Chapter 3: HARD ROCK SHOW
In modes of occurrence, rocks are classified into three main types: igneous (solidification from magma in the Earth’s interior or on its surface), sedimentary (aggregation of grains disintegrated from pre-existing rocks and transported by water, glacier, or wind; or accumulation from chemical deposition and biologic debris on the Earth’s surface), and metamorphic (solid-state transformation of pre-existing rocks in the Earth’s interior). This chapter encompasses naturally broken pieces of igneous and metamorphic rocks, which have been shaped by water and wind. Both types are informally named hard rocks, but like hard wood, not all are necessarily hard.
Igneous rocks are classified into intrusive ‘plutonic’ for magma solidified in the Earth’s interior and extrusive ‘volcanic’ for magma erupted or oozed to the Earth’s surface. Each subtype is further divided in terms of texture and mineral composition. I will not dwell on the details of naming a rock. In fact, some cannot be easily identified with the naked eye unless one has experienced with those rocks, especially with the aid of instrumental analyses. Generally, slow cooling of magma yields coarse crystals whereas rapid cooling produces fine crystals. Silicates rocks of high magnesium and iron contents are usually greener or darker whereas those of high sodium, potassium, or calcium contents are lighter in color. Visual identification is relatively easy for light-colored, coarse-grained plutonic rocks such as granite, of which the major mineral constituents can be identified visually. Although colors of rocks can be correlated positively with composition, color is not a sure way for identification, especially challenging for dark rocks with indiscernible mineral crystal shapes to the unaided eye. Identification by picture alone can be faulty too.
Metamorphic rocks are classified according to texture, namely schistosity (foliation or cleavage). The rocks range from easily cleaved slate, through finely foliated schist, to coarse-layered gneiss, and to granular hornfels under low pressure but high temperature condition and to granulite at high temperature and pressure. In cases of contact metamorphism arisen from thermal contact with magma, the rocks are metamorphosed in massive forms like marble or quartzite without schistosity. Commonly a metamorphic rock is prefixed with mineral names or an adjective modifier.
Nature does not follow our design of rock classification. Some transition zones further compound the uncertainty or our confidence in calling the names. For example, migmatite is a hybrid of the solid-state transformed metamorphic gneiss and recrystallized igneous granite as solidified from the overheated and partially melted gneiss.
I have jumbled many terms in this brief introduction. My interest lies in nature’s carving, polishing, staining, and varnishing. My stories relate typically what have happened after those stones broke loose from their parental rocks, rather than their parents’ origin and subsequent evolutionary history. In this regards, proper naming of rocks, although desirable, is not very critical for most stories herein.
Volcanic bombs are essentially projectiles ejected from a volcanic eruption into the air and returned to the ground as a solid or semisolid block, of which the size exceeds a few mm. (Finer ones are cinder, scoria, or volcanic ash.) It can be found among piles of volcanic debris (namely pyroclastics). It is commonly classified according to its descriptive shapes, like a volcanic block for one piece of country rocks (which have been invaded by magma) without significant coating of magma, a cow pie bomb for its splash-spread shape when the ejecta is still in liquid form or of low viscosity when it falls to the ground.
The stone here is the most distinctive type of volcanic bomb – a spinner volcanic bomb (like a spinner of thread). Upon eruption, it goes airborne and spins as it is solidifying in the air to generate two tapered ends before landing. At touchdown, the bottom of the leading front is usually flattened a little but it can also be crushed, being fated on the angle of landing, viscosity of the solidifying ejecta, and landing site, among many factors.
What is inside a bomb? The core of a bomb may not be readily identifiable until one sees a broken piece or cuts it open to reveal. In the literature, the core is referred as xenolith (alien rock inclusion), i.e., a piece of rock captured by the magma on its ascending journey from the Earth’s deep interior to shallow depth. The xenolith in this particular bomb is peridotite – an olivine-rich rock (the olive-green olivine is a magnesium silicate mineral; peridot is its precious variety). Xenoliths provide a window for glimpsing at what constitutes the Earth’s interior. There are other methods such as using seismic or electromagnetic wave propagations to infer the interior’s composition but study of xenoliths is the only means to see directly what the materials are, and to confirm geophysical inferences and understand physical and chemical processes in the formation of mantle and crust, or continents and seafloors.
