VOLCANIC CURIOSITIES

Volcanic Bombs

These are fragments of liquid and/or plastic state lava, ejected into the air by means of a volcanic explosion (s). As the fragments move through the air, they twist and turn and can form spindle-shaped masses with surface markings. They range in size from a few millimeters to more than a meter in diameter. 

Two volcanic “bombs” from a relatively young volcanic eruption in central Arizona, USA are shown below. This first “bomb,” 6 cm width, is shown in two views, front and back. 

The second "bomb" is 9 cm wide.

Lava Stalactites

These are associated with lava flows. The lava was originally molten liquid matter  (magma). The stalactites shown below formed when the red-hot lava infiltrated cracks in the ceiling of a hollow lava tube and dripped down into the empty space. 

Two stalactites from a young lava-flow tube in central New Mexico, USA are shown.

The first stalactite (front and top views) is 7 cm high and 5 cm wide. The holes visible in the top view of the interior were caused by trapped air bubbles.

  

The other lava stalactite (front view) shown below is 9.5 high and 7 cm wide. If you look carefully at its side, you can see some green surface staining resulting from deposition of the copper mineral malachite.

Xenolith

The word “xenolith” is Greek, meaning a foreign (stranger) or guest rock. It is an inclusion of an older rock in a younger volcanic rock. The older rock is referred to as “country rock” or “wall rock” by geologists.

The following example of a xenolith is from the Mojave Desert in eastern central California. The xenolith is a small piece of granite, which is a plutonic igneous rock that formed inside the Earth. This xenolith has been incorporated as an inclusion into black volcanic rock (basalt lava, which is a volcanic igneous rock that formed from a lava flow or volcanic neck near the surface of the Earth). The age of the xenolith can be very old and indicative or an entirely different geologic temperature and pressure than the surrounding basalt. The whitish xenolith is 4.5 cm high and 2.5 cm wide. 

There is no indication that the temperature of the basalt lava affected the granite xenolith (inclusion) because there is no alteration of the granite where it made contact with the basalt. I have also included below a close-up image of a portion of this contact.  

SPONDYLUS, The Thorny "Oyster"

The bivalve (clam) Spondylus is a favorite among seashell enthusiasts, who like these clams because of their picturesque shape, potentially vivid colors, and, in some species, long spines. The shells of many specimens, however, can be obscured because of attached corals and other sea life. Spondylus is indicative of warm (tropical to subtropical) seas, and the geologic record of this genus ranges from the Jurassic Period to Recent times. Its distribution today is pantropical, with about 40 or so species. The number is inexact because of the notoriously high degree of variability in the shape and features of the shells.

Spondylus has been referred to as the “thorny oyster” or the “spiny oyster,” but it is not an oyster. Some species can resemble the bivalve Lima. In terms of the development of its hinge morphology, however, Spondylus has some degree of affinity with Pecten clams (scallops). Spