Basaltic Volcanoes, G. Walker

January 2025

I am told this is a classic papper. Here are some notes / excerpts:

“Basaltic magma is derived by incongruent partial melting of mantle peridotite, favoured in tectonic settings (e.g. hotspots and rifts) where mantle rock rises adiabatically to relatively shallow levels, or in subduction-zone settings where volatiles decrease the melting temperature of mantle rock.”
Important magma parameters (pretty uniform for basaltic volcanos)
- Magma density relative to lithosphere density — helps deter- mine the positions of magma chambers and intrusions;
- Viscosity and yield strength determine the geometry and structures of lava flows and intrusions;
- Gas content + viscosity + rheology controls the explosive violence of eruptions by determining the ease with which gases escape from magmas.
Parameters responsible for diversity: magma-supply rate and involvement of non- magmatic water.
”Basaltic systems have a source in the mantle from which magma ascends, mainly because of its positive buoyancy but sometimes aided by tectonic forces, toward the surface. They have one or more conduits by which the magma ascends. Polygenetic volcano systems generally possess a high-level magma chamber, situated at a neutral buoyancy level, which stores magma and modulates its delivery to the volcano and to sub-volcanic intrusions. Deep storage reservoirs may also exist.”
Types of volcanos
- Shield volcanos
- Stratovolcanos
- Central Volanos.
- Monogenetic volcanoes. These consist of clusters of scattered and mostly small (> 2 km3) volcanoes, each generated by a single eruption. Most commonly a volcano con- sists of a cinder cone associated with outflows of aa lava, but some are lava shields of scutulum- type (e.g. Rangitoto Island, Auckland, and Xitle in M…, and many that occur near the coast or close to lakes are phreatomagmatic tuff-rings or maars.
- Flood basalt fields consist of monogenetic volcanoes erupted from widely scattered vents, but their lava flows cover wider areas, overlap or are superposed to form parallel-stratified successions, and have much greater volumes. Giant flood-basalt fields are distributed through geological time at average intervals of 32 Ma (Rampino & Stothers 1988), and each one formed at the time of inception of a hotspot, on arrival of an ascending mantle plume at the asthenosphere/litho- sphere boundary.
Volcano Collapse due to instable foundations, layers of pyroclastic or hydrothermally-altered material, intrusive dykes, local updomings in central volcanoes, severe marine erosion.
Polygenetic vs. monogenetic. “In the polygenetic volcano systems, magma batches ascend sufficiently frequently along the same conduit that the conduit walls are maintained in a hot condition and provide magma with a thermally and mechanically very favour- able pathway toward the surface. In the monogenetic and flood basalt systems magma batches ascend at such long time inter- vals that the pathway taken by one batch has effectivelycooled by the time that the next batch is ready to ascend.”
Fissures / Rift systems. ‘Most basaltic eruptions occur from fissures, and virtually all basaltic volcano systems have eruptive fissures. Fissures are opened very easily by the hydraulic jacking action of magma, and are the ‘natural’ underground conveyance for low- viscosity magma (Emerman & Marrett 1990). They commonly extend for tens of kilometres and are typically concentrated into rift zones. Magma solidified in fissures forms dykes. Dykes have a high survival potential, and in deeply eroded areas may be virtually all that survives of the volcanic system.
xxx

Vesper Flights, Helen Macdonald

January 2025

I picked up this book, probably about a year ago at the recommendation of Dan Russell. In terms of single-author collections, I’ve liked this more than anything I’ve read in years, perhaps with the exception of Loren Eisley’s essays. Regardless, Macdonald is a superb writer, and in particular her descriptions of the natural world are remarkable. I intend to seek out her other books.

I like, as well, her view of what literature ought to do:

What science does is what I would like more literature to do too: show us that we are living in an exquisitely complicated world that is not all about us. — Helen Macdonald, Vesper Flights. p. ix

Favorites

2. Nothing Like a Pig
9. Ants
10. Symptomatic
12. Winter Woods
18. Deer in the Headlights
35 Eulogy
38. Dispatches from the Valley

The Craft of Writing — things I’ve learned here

Describing a Moment: “And then it happens: a short, collapsing moment.” This passage, by foregrounding the nature of the moment, and the movement from uncertainty to realization, does a superb job of highlighting and intensifying an epiphany. (In Nothing like a Pig.)
A beautiful resonating description: “a long, yawning burr that dopplered into memory and replayed itself in dreams.“
Use of incongruity for stream-of-consciousness. In Deer in the Headlights she does a great job of conveying the incongruity of two worlds — the forest and the highway — in a single sentence that juxtaposes glimpses of the nature of each. Similarly, in The Student’s Tale, the first sentence, with independent clauses connected by a series of “ands,” really conveys an immediate stream-of-consciousness experience, where the attention is hopping around, and making non-rational connections as it does so (.e.g, the grapes on the table are black, and so is the taxi out front).
Transforming dynamic movement into a pattern.“The hitching curves of the gulls in a vault of sky crossed with thousands of different flightlines…” (Ants). For me, this generates a pattern — a vault of sky circumscribed by imagined flightlines – that extends over time and creates a persistent space which frames other happenings…

1. Nests

This essay describes nests. She begins with her feelings about nests develop when she was a child, and encountered them in her yard. She then goes into the present, and reflects more on this than their meanings.

