by ice: ~8% Climate records, climate effects feedbacks: more in climate lecture Postglacial rebound helps constrain mantle viscosity Major driver of erosion, sediment transport, and landscape evolution But how important have glaciers been over Earth’s history? Over very long timescales, there seems to be a rhythm to glaciations From geologic evidence: 950 Ma, 750, 650, 450, 350-250, 4-? Proposed to reflect arrangement of continents (supercontinents promote accumulation of continental ice). But we don’t even have a very good record of this beyond a few hundred Myr, let alone precise climate records We have better records for more recent intervals Antarctic glaciation began 25 Ma, Greenland 20Ma, Alaska 7Ma. Major Northern hemisphere glaciations began ~4Myr. And this has been the typical climate state since! Again, more in climate lecture. For now, suffice to say that glacial periods have dominated recent climate history. We’re in an interglacial, the first major one since ~125 ka. And our interglacial is an anomalously warm one. We must remind ourselves of this when examining landforms today. What was North America like during glacial conditions? During LGM, we know there were Extensive ice sheets [PPT] Alpine glaciers in mountains beyond the ice margin Large glacially-dammed lakes (Missoula, Bonneville) [PPT] Huge floods from these lakes (formed Channeled Scablands in WA, Snake River gorge in ID) Pleistocene megafauna! [PPT] Glaciers change drainage networks [PPT: North America] II. Glacial features terminology Definition of glacier: perennial ice that has moved Ways to classify glaciers Morphologic [PPT: Cirque, valley, ice sheet] Confined by topography: valley glacier, cirque glacier Unconfined: ice sheet/cap, ice shelf, ice strea