Subaerial Felsic Lava Flows and

Domes

Occurrence• Alone or in linear and arcuate chains up to 20 km

long

• Margins of calderas or volcanic depressions. Feeder

occupies synvolcanic fault (ring fracture). Extrusion

of lava preceded by explosive eruptions- thus

associated with pyroclastic rocks.

• Large caldera- central portions- product of magma

resurgence after cataclysmic eruptions.

• Crater of Stratavolcano- Mt. St. Helens

• Felsic (Dacite – Rhyolite) flows grade

laterally into domes.

• Magma so sticky doesn’t flow far (10,000

to 100,000 times more viscous than

basalt) [important exception]

• Thus relatively short flows

Flows:

10’s of meters to more than 500 meters thick

Average 100 meters (much thicker than mafic lava flows)

< ½ km to 6-8 km length (Hawaiian lavas- 20-25 km)

Two Main Shapes:

A)Coulees-

B) Asymetric about a central

vent- short, steep flows

talus covered margins

Flow Morphology:

1) Obsidian (devitrified =)-

Glassy, chilled carapace around flow

6-10 m thick at top, thinner at bottom

2) Interior- Lithic (cryptocrystalline

rhyolite-dacite. Some flows all

obsidian

3) Top- Flow Bx- vescicular lava,

Obsidian

4) Bottom- flow Bx, similar to top

but thinner.

Surface Features

• Surface- blocky and rough. Flow fronts are steep and normally terminated by a talus apron of bx.

• Breccia results from flow advance and mass wasting

• Basal Breccia- flow overrides frags that avalanche down front

Internal Features:

Flow Banding- foliated layers

Of obsidian and vesicular-massive

Lithic rhyolite-dacite

Due to friction along conduit walls

During ascent then deformation in

Response to velocity changes as lava

Advances.

Flow lamina mm’s to several

meter’s thick. Most < 2cm.,

Individual Lamina-bands recognized by:

1) Color differences

2) Textural differences:

Spherulitic vs non spherulitic

Crystalline vs glass

Ideally flow banding is

Parallel at base of flow (Flow

Contact) and vertical-contorted

In core

Spherulites- Devitrification features

Obsidian to crystalline rock

Spherulites grown together

Perlitic Rhyolite: Hydrated Obsidian

Gives onion skin appearance due to

expansion of glass during hydration

Lithophysae (stone lilies):

Gas Phase- gas cavities filled by

Vapor phase minerals- Sanidine,

Tridimite, Fayalite

Domes

• Steep sided, mound

shaped masses of

viscous lava that pile up

over and around vent or

fissure

• Domes usually form after

explosive eruptions

• Domes are volatile poor

Domes have an exterior composed mostly of rubble with banks

of talus covering sides and top.

May make dome unrecognizable

Rubble

In plan view domes. Like flows,

are either circular to oval in shape-

and related to a central vent

Or- elongate and related to a fissure

Single Domes- occupy explosion

Craters at tops and sides of

Stratovolcanos or

Center of tuff rings and cones

Clusters of domes- above

And along fissures-

Margins of calderas

Rift Valleys

More rarely in stratovolcano

Surface Features-3 Texturally Distinct Parts

• Outer and upper zone of breccia- talus

• Intermediate zone of fractured rock- coherent

• Inner zone- massive to flow banded

• Gradational

• Bx Fct Massive

Talus Breccia:

Ash- block-size fragments

Form due to expansion and

Cracking of outer surface as

Dome rises, expands, grows

Magma-semisolid shell

Along with explosions.

Top and sides

• Continued growth leads to constant

formation of breccia and a continuous

increase in the thickness of the breccia

pile on top and sides.

• Also get fragments from collapsed spines.

• Fragments cemented by secondary

minerals.

Spines-

Monoliths of solid magma pushed

Up through dome rubble-

Dykes

Slickensides

Explosions

• Rain or sea-water entering fractures at top

and sides of the dome- hot rocks-cold

water

• Vesiculation and violent escape of gas

Dome covers vent

Hazard

• Talus rubble plus rapid dome growth-expansion

• Gravitationally unstable- collapses

• Avalanche down slope of talus and hot magma-OR

• Explosion at base of growing dome- avalanche of material down slope

• Block and ash flows

• Can collapse to form debris flows 100’s of years after activity-Chaos Crags-5km to form Chaos Jumbles

Domes Characterized by:

• High glass content-> 60 % obsidian

• Cores may be cryptocrystalline to

crystalline and massive or vesicular

margins often more vesicular

• Phenocryst content- 0-40%’

crystals typically quartz, feldspar,

biotite and/or hornblende.

crystals can parallel flow banding

or define it.

Domes associated with calderas have

Low crystal contents- blown out

Typs of Domes

• Can be classified by:

– Morphology

– Surface texture

– Eruptive style

Types of Domes

• Plug

• Spiny to Lobate to Axisymmetric -Vary

based on extrusion rate and cooling rate

• Crypto

• Either case- domes grow rapidly- 0ne to

five years

Types of Domes

• Plug- Lassen: extruded

lava solidifies as a plug

(steep-sided) and is not

much wider than the

central vent.

Types of Domes

• Spiney-lobate-axisymetric- Steep sided and circular to irregular in plan view. smooth to block covered upper upper surface punctuated by spines and talus aprons,

Lots of avalanches and block and ash flows

Explosions may occur

Spines up to 700 feet tall

Lobate

Types of Domes

• Cryptodomes-High level intrusions

May not breach surface

• Subaqueous- important

Fumerolic activity

Types of Domes

• Domes have a great passion for self-

destruction- throughout formation constant

fragmentation- explosions, spines, talus,

avalanches, earthquakes

• End up with a pile of rubble

• Can happen quickly then weathering and

mass wasting finish dome off