*2. Nothing like a Pig

This essay describes an encounter with a boar. She reflects both on the boar, and more in general on animals in particular, and how the conception of an animal differs from the reality of the animal

Then it happens: a short, collapsing moment as sixty or seventy yards away something walks fast between the trees, and then the boar. The boar. The boar.

– Vesper Flights, Helen Macdonald, p 11.

A great bit of writing. The “short, collapsing moment.” The uncertainty about distance — “sixty or seventy yards” — and what she is seeing — “something.” The revelation: “and then the boar.” And the repetition: “The boar. The boar.“

3. Inspector Calls

A very nice short piece about an encounter with autistic boy, who is visiting her flat with his parents. In particular he connects with her bird and the bird with him.

4. Field Guides

“Field guides made possible the joy of encountering a thing I already knew but had never seen.”

5. Terkels Park

An essay on the place where she grew up. A bit nostalgic, but it was unusual, and had interesting reflections, so I found it worth reading. Some very nice writing:

I could lie awake in the small hours and hear a single motorbike speeding west or east: a long, yawning burr that dopplered into memory and replayed itself in dreams.

— ibid. p 12

My eyes catch on the place where the zoetrope flicker of pines behind the fence gives way to a patch of sky with the black peak of a redwood tree against it and the cradled mathematical branches of a monkey puzzle, and my head blooms with an apprehension of lost space,

— Ibid. p 13

6. High-Rise

About watching migrating birds at night from the top of the Empire State Building. An interesting discussion of how birds migrate — the height and speeds at which they fly, and the way they navigate — and the problems that the lights and tall buildings of the city give them.

7. The Human Flock

“Overhead a long wavering chevron of beating wings is inked across the darkening sky.“

Recounting the observation of large flocks of migrating cranes, and continuing to a discussion of the dynamics of swarms and murmurations. “Turns can propagate through a cloud of birds at speeds approaching 90 miles an hour…” This segues into a concluding comment on refugees, and a plea to regard them as individuals rather than masses.

8. The Student’s Tale

An account of meeting a student who is a refugee and spending time in camps…

A great opening sentence:

There’s a window and the rattle of a taxi and grapes on the table, black ones, sweet ones, and the taxi is also black and there’s a woman inside it, a charity worker who befriended you when you were in detention, and she’s leaning to pay the driver and through the dust and bloom of the glass I see you standing on the pavement next to the open taxi door and your back is turned towards me so all I can see are your shoulders hunched in a blue denim jacket.

— The Student’s Tale, Vesper Flight, Kate Macdonald, p. 53

I think this is a marvelous stream-of-consciousness sentence, with the writers attention shifting from taxi to grapes to taxi to the woman and then to the student whose shoulders are hunched. The second person is also very effective.

*9. Ants

About the mating flights of ants, and the birds that prey upon them. Also reflects on the power of scientific understanding to enhance the beauty of things, rather than detract: “…it’s things I’ve learned from scientific books and papers that are making what I’m watching almost unbearably moving.”

A red kite joins the flock, drifting and tilting through it on paper-cut wings stamped black against the sky.

[…]

The hitching curves of the gulls in a vault of sky crossed with thousands of different flightlines, warm airspace tense with predatory intent and the tiny hopes of each rising ant.

— Helen Macdonald, Vesper Flights, p. 63

*10. Symptomatic

Discusses her experiences with migraines. The writing is beautiful and ranges from describing the onset and symptoms of her migraine, to the way in which she has come to live with them. Ends with a partial analogy to earth undergoing climate change…

I was busily signing books when a spray of sparks, an array of livid and prickling phosphenes like shorting fairy lights, spread downwards from the upper right-hand corner of my vision until I could barely see through them.
—Helen Macdonald, Vesper Flights, p. 66

11. Sex, Death, Mushrooms

On mushroom hunting: “It is raining hard, and the forest air is sweet and winey with decay.“

The air is damp and dark in here. Taut lines of spider silk are slung between their flaking trunks; I can feel them snapping across my chest. Fat garden spiders drop from my coat on to the thick carpet of pine needles below.
—Helen Macdonald, Vesper Flights, p. 80

I like feeling the snapping, and the spiders dropping from her coat to the forest floor. It animates the scene, and tells us she is moving through it.

