6. DINAMICA SEDIMENTELOR. TRANSPORTUL SUBNIVAL SI CURGERI PIROCLASTICE. STRUCTURI SEDIMENTARE SPECIFICE

Conf. dr. Relu D. ROBAN

TRANSPORTUL SUBNIVALSTRUCTURI SEDIMENTARE SPECIFICE

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

Strahler & Strahler, 2006

calote glaciare actuale

Izoliniile- curbe de nivel

Strahler & Strahler, 2006

calote glaciare pleistocene

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

(A) Typical crevasse types.

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.

Images (C) and (D) are of glacier

termini, showing dark bands of

sediment, Portage Glacier,

Alaska.

Strahler & Strahler, 2006

Morene:

• Laterale

• Frontale

• Mediane

Sedimente bazaleTilite:

Lodgement tillAblation till

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.

CURGERI PIROCLASTICE STRUCTURI SEDIMENTARE SPECIFICE

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

A section through pyroclastic flow deposits from the

1990 eruptions.

Carbonised wood

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.

PYROCLASTIC SURGES

Base surge

Low particle density particle/gas suspension flows

Chough and Sohn, (1990)

Depozite tipice de val piroclasticSongaksan tuf ring

LAHAR DEPOSITS

Mt St Helens, 2003

Lahar deposits caused

by melting of ice and

snow in 1981 eruption.

Dune Ondulatii-ripples

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