Are xenoliths really representative of the Earth’s interior? They represent where they originate provided that the xenoliths have not been chemically and physically altered by the carrier magma. It is a race between ascending rates (travel distance/time) and the rates of chemical and thermal reactions between magma and xenoliths as well as chemical diffusion rates. In short, the transport speed has to be fast enough to keep xenoliths from making significant interaction with the enclosing magma. So, a site of violent volcanic eruption is the choice for sampling rocks in the deep interior.
Even so, how can one insure that the xenoliths have not been influenced by the host magma? One can examine mineralogical and chemical variations as well as trace-element distribution along profiles across the reaction rims around the xenoliths. Furthermore, one can compare those spatial variations among xenoliths of different sizes. Small xenolith can be readily engulfed by the magma beyond recognition but the larger ones are much harder to assimilate, especially their inner parts which tend to preserve what they are in chemical and mineralogical compositions.
Unlike the mantle-derived stone described here, many bombs bear crustal xenoliths. Depending on localities, mantle xenoliths originate from depths of a few tens to 200 kilometers while the crustal ones come from depths up to a few tens of kilometers. Some spinner bombs do not nucleate around any core. Those are simply blobs of magma that are catapulted into the air to twist and solidify. A xenolith-free bomb represents a small pinched stringer as split from a long stringer of lava that is ejected into the air. Figure 3-12a depicts the internal flow patterns along transverse sections of three small bombs, whose bigger sister bomb is depicted in Figure 3-2. All have their distinctive external shapes and internal flow patterns.
Reflection: Like human beings, each bomb has its own individual life history and hence characteristics.
This crustal bomb is from a 10 million-year old volcano. It is remarkably well preserved. I photographed it upside down to show its belly and the ‘gill-like’ tension cracks.
The ratio of the bomb’s length to diameter happens to coincide with the equivalent ratio for a toy football at my home. This coincidence inspired me to go back to the crater, with my wife, searching for more bombs. I cut some bombs to reveal their internal structures. I also examined some museum specimens. Unfortunately, at the end, I could not decipher a common theme for dynamic modeling of the bomb’s flight. (See also Figures 3-1 and -12.)
Reflection: As a scientist, I hypothesize theories to explain observations and at the end of further exploration, some are fruitful but others fail. Appreciate success and enjoy the laugh at failure.
Figure 3-3a, b, c, and d
At first sight, the stone is a piece of lava rock. Its rugged outlook seems to have been sustainable only by the skeleton of hardened lava. But, the stone appears fragile and grainy to the touch. It lacks typical luster and ropy texture of lava. Most strangely, it shows fine layering (each about 1 mm thick, side view, 3c & 3d). The rock is magnetic – a good indicator of lava or an aggregate of magnetite-rich sediment. What is this stone’s geologic history? Especially, what has it happened after the lava solidified?
Sitting on the top and bottom surfaces are a plethora of mini-erosion remnants: mesas, ridges, and valleys superimposed on horizontally layered strata (side view). The landform is a quasi-fractal microcosm of desert landscape. However, those features appear on both the upper side and underside of the stone. One cannot fancy a desert with an upside down landscape, excepting a mirage or a mirror image over a water-standing lake (not dry lake).
Now we need a story for carving the double sided landscape. Let’s call upon water to come up with the miracle on a piece of free-standing lava immersed repeatedly in a small, shallow, intermittent pond over a long time. First, the fragility on some parts of the stone and the grainy textures are indicative of water at work through slow weathering and erosion. Rain does come and go in the desert. Ponding of water occurs occasionally and locally (as in mud pot); and the pond water interacts with rocks. The potency of water’s power is enhanced by the presence of bicarbonic acid that originates from dissolving carbon dioxide from the air and soils.
Water level in the pond rises and recedes; and the rates of erosion and weathering of lava increase and decrease accordingly. But the rates are not so high as to spoil the game. For sure, the piece cannot reside in flash flood channels where any features like this piece, if ever present, will not survive. Hence, a delicate balance must be sustained – the rates must be high enough to create the landscape but not so high as to destroy it. The apparent layering is reminiscent of ancient lake shorelines that mark episodes of change in water level by chemical alterations of underwater rocks. (Ancient shorelines commonly appear as bleached zones; also, note the modern analogy: multiple-level shoreline markings on steep wall of a lake or reservoir as the water recedes, especially during drought.) From the viewpoint of shoreline observables or photographic documentation, the marking of ancient shorelines is more conspicuous on hillside with steep slope (a cliff as here imagined) than on gentle slope. The erosional remnants on the upper side are therefore islands in a pond of changing water level.