* 12. Winter Woods

Beginning with her custom of walking in the woods every New Year’s day, she reflects on the things that are distinctive about forests in winter. From the revelation of the landscape, to the bark textures and angled branches of leafless trees, to the sometimes transitory life that becomes evident. Winter woods, she suggests, are full of potential:

So often we think of mindfulness, of existing purely in the present moment, as a spiritual goal. But winter woods teach me something else: the importance of thinking about history. They are able to show you the last five hours, the last five days, the last five centuries, all at once. They’re wood and soil and rotting leaves, the crystal fur of hoarfrost and the melting of overnight snow, but they are also places of different interpolated timeframes. In them, potentiality crackles in the winter air.
—ibid., p. 85

13. Eclipse

On viewing a solar eclipse. The phenomenology of the event, but also the deep, irrational, fundamental, emotional impact. The essay is reminiscent of Joan Didion’s essay, and in particular the way in which the fading daylight alters the colors in ways that cast the landscape in an alien light. It ends, beautifully, with a description of the light returning, and the emotions that brings.

14. In Her Orbit

A description of a trip with an astrobiologist to study extremophiles at very high altitudes in the Andes. Some beautiful descriptions of desolate and unworldly environments.

15. Hares

A description of the phenomenon of boxing hares, their place in English thought and mythology, and their decline due to environmental change.

16. Lost, But Catching Up

A very short essay description her glimpse of a hound that was trying to catch up to the pack during a fox hunt.

17. Swan Upping Nestboxes

About the English tradition of “Swan Upping,” and her experience observing the activity; all interladen with reflections on the role of tradition and its uneasy releationship to Brexit, which had recently occurred.

18. Deer in the Headlights

Discusses her changing feelings about deer, from initially wishing to known nothing about them and valuing them as a source of surprise and delight, to a desire to understand them. She says it better, though:

Deer occupy a unique place in my personal pantheon of animals. There are many creatures I know very little about, but the difference with deer is that I’ve never had any desire to find out more. They’re like a distant country I’ve never wanted to visit. I know the names of different deer species, and can identify the commonest ones by sight, but I’ve always resisted the almost negligible effort it would take to discover when they give birth, how they grow and shed their antlers, what they eat, where and how they live. Standing on the bridge I’m wondering why that is.
– Helen Macdonald, Vesper Flights, p. 141

As the title suggests, much of the essay is about deer-vehicle collisions; and also about how people react to them, in the moment, and, sometimes in cruel ways, on the internet. It is a complex essay. It doesn’t really speak to me, but there are a lot of great turns of phrase and passages.

Here is how the essay begins:

The deer drift in and out of the trees like breathing. They appear unexpectedly delicate and cold, as if chill air is pouring from them to the ground to pool into the mist that half obscures their legs and turning flanks. They aren’t tame: I can’t get closer than a hundred yards before they slip into the gloom.

– Helen Macdonald, Vesper Flights, p. 140

And here is a passage I admire for the way it highlights the incongruity of the two worlds: nature and the highway. It moves from the forest, to the road, to the forest, to the road, to her standing, embodied, on the bridge.

For a while the road doesn’t seem real. Then it does, almost violently so, and at that moment the bridge and the woods behind me do not. I can’t hold both in the same world at once. Deer and forest, mist, speed, a drift of wet leaves, white noise, scrap-metal trucks, a convoy of eighteen-wheelers, beads of water on the toes of my boots and the scald of my hands on the cold metal rail.

– Helen Macdonald, Vesper Flights, p. 141

19. The Falcon and the Tower

She is watching birds — falcons — in an abandoned industrial plant in Dublin. The essay discusses falcons, and how they have adapted to living in cities and their infrastructures. Moves from their behavior and natural history, to the ways in which people have viewed them, to their change in habitat given the ‘advance’ of civilization. Ends with a reflection on the brevity of life, and a note of hope.

20. Vesper Flights

The essay that gives the collection a title. Begins with her finding a dead Swift and not knowing what to do with it. Segues into a description of Swifts and how they are somewhat “magical” — “the closest things to aliens on earth.” After describing their natural history, describes the phenonmenon of “vesper flights,” where they gather in the evening and fly up to 8,000 feet. She describes how this behavior was discovered, and goes through the history of this behavior being observed and understood. Interleaved with this is her accounts of how, as a small child, she sought comfort in the evening (her own private vespers) by imagining herself as embedded in layers of the earth below her and the atmosphere above her. This comes together as we learn that vesper flights, for Swifts, help them take account of where they are and the oncoming weather conditions, and as Macdonald reflects on ways in which she (we) can adopt practices that enable us to locate ourselves and think about what comes next.

