6. DINAMICA SEDIMENTELOR. TRANSPORTUL SUBNIVAL SI CURGERI PIROCLASTICE. STRUCTURI SEDIMENTARE SPECIFICE
Conf. dr. Relu D. ROBAN
SUMAR
• Introducere
• Formarea si proprietatile fizice ale ghetii
• Mecanica curgerilor de gheata:• Viteza, stress de forfecare, deformarea interna a ghetii• Fracturi, falii si cute in gheata• Curentii de gheata topita• Icebergs
• Eroziunea, transportul si acumularea sedimentelor• Eroziunea ghetii• Transportul sedimentelor de catre gheata• Depunerea sedimentelor transportate de gheata• Eroziunea , transportul si acumularea din curentii de gheata topita• Transportul sedimentelor si acumularea din iceberg-uri
Releful glaciar : conditionat de prezenta ghetii
Conditia existentei ghetii: cantitatea de zapada cazuta > ablatia (topire+
evaporare)
Factori principali: clima
- climat polar si subpolar (latitudini mari> 60o)
- climat montan (altitudinile inalte) (>5000 m zona calda; >3000 m zona
temperata)
Introducere
Formarea si proprietatile fizice ale ghetii
Schimbări ale densităţii apei lichide şi gheţii între 4 şi 10oC. Bridge şi Demicco, (2008)
Punctul triplu al apei.
(0,01oC; 0,00603 atm)
(374oC; 218 atm)
Formarea si proprietatile fizice ale ghetii
Nichols (2009)
Climat favorabil dezvolatarii sistemului depozitional glaciar: glaciar, subglaciar si montan)
Tipuri de ghetari
Ghetar- acumulari mari de gheata
de vale, montani, alpini
de calota
continentali
marginali, de self
Strahler & Strahler, (2006)
Elementele unui ghetar de vale:
Zona de acumuare:
-Circul glaciar
Zona de ablatie:
- Fruntea ghetarului
Posibile cauze
ale glaciatiunilor
Cauze cosmice- ciclicitatea Milancovitch
Cauze terestre-
tectonica placilor- dispunerea
continentelor
vulcanism: variatia CO2 si a cantitatii
de cenusa vulcanica
Mean global surface atmospheric temperature, carbon dioxide and oxygen levels during the Phanerozoic. During this Eon
surface temperatures generally decreased by about 8°C with four mayor peaks, CO2 decreased from 7300 ppm to 380 ppm,
and O2 was up to 30% during the Permian. Dotted lines represent current values. Temperature data from Royer et al., 2004
assuming a current mean global surface temperature of 15°C. Carbon dioxide levels from Berner et al., 2001 and oxygen levels
from Berner, 2009.
http://phl.upr.edu/_/rsrc/1312563142291/library/notes/habitabilityofthepaleo-
earthasamodelforearth-likeexoplanets/VPE_Temp-CO2-O2.png
Glaciaţiunea Ordoviciană
Glaciaţiunea Proterozoicului superior
Glaciaţiunea Huroniană
Glaciaţiunea Gondwaniană
Glaciaţiunea Cuaternară
Estimări privind temperatura medie anuală globală şi media precipitaţiilor de-alungul istoriei Pământului, Frakes (1979)
Mari glaciatiuni in istoria Pamantului
Mecanica curgerilor de gheata:
Viteza, stress de forfecare, deformarea interna a ghetii
Fracturi, falii si cute in gheata
Curentii de gheata topita
Icebergs
An along-stream section of flowing ice, showing
(A) the velocity profile involving ice, basal water, and sediment;
(B) the ice rheology;
(C) the mechanics of basal pressure melting, refreezing, and flow of meltwater into a
separation-zone cavity.
Mecanica curgerilor de gheata:
• viteza,
• stress de forfecare,
• deformarea interna a ghetii
Strahler & Strahler, 2006
Deplasarea ghetii:
- Partea superficiala- rigida- crevasse
- Partea interna - stare plastic -cute
Daca substratul este apos eroziunea este mai puternica
Gheata- stare pseudoplastica-
curge
c-va m/zi- ghetari de vale activi
c-va cm/zi- ghetari de continentali
(A) The measured distribution of ice velocity within a cross section of Athabasca Glacier, and
(B) Comparison with the theoretical model of Nye (1957, 1965). From Paterson (1994). In the model, the basal
sliding velocity is assumed to be uniform, but measurements indicate that the basal sliding velocity
decreases from the thickest part to the edges. The friction at the sides is much greater than predicted.
(A) A longitudinal section of a glacier showing zones
of accumulation and ablation (loss), the
equilibrium (snow) line, annual layers, and ice-
velocity vectors.
(B)The snowline (arrowed) on South Cascade
Glacier.
(C) Annual accumulation layers, Blue Glacier,
Olympic Mountains.
Images (B) and (C) are from Post and LaChapelle
(2000).
Structura ghetii
(C) Folds in ice (outlined by dark sediment
bands) related to glacier surging.
From Post and LaChapelle (2000).
Cute si fracturi in gheata
Meltwater streams
Curenti din gheata topita
(A) A model for meltwater drainage routes. From Rothlisberger
and Lang (1987).
(B) Dendritic meltwater streams and small hollows, Black
Rapids Glacier, Alaska range.From Post and LaChapelle (2000).
http://25.media.tumblr.com/tumblr_m8n6s7mnsu1rsfmu1o1_1280.jpg
Eroziunea, transportul si acumularea sedimentelor
Eroziunea ghetii
Transportul sedimentelor de catre gheata
Depunerea sedimentelor transportate de
gheata
Eroziunea , transportul si acumularea din curentii
de gheata topita
Transportul sedimentelor si acumularea din
iceberg-uri
Procese
erozionale
exharatie: eroziunea detritusului din masa de gheata
(zgarieturi)
eroziunea masei de gheata
“spinare de berbec”
Acumularea sedimentelor
Depuneri din masa ghetii:
• Tilluri (depozite haotice, slab sortate)
Morene
Depuneri din apele curgatoare (melt) sau lacurile
periferice (alimentate de ghetar):
• Sedimente organizate (stratificate)
Bazale
Frontale
Laterale
Recesionale
Intraglaciar: incastratDomenii
Subglaciar: sediment bazal (lodgemet till)
Proglaciar:
Supraglaciar: debrite, canale, sediment eolian
Morene
Glaciofluviatil
Glaciolacustru
Glaciomarin
Sediment transport in flowing ice
(A) Cross sections through terrestrial
and coastal glaciers, showing locations
and pathways of sediment transport,
and the nature of deposits.
Types of subglacial till.
(A) Compacted, fissile
lodgement till with bullet-shaped and faceted clasts, from
Skye, Scotland.
(B) Deformation till with fold structures from Skipsea
Till at Filey, England.
Sediment transport and deposition from icebergs
Dropstone in laminated glacimarine silts,
Ellesmere Island, Arctic Canada. Photo courtesy of Dave
Evans.
IGNEOUS PETROGENESIS
1. Mid-ocean ridges
2. Continental rifts
3. Island Arcs
4. Active continental margins
5. Back-arc basins
6. Ocean Islands
7. Intraplate hotspot activity, carbonatites, or kimberlites
TYPES OF PYROCLASTIC ERUPTION
Eruption types are based on height of the column and the degree of fragmentation
PYROCLASTSBy Type
Juvenile fragments – samples of quenched glassy/devitrified magma,
Crystals – phenocrysts from the magma
Lithic fragments – clasts of pre-existing rock, from the walls of the conduit.
By Size
blocks or bombs (>64 mm),
lapilli (64-2mm)
ash (>2mm).
DOMENII SI PROCESE
• Domeniul continental
• Someniul subacvatic
• Fall out (decantare)
• Pyroclastic flow (curgeripiroclastice)
• Pyroclastitc surge (valuri)
• Lahar
• Resedimentari
• Submarine pyroclastic flow
• Ash turbidite
• Pelagite cu cenusa vulcanica
PYROCLASTIC DEPOSITS: AIR FALL (TEPHRA)
Ballistic ejecta
Air Fall
Pyroclastic air fall deposits (tephra) are poorly sorted (except at large
distances i.e. distal deposits)
Proximal Distal
Bomb sags in bedded ash/lapilli.
Stratification due to pulsing of an eruption observed closer to the vent
Reverse grading occurs due to increasing vent diameter due to erosion
The diagram demonstrates the sequence of events associated with
the 1991-95 nuée ardente eruptions from Mt. Unzen, Japan.
Collapse of a growing lava dome generates the nuée ardente.
Within seconds a faster-moving cloud of smaller ash-sized
fragments (the ash-cloud surge) forms above and in front of the
nuée ardente.
The terminology of pyroclastic flows and pyroclastic
flow deposits can be complex and confusing. In
general, there are two end-member types of flows:
NUÉE ARDENTES -- these contain dense lava
fragments derived from the collapse of a growing lava
dome, and
PUMICE FLOWS -- these contain vesiculated, low-
density pumice derived from the collapse of an
eruption column.
A nuée ardente deposit is called a block-and-ash flow
deposit, whereas a pumice flow deposit is called an
ignimbrite.
PYROCLASTIC FLOWS
PYROCLASTIC FLOWS
• Emplaced hot (not usually molten).
• Restricted to topographic lows.
pumice
lithics
Pumice flows = ignimbrites
Pyroclastic flow during the August 7, 1980,
eruption of Mount St. Helens. A billowing
ash cloud rises from the pyroclastic flow.
BLOCK & ASH FLOW DEPOSITS
Massive, poorly sorted facies of a Permian block & ash flow depositwith dacitic composition, Saar-Nahe Basin/Germany
Ignimbrites are pumice-dominated pyroclastic flow deposits with subordinate ash. There are many historic
examples, most of which are restricted to valleys emanating from summit craters.
Pumice flow, Mt. St. Helens, one
day after its deposition on October
22, 1980.
(USGS).
PUMICE FLOW DEPOSITS
IGNIMBRITE ROCK TEXTURESIgnimbrites are typically characterized by pumice fragments floating in a finer grained matrix. Many
ignimbrites, however, are still hot when they are emplaced so that the pumice and ash fragments are
still plastic. Here we distiguish between welded and unwelded ignimbrites.
The unwelded 4.6 million year old Real Grande ignimbrite, Argentina which has a volume of over 55
km3. (Peter Francis)
Nonwelded tuff from Sumatra with very slightcompaction of glass shards. Note the unusuallymassive shard center right.
Rattlesnake tuff from central Oregon, displayingslightly flattened shards with unbroken glassbubbles, now in oval outline
NON-WELDED IGNIMBRITESGlass shards, derived from the fragmentation of the vitric bubble walls of pumice vesicles, are well-
preserved. They occur as slender branches having platy to cuspate forms, many of which display triple
junctions marking the site of the coalesced bubble walls. In many cases, entire vesicles are well-
preserved.
Welded tuff from SE Idaho. Note markedcompression of the shards, but goodretension of the shard structures
Fine-grained, glassy welded tuff showingextreme compaction and molding againstcrystal fragments.
WELDED TEXTURESCompaction and welding is evident by:
(1) the collapse of Y-shaped shards and bubble walls,
(2) the alignment of elongate crystal and lithic fragments,(3) the folding of shards around lithic and crystal fragments, and
(4) the collapse of pumice fragments into glassy lenticular masses called fiamme.
LAHAR DEPOSITS
Mt St Helens, 2003
Lahar deposits caused
by melting of ice and
snow in 1981 eruption.
EPICLASTIC DEPOSITS
Poorly consolidated
volcaniclastic deposits are
rapidly reworked by runoff
to form epiclastics.
Flood plain
EPICLASTIC DEPOSITS -RESEDIMENTARI
Volcaniclastic deposits are often reworked to become epiclastic sediments.
Someniul subacvatic Submarine pyroclastic flow
Ash turbidite
Pelagite cu cenusa vulcanicaEinsele, 1992