The underside landscape is also a product of weathering and erosion. For its development, the piece must remain free standing in the pond without collapsing on itself. This condition can be sustained on a frame of lava rock, not on sediments as the layering might have implied. In short, this stone is shaped in a so-called condition of Goldilocks: all influential factors just about balance out; not too much, not too little. Everything works out just about right and that is why this piece of lava with layer appearance is rare. The layering is surficial, not ramification of an internal or intrinsic layer structure.
The layer-like yellowish brown, rusty material is the weathered product of the original, black lava. The black is magnetic but the brown is not, as tested with powder scratched from the stone. Apparently, some magnetite has been oxidized to become the rusty, non-magnetic goethite or limonite.
The dark green piece is corrugated or wrinkled with obtrusive dark ridges and recessive green grooves. The green is olivine spread in patches without any visible crystal forms. The rock is likely olivine basalt.
Figure 3-4a and b
Basalt is solidified from lava oozed out of volcanoes; and it can also come out subaerially during pyroclastic eruption. Wind has chiseled one detached piece into a dome with multitude of facets (here, three dominant facets, or a dreikanter ventifact in German). The variations in orientations of facets have resulted from wind blasting that has swung but stayed persistently and repeatedly on course long enough to make an imprint. The competency in basalt and the holding duration of wind direction conspire to retain the sharp ridges (aka keels) between facets.
Sometimes I wonder if all facets in a ventifact represent changes in wind directions. I believe blasting by a steady windy-sand stream can be diffracted or deflected to create multiple facets.
Oxidation of dark iron-bearing minerals in basalt yields red pigment to self-stain the exposed surface. However, this rock’s underbelly is bleached (lower-right insert) instead of being red stained. The bleaching is odd because most rocks in the area (Figures 3-5, 1-8 & 1-12) have red stain in the bottom face.
Figure 3-5a, b, and c
This stone is a piece of granite; ‘aplite’ is used here to emphasize its small grain size (on the order of a couple mm) and uniform size distribution. The underside or ground-facing side (Figure 3-5c) is stained red, in stark contrast with the pale, reddish white of the exposed surface. Outside the red-stained bottom: the brownish red is orthoclase; the gray is quartz; and the dark is mainly mica or hornblende. The upper-left insert is the rear view. The front and rear faces are essentially miniature vertical cliffs.
Wind has sculptured one break-away block of aplite into a magnificent piece of stone art. Now, how does the wind abrasion work in landscaping? At a given site under the same environmental elements, there are innumerable pieces of loose rocks but few ends up being artfully distinctive. So, the intrinsic properties of individual rocks make the difference under similar extrinsic circumstances. What are the telltale characteristics?
I believe that the ‘outstanding appearance’ for this stone is attributable to its uniformity in grain size distribution and homogeneity in mineralogical composition. The two factors allow all parts of the exterior surface to respond uniformly to natural polishing and varnishing. A piece of granite with different coarse-sized minerals will exhibit area-varying resistance to weathering and erosion, leading to rough surface. Furthermore, the desirable piece must also be sufficiently competent (neither pliable nor fragile) to maintain soft sharpness along linear ridges or ledges. (The ‘ledge’ would be more apparent if the image in Figure 3-5a were looked at upside down.) The ‘soft sharpness’ is coined here to mean sharp edge but still touchable, unlike the untouchable, sharp edge of broken glass. For depressions such as the ‘trough’ on the front face of the stone and eastern overhang, I invoke micro-cracks as the pre-existing weakness for the wind and water to preferentially breach and carve although there is no visible fracture now. This is tantamount to eave or cave forming at field scale (see Figure 3-6) rather than at the foot-scale as depicted here.
The underside is stained red by occasional, passing soil water. However, the red coating under the eastern, overhanging portion of the stone (to the right) calls for a second opinion. Here are some additional scenarios. One, the rock had been buried deeper in the past so as to have the underside of its overhang stained by soil water. In this scenario, the coating represents the residual after the enclosing soil had been stripped away (and the red coating at equivalent depth range around all faces would have to be wiped off as well).
In the second but more convincing scenario, the staining or coating originates from weathering of the dark iron-bearing minerals in the aplite itself; deeper burial in the past is not necessary but it does require wiping out any red coating on the rest of the exposed surface. (Orthoclase here has an intrinsic reddish brown color and does not need staining to attain the reddish brown hue.)
If the second scenario is credible, how does the coating under the eastern overhang stay? Likely, it sits in the lee of the prevailing wind that blows rightward. The stain-bearing moisture adheres long enough in the lee to have effectively stained the underside of the overhang on the right. Similarly, greater moisture-retention capability contributes to the red staining at the bottom face, where evaporation of moisture is the least. In short, microclimate around the stone plays a significant role in shaping and staining the stone.
However, the moisture content should not be high enough as to drip away. Otherwise, the staining agents would be leached, resulting in a bleached bottom as seen in Figure 3-4.
Figure 3-6a and b
The stone in Figure 3-6a was carved naturally from granodiorite, which is composed mainly of feldspar (white) and quartz (gray and somewhat translucent) with minor amount of dark minerals. As compared to the well-polished aplite in Figure 3-5, this stone’s coarse-grained texture and hence uneven resistance to erosion and weathering prevent it from being polished in the desert. Nevertheless, erosion does yield sharp peaks, steep ridges, and pits for this hand specimen.
At outcrop scale, granite in the desert can suffer from severe pitting during wind erosion to yield holes or pits, as aided with weathering through occasional rain fall. Joshua Tree National Park in southeastern California is famous for such eroded but picturesque granite landscape, which is dotted with Joshua trees and altitude-zoned cacti. The picture in Figure 3-6b depicts holes in granite, as excavated by weathering at Mt. Rubidoux in Riverside, California – an area of Mediterranean climate or semi-desert. The white patch at its bottom center is a restoration mask over graffiti – a shameful defacing act by some ruthless youngsters.
Reflection: An object as solid and competent as granite can crumble for bearing weak components – feldspar and biotite which succumb easily to chemical weathering. Once the weaker crumbles, the stronger quartz has to yield too.
The black stone is a master piece of desert magic power of wind, water, and staining agents. Overall it is a tri-faced monument of unknown rock type, of which the identity is masked by dark greenish staining agents. Its faces are meticulously carved and varnished. It cannot be a porous or vesicular volcanic glass (e.g., pumice) because it is too dense to be one.
Shined under the sun light, the material appears fairly homogenous; yet there are protrusive nipples and receding dimples spread over all faces. The nipples are olive-green translucent and the dimples are much darker. The two features manifest differential resistance to erosion and weathering. Is the nipple-dimple difference indicative of mineralogical difference or structural defect? Is it a rock consisting of quartz and feldspar as represented by the nipples and dimples, respectively? A saw-cut to the base could clear the uncertainty, but let us keep the stone intact and speculate.
Alternatively it can be a piece of quartz with micro structural defects. Nipples and dimples have resulted from differential weathering and erosion through cycles of water-freezing and ice-thawing. See Figures 1-4 and -5 for comparison.
The white piece of rock on the left is marble. It was highly eroded in the desert. Like the piece on the right, it is tri-lateral faced. (Sometimes, nature seems to favor isolated, pyramid-shaped stones for no obvious reason.) Tetrahedron has the least number of faces that enclose a solid object (one may take an exception for a sphere but a sphere can be approximated by numerous tetrahedrons as engineers or scientists working on finite element analysis can attest.) The two stones are presented here for their common shape of elongated tetrahedron.
The stone was split along bedding plane to show color banding (Liesegang bands) on the opposing faces. Such banding happens frequently in porous sandstone too. Some scientists suggest the banding is caused by ferric oxide-bearing pore fluids (trapped in case of sandstone), which have evolved to supersaturation through dehydration, then followed by simultaneous nucleation (heavy rings) and depletion of solutes (light colored rings). I believe the banding reflects advective diffusion of staining agents that have been extracted from the host rock, through which water, originated from occasional rainfall, has percolated. Stain coloring occurs through repeated percolation, dissolution, dehydration, and deposition until the rock becomes impervious. For each band, the intensity fades inward and the pattern is repeated from band to band.
Was there any chronological order in color banding? See next figure for an outcrop of Liesegang rings.
Reflection: Who is to believe this is a memento that my wife had played a minuscule role during the height of the Cold War between the United States of America and the former Soviet Union?!
This outcrop picture serves as a supplementary explanation to the Liesegang rings depicted in Figure 3-8. First, the granite body is fractured into blocks of cube, parallelepiped, etc. Then, water seeps or diffuses into each block from the block-defining joints. The water scavenges the staining agents along its travel path to milestone the migrating fronts. (This is part of the weathering process; hence the name: spheroidal weathering.) Around the corners of a fractured rock block, the migrating fronts advance faster because more water per unit area enters the block. Consequently the inward migrating fluid front rounds the sharp corners to have circular or oval patterns instead of mimicking the original block geometry. The entry water was either chemically captured by the rock through hydration of some minerals or dissipated from the rock during dry season. The center ‘eye’ is neither fluid entry nor exit point.
It is noted that, the fluid diffuses radially inward (include upward-flow component), following the potential hydraulic gradient of water, not simply following the gravity downward.
Multiple rings signal stop-and-go operation, pending on the water supply or precipitation. Unlike the tree-ring chronology, the geometric order of the rings does not necessarily follow the chronological order; a major ring may record many but not necessarily all migrating events.
Figure 3-10a and b
My father, who was once shackled away from home as a political prisoner, proclaimed ‘Rings are degenerative shackles’. Think about it, indeed, a wedding ring is a shackle worn by willingly shackled partners.
This granitic stone on the left appears to be girdled with an aplite ring. The ring is actually a plate with greater resistance to weathering and erosion – a disk-like remnant of a dike or vein sandwiched in the granite, not a ring.
The stone on the right is sandstone with an intervening white quartz dike or vein. The picture was taken from a Taoist temple yard in the village where I grew up. I was appalled by the sight that temples of various shades have engaged in the business of hoarding rare rocks and plants in recent economic booming time in Taiwan and China. The stone pictured here is a less egregious one.
The pink, lenticular mineral is orthoclase; the light one is quartz grain; and the black streaks or seams are mainly biotite.
This lava piece (basalt) is roughly shaped like a tetrahedron. Outside the pinnacle cap, the stone exhibits three subtle color zones. The whitish zone has resulted from chemical interactions with soil water around its ground-level footing. The soil-water bleaching is most intense near the contact with the ground surface, suggesting fairly limited availability of the intermittent, fluctuating soil moisture. The dull, dark-brown colored face (on the left) contrasts with the shining black, fresh-looking face, suggesting different exposures to snow or moisture cover. Yes, vegetation on the north- and south-facing slopes of a hill in areas of low precipitation can response differently to the availability of moisture. Here we see equivalent response in this free-standing rock at microcosmic scale when water is scarcely available in the desert.
The three basalt cobbles come from a desert wash in a volcanic terrain, where jasper also occurs (see Figure 5-13).
The corrugation appears fairly linear, reflecting influence of subtle layering. All appear uni-directional to the view and finger touch, suggestive of lava flow direction. The corrugation in the middle one is influenced by the presence of bubbles in the lava.
For a short period of time, I was interested in the dynamics of volcanic bombs. After I cut open some bombs, I gave up because there is no consistent internal structure. Each one deserves a specially tailored story even though all came from the same crater.
This piece supplements the basalt piece in Figure 3-3.
Spinner volcanic bombs – crustal bombs without xenolith. The one in Figure 1-12c1 came from eastern California and the one in Figure 1-12c2 came from Oregon.
Figure 3-13a and b
The white ‘dike’ in the cluster of brown garnet in Figure 3-13a is composed of calcite (for its reaction with weak acid) but there is no visible crystal form. Likely the dike is a crack filling — secondary calcite that formed after crystallization and cracking of the garnet. The white flakes are veneers of residual (peel-off) calcite. The stone comes from contact metamorphic zone.
The garnet in Figure 3-13b also occurs in contact metamorphic zones. The purple garnet crystals are almost isolated from one another and appear to have immersed in the black biotite and hornblende (unknown source location).
The single garnet crystal at the lower right corner of Figure 3-13a is unrelated to these two garnet clusters.