21. In Spight of Prisons

A very nice, short essay about her annual practice of going to see glowworms in a quarry.

* 22. Sun Birds and Cashmere Spheres

About her efforts to observe Oriels at the single place in Britain where they can still be found. Over time, their habitat is degraded, and at last there is only one… but, at the last moment, she is able to get a glimpse of it. She has a lovely sentence where she describes the song (or a song) or the oriel: “Wo-de-wal-e, wo-de-wal-e, a phrase like the curl of the cut ends of a gilded banner furling over the page of an illuminated manuscript.“

In this essay, she excels at capturing the fragmentary, mosaical nature of perception.

…what I saw became something like looking into a Magic Eye picture. Here was a circle, and in it a thousand angles of stalk and leaf and scraps of shade at various distances, and every straight stalk or branch was alternately obscured and revealed as the wind blew. I began to feel a little seasick watching this chaos, but then, as magically as a stereogram suddenly reveals a not-very-accurate 3D dinosaur, the muddy patch just off centre resolved itself into the nest.

[…]

Finally, I saw my oriole. A bright, golden male. It was a complex joy, because I saw him only in stamped-out sections, small jigsaw pieces of a bird, but moving ones, animated mutoscope views. A flick of wings, a scrap of tail, then another glimpse – this time, just his head alone – through a screen of leaves. I was transfixed. I had not expected the joyous, extravagant way this oriole leapt into the air between feeds, the enormously decisive movements, always, and the little dots like stars that flared along the edge of his spread-wide tail.

– Helen Macdonald, Vesper Flights, p. 177 & 179

23. The Observatory

About swans, beginning with an odd experience she had with one approaching her, and sitting beside her, in a moment of grief.

24. Wicken

About a visit to a nature preserve with her young niece, and her niece’s puzzlement about why there were so many animals here — ‘did they bring them from a zoo?’ Reflections on the shift from a time when nature and animals were all around us, to the present, when they are mostly found in special preserves.

25. Storm

A short essay describing a thunderstorm, and also reflecting on storms as metaphors, in particular, in this essay, for the onset of Brexit.

26. Murmurations

Begins with getting a passport replaced at the last minute, and then moves to how birds were seen during war time, and the rise and evolution of the notion of birdwatching.

27. A Cuckoo in the House

On cuckoos, how people perceive them, and in particular a rather eccentric British intelligence agent — Maxwell Knight — who raised a cuckoo. Didn’t grab me, but others might well find it a fascinating tale.

28. The Arrow-Stork

About tracking migrating birds. Makes this interesting point:

Projects like this give us imaginative access to the lives of wild creatures, but they cannot capture the real animals’ complex, halting paths. Instead they let us watch virtual animals moving across a world of eternal daylight built of a patchwork of layered satellite and aerial imagery, a flattened, static landscape free of happenstance. There are no icy winds over high mountain passes here, no heavy rains, soaring hawks, ripening crops or recent droughts.
– Helen Macdonald, Vesper Flights, p. 217

29. Ashes

About destroying diseased trees, beginning with elms with Dutch Elm disease in her childhood, and ending with ash trees and the emerald ash borer.

30. A Handful of Corn

About feeding animals. Starts with a nice anecdote about an elderly woman who put out corn to attract badgers at night. Continues into the practice of feeding animals, and makes the interesting point that there are some animals it is socially acceptable to feed, and others — foxes, rats, pigeons — that it is not.

31. Berries

This short essay begins with her decorating a Christmas tree, and sprucing up its decorations with berries from outside, but feeling slightly guilty because berries exist as food for birds. Segues into natural history of both birds and berries.

A great bit of description: “…like a gravity stricken whirlwind, a pack of fat birds swirled down from the blank sky…“

32. Cherry Stones

About the return of hawfinches to Britain, the excitement it engenders, and the ways in which their behavior seems to be changing vis a vis what habitat they prefer. Also touches on the blurring of natural history and national identity.

33. Birds, Tabled

About the practice of capturing and keeping birds, which in England is mostly done by the working classes, and which is, it seems, looked down upon by others. She discusses the practice, how bird keepers feel about it and their birds, and the class differences and that this highlights. Interesting.

34. Hiding

An interesting piece about hides (what we in the U.S. call “blinds”). It touches both on the aims and experience of watching animals from blinds, as well as the human experience within blinds.

* 35. Eulogy

A eulogy for a friend: a description of the her friend is interleaved with a night outing to see nightjars. A beautiful piece of writing.

The essay begins with a description of the outing, setting out while it is still light, but with the darkness coming:

As night falls, our senses stretch to meet it. A roebuck barks in the distance, small mammals rustle in the grass. The faintest tick of insects. The scratchy, resinous fragrance of heathland grows stronger, more insistent. As we pass clumps of viper’s bugloss we watch the oncoming night turn their leaves blacker, their purple petals bluer and more intense until they seem to glow. The paths become luminous trails through darkness. White moths spiral up from the ground, and a cockchafer zips past us, elytra raised, wings buzzing.

– Helen Macdonald, Vesper Flights, p. 255

After this, she makes the connection to her friend: “Soon all color will be gone. The thought is a hard one.” And then, after writing about him: “Now, watching the slow diminishment of sense and detail around me, I’m thinking of Stu and what is happening to him, thinking of his family, of what we face at the end of our lives’ long summers when the world parts from us, of how we all, one day, will walk into darkness.“

A somber essay, but ending with a note of, not hope, but acceptance. Stu says, “It’s OK. It’s OK. It’s not hard.”

It’s OK, he said. It’s not hard. Those are the words I am remembering as we walk onward, as the minutes pass, until night thickens completely and there is starlight and dust and the feel of sand underfoot. It’s so dark now I cannot see myself. But the song continues, and the air around us is full of invisible wings.
– Helen Macdonald, Vesper Flights, p. 259

36. Rescue

An account of a visit to the house of a friend who rescues and rehabilitates swifts. It begins with her friend feeding nestlings, and ends with the release of a swift, and a haunting description of the swift’s transformation as it is about to take to the sky.

37. Goats

A brief, funny story about her, her dad, and pushing goats. Wouldn’t call it an essay though.

* 38. Dispatches from the Valleys

A curious essay centered around her experiences in her first job out of college, working on a falcon conservation-breeding farm. She describes what it was like — it sounded unpleasant to me, but she clearly got to do many things she loved and valued. She describes what led her to leave the farm, and does a good job of creating tension by naming two incidents, first “the dreadful incident with the ostrich,” and then “the cattle on the hill,” and describing each played out.

The ostrich incident — euthanasia of a horribly injured bird — was straightforward, if unpleasant. The “cattle on the hill” incident is quite strange: it involves her spending hours sneaking up on them, and then jumping up and scaring them into stampeding, though she does not know why.

At the end of the essay, though, she recounts an epiphany, and, for me, it resolves not just the ‘cattle on the hill’ incident, but the whole essay:

And then I thought of the day I stalked the steers on the hill and it resolved into perfect clarity. For I had seen myself as one of those steers, one of a feral and uncared-for herd enjoying life in the middle of nowhere, not thinking about what would happen in the future, and not much worried about it, but knowing deep down that one day I was headed for the abattoir. There would be no escaping the deep sea for the shore. And my stalking and shouting was not mindless. It had been an inchoate attempt to knock them out of their contented composure. It had been a warning to make them run the hell out of there, because the valley we were all in was dark and deep and could have no good end.
– Helen Macdonald, Vesper Flights, p. 282

39. The Numinous Ordinary

An interesting essay with some nice passages in it, but it didn’t really resonate with me.

40. What Animals Taught Me

Discusses the author’s changing conceptions of and relationships to animals. She liked caring for them, as a child, but came to recognize that was about her feeling good about herself, rather than about the animals. As she grew older, she found that an intense focus on animals was a way to make herself disappear, to allow herself into a separate world that did not contain the difficulties she was faced with. Later, with respect to falconry, she speaks about how she learned that the other party in a relationship might see it very differently — a lesson she was slow to apply to humans. The “deepest lesson animals have taught me is how easily and unconsciously we see other lives as mirrors of our own.” And “None of us sees animals clearly. They are too full of the stories we have given them.“

Towards the end of the essay, speaking of a rook, she comments that now what she enjoys is not imagining that she can feel what the rook feels, know what it knows, but that it’s slow delight in knowing that she cannot.

As it passes overhead, the rook tilts its head to regard me briefly before flying on. And with that glance I feel a prickling in my skin that runs down my spine, my sense of place shifts, and the world is enlarged. The rook and I have shared no purpose. We noticed each other, is all. When I looked at the rook and the rook looked at me, I became a feature of its world as much as it became a feature of mine. Our separate lives coincided, and all my self-absorbed anxiety vanished in that one fugitive moment, when a bird in the sky on its way somewhere else sent a glance across the divide and stitched me back into a world where both of us have equal billing.
– Helen Macdonald, Vesper Flights, p. 299

# # #

2025 ILSG Kilauea &c Field Trip Prep Notes

Volcanic Rock Types

B (Basalt)—Use normative mineralogy to subdivide.

O1 (Basaltic andesite)

O2 (Andesite)

O3 (Dacite)

R (Rhyolite)

T (Trachyte or Trachydacite)—Use normative mineralogy to decide.

Ph (Phonolite)

S1 (Trachybasalt)—*Sodic and potassic variants are Hawaiite and Potassic Trachybasalt.

S2 (Basaltic trachyandesite)—*Sodic and potassic variants are Mugearite and Shoshonite.

S3 (Trachyandesite—*Sodic and potassic variants are Benmoreite and Latite.

Pc (Picrobasalt)

U1 (Basanite or Tephrite)—Use normative mineralogy to decide.

U2 (Phonotephrite)

U3 (Tephriphonolite)

F (Foidite)—When possible, classify/name according to the dominant feldspathoid. Melilitites also plot in this area and can be distinguished by additional chemical criteria.

(*)Sodic as used above means that Na₂O – 2 is greater than K₂O, and potassic that Na₂O – 2 is less than K₂O. Yet other names have been applied to rocks particularly rich in either sodium or potassium—as are ultrapotassic igneous rocks.

Lava flows from Mauna Loa

Palagonite

About Palagonite

Palagonite is an alteration product formed from basaltic glass (tachylite); concentric bands of it often surround kernels of unaltered tachylite, and are so soft that they are easily cut with a knife. In the palagonite the minerals are also decomposed and are represented only by pseudomorphs.

Palagonite soil is a light yellow-orange dust, comprising a mixture of particles ranging down to sub-micrometer sizes, usually found mixed with larger fragments of lava. The color is indicative of the presence of iron in the +3 oxidation state, embedded in an amorphous matrix.

Palagonite tuff is a tuff composed of sideromelane fragments and coarser pieces of basaltic rock, embedded in a palagonite matrix. A composite of sideromelane aggregate in palagonite matrix is called hyaloclastite.

Formation of Palagonite

Phreatomagmatic. Palagonite can be formed from the interaction between water and basalt melt. The water flashes to steam on contact with the hot lava and the small fragments of lava react with the steam to form the light-colored palagonite tuff cones common in areas of basaltic eruptions in contact with water.

Weathering. Palagonite can also be formed by a slower weathering of lava into palagonite, resulting in a thin, yellow-orange rind on the surface of the rock. The process of conversion of lava to palagonite is called palagonitization.

Tachylite

About Tachylite (tachylyte)

Tachylite (from ταχύς, meaning “swift”) is a form of basaltic volcanic glass formed by the rapid cooling of molten basalt. It is a type of mafic igneous rock that is decomposable by acids and readily fusible. The color is a black or dark-brown, and it has a greasy-looking, resinous luster. It is often vesicular and sometime spherulitic. Small pheoncrysts of feldspar or olivine are sometimes visible. Fresh tachylite glass often contains lozenge-shaped crystals of plagioclase feldspar and small prisms of augite and olivine, but all these minerals occur mainly as microlites or as skeletal growths with sharply-pointed corners or ramifying processes.

All tachylites weather easily and become red to brown as their iron oxidizes.

Formation

Three modes of occurrence characterize this rock. In all cases they are found under conditions which imply rapid cooling, but they are much less common than acid obsidians. (Alkaline rocks have a stronger tendency to crystallize (i.e. not form glass), in part because they are more liquid and the molecules have more freedom to arrange themselves in crystalline order.)

Scoria

The fine scoria (aka cinders) thrown out by basaltic volcanoes are often spongy masses of tachylite with only a few larger crystals or phenocrysts imbedded in black glass. Basic pumices of this kind are exceedingly widespread on the bottom of the sea, either dispersed in the pelagic red clay and other deposits or forming layers coated with oxides of manganese precipitated on them from the sea water. These tachylite fragments, which are usually much decomposed by the oxidation and hydration of their ferrous compounds, have taken on a dark red color (scoria is from σκωρία, skōria, Greek for rust.); this altered basic glass is known as “palagonite.” [see above]

Lava flows

In the Hawaiian Islands volcanoes have poured out vast floods of black basalt, containing feldspar, augite, olivine, and iron ores in a black glassy base. They are highly liquid when discharged, and the rapid cooling that ensues on their emergence to the air prevents crystallization taking place completely. Many of them are spongy or vesicular, and their upper surfaces are often exceedingly rough and jagged, while at other times they assume rounded wave-like forms on solidification. Great caves are found where the crust has solidified and the liquid interior has subsequently flowed away, and stalactites and stalagmites of black tachylite adorn the roofs and floors. On section these growths show usually a central cavity enclosed by walls of dark brown glass in which skeletons and microliths of augite, olivine and feldspar lie embedded

Dikes and Sills

A third mode of occurrence of tachylite is as margins and thin offshoots of dikes or sills of basalt and diabase. They are often only a fraction of an inch in thickness, resembling a thin layer of pitch or tar on the edge of a crystalline diabase dike, but veins several inches thick are sometimes found. In these situations tachylite is rarely vesicular, but often shows pronounced fluxion banding* accentuated by the presence of rows of spherulites that are visible as dark brown rounded spots. The spherulites have a distinct radiate structure and sometimes exhibit zones of varying color. The non-spherulitic glassy portion is sometimes perlitic, and these rocks are always brittle. Common crystals are olivine, augite and feldspar, with swarms of minute dusty black grains of magnetite. At the extreme edges the glass is often perfectly free from crystalline products, but it merges rapidly into the ordinary crystalline diabase, which in a very short distance may contain no vitreous base whatever. The spherulites may form the greater part of the mass, they may be a quarter of an inch in diameter and are occasionally much larger than this.

Fluxion banding

See: https://en.wikipedia.org/wiki/Flow_banding

Flow banding is caused by friction of the viscous magma that is in contact with a solid rock interface, usually the wall rock to an intrusive chamber or the earth’s surface.

The friction and viscosity of the magma causes phenocrysts and xenoliths within the magma or lava to slow down near the interface and become trapped in a viscous layer. This forms laminar flow, which manifests as a banded, streaky appearance.

Flow banding also results from the process of fractional crystallization that occurs by convection if the crystals that are caught in the flow-banded margins are removed from the melt. This can change the composition of the melt in large intrusions, leading to differentiation.

From GPT:

Fluxion banding results from shear forces within a moving magma body. This can happen in several ways:

1. Differential Flow in Lava. As lava moves, its viscosity varies due to cooling and crystallization. The outer layers, which cool faster, may develop a plastic or solid crust, while the inner material remains fluid. This difference in viscosity causes layers of magma to stretch and deform, forming elongated bands.

2. Crystal Sorting and Alignment. “ As magma flows, mineral crystals within it may become aligned due to shear stress. This is common in silicic lavas like rhyolite and dacite, where feldspar and quartz can form parallel bands.

3. Magma Mixing and Compositional Banding. If two magmas of different compositions mix, they may not completely homogenize, leading to streaks of contrasting compositions that appear as bands.

4. Intrusive Settings. In some plutonic rocks, fluxion banding may form as a result of late-stage magmatic flow, where crystals and melts segregate due to convection or deformation.

Kilauea: Dynamics of eruptions; Magma types

over the last 4 decades Kilauea has been very active, erupting both from Haumaumau crater on its summit and various rifts on its east side.
in 1983 Kilauea longest and most voluminous outpouring of lava from Kīlauea’s East Rift Zone in over 500 years. It resulted in the creation of the Pu‘u ‘Ō‘ō cone and extensive lava flows that covered significant areas, destroyed numerous structures, and added new land to the island.
Kilauea erupted on 4 May 2018 — it was an east rift zone eruption following the collapse of the Pu’u O’o vent. The rift eruption was driven by collapse of the central (shallow) magma chambers
The 2018 rift eruption had at least three different magmas:
- a highly evolved cool (1110°) viscous lava presumably from sources in the rift system [May 3-9]
- a less evolved hot (1130°) more fluid lava [May 17-18…]
- a very hot (1145°) magma lacking the cargo of low temperature crystals of the previous lavas, but with olvine with high levels of MgO indicating magma > 1250° somewhere in the feeder system

“The first two were the chemically evolved basalt of the initial fissures and the highly viscous andesite. Both are volumetrically minor sources that represent distinct pockets of old residual magma from Kīlauea’s east rift zone that evolved for more than 55 years, cooling and crystallizing at depth. The third and volumetrically more substantial source was less-evolved and hotter basalt of fissure 8. This source was similar in composition to the magma erupted at Kīlauea in the years before 2018 and was ultimately derived from the summit region. Draining and collapse of the summit by this voluminous eruption may have stirred up deeper, hotter parts of the summit magma system and sent mixed magma down the rift..”

Things I’ve learned re eruption dynamics and magmas

Not all lava from Hawaiian volcanoes is basaltic
Even that that is basaltic, changes in composition; each eruption features at least one, and often several, unique lava compositions.
Magma chambers are not homogeneous; this is presumably even more true of rift systems, where greater cooling can generate mushes of crystals
The 2018 Kilauea rift eruptions were driven by collapse of summit magma chambers.
The 2018 Kilauea rift eruption exhibited periodicity of 2-3 days (surges that began within minutes of caldera collapses 40 K upslope) and 5-10 minutes (pulses driven by local outgassing changes )
The dynamics of an eruption can be mapped into several stages
Lateral injection of magma into a rift zone (which forms, in Hawaii, due to volcano flanks sliding into ocean) leads to initial eruption
Pressure in the rift system leads to its elaboration – advancing dikes may capture pockets of highly evolved magma with mushes of low temperature crystals.
Magma injection into rifts, if large enough, can trigger slip on caldera ring faults
Ring fault slippage can add pressure to rift system and drive eruptive behavior at the rift
The central magma chamber appears to be vertically zoned. Initial eruptions of the rift zone (after flushing out pockets of magma that have evolved in the rifts) are composed of younger magmas from lower in the chamber; summit eruptions are fed by older, more evolved magma, higher up in the chamber.
The 2018 Kilauea eruption produced lava at volumes of 100 meters³/sec
Stages of Hawaiian volcanoes: pre-shield (alkalic basalt & basanite); shield (thoelitic basalt derived from both shallow plumbing system and deep plumbing system adjacent to mantle); post-shield (alkalic basalt from deep plumbing system adjacent to mantle (shallow plumbing has crystalized)); post erosional/rejuvenated (alkalic basalt, basanite & nephelinite from ???)

Order and nature of basaltic mineral & crystals

Common minerals that crystallize from basaltic magma, ordered by the temperatures at which they typically form:

*Olivine (Ca2(Mg,Fe)4O4): This is one of the first minerals to crystallize at the highest temperatures, typically around 1,200°C to 1,300°C. Olivine is rich in magnesium and iron and is often found in the earliest stages of crystallization in basaltic magmas.
*Olivine crystals are olive-green to yellow-green color. It often has a glassy or vitreous luster, and the crystals can be angular or rounded, with a granular texture when present in volcanic rocks. When olivine crystals are large enough, they often appear as transparent or translucent, sometimes with visible crystal faces, which are usually in a near-rectangular shape.
When olivine is exposed to oxidation, especially under conditions of high temperatures or in the presence of oxygen, it can alter to a yellowish or brownish hue, sometimes developing a reddish or rusty tint due to the formation of iron oxide minerals.
*Pyroxene (e.g., augite, diopside): Pyroxenes crystallize at slightly lower temperatures, generally around 1,100°C to 1,200°C. These minerals are composed of chains of tetrahedra and are rich in iron and magnesium.
     *Augite crystals are dark green to black, often with a shiny, almost metallic luster. It crystallizes in short prismatic crystals, which are often rectangular or blocky in shape. Augite crystals are typically larger than many other basaltic minerals and can be quite visible in coarse-grained basalts.
     Augite, being rich in iron, may undergo partial oxidation upon exposure to the atmosphere. The oxidation often causes a darkening of the color to a more brownish or reddish tint, though it rarely forms the rusty, reddish color seen in olivine. Augite may also exhibit a duller or more matte luster when oxidized.
     *Diopside is another pyroxene mineral, typically appearing as light green to pale green, although it can also be colorless or pale yellow. It forms prismatic crystals that are often transparent or translucent. Diopside crystals have a glassy or vitreous luster and typically display distinct striations or fine parallel lines on their crystal faces.
*Plagioclase feldspar (labradorite, anorthite): Plagioclase forms between 1,000°C and 1,100°C in basaltic magmas. This mineral can range from calcium-rich (anorthite) to sodium-rich (albite) compositions, with the more calcium-rich varieties crystallizing at higher temperatures.
* Plagioclase crystals vary from white to gray, and often have a glassy luster. They are typically tabular or blocky in shape and can show distinctive twin planes (known as albite twinning).
Magnetite (Fe3O4): Magnetite crystallizes at around 1,000°C to 1,100°C and often forms alongside other iron-rich minerals. It is a common accessory mineral in basaltic magmas.
* Crystals not typically visible in lavas
Ilmenite (FeTiO3): Ilmenite forms at slightly lower temperatures, typically around 900°C to 1,000°C. It is a titanium-iron oxide mineral and often occurs in basaltic lavas.
* Crystals not typically visible in lavas
Spinel (MgAl2O4): Spinel crystallizes at lower temperatures, usually around 900°C. It is a common accessory mineral in basaltic rocks, often forming in the lower temperature range of basaltic crystallization.
* Crystals not typically visible in lavas

These minerals crystallize according to Bowen’s reaction series, where early-formed minerals (like olivine and pyroxene) are typically more magnesium- and iron-rich, while later-formed minerals (like plagioclase and spinel) are more silica-rich due to depletion of Mg and Fe.

Other Notes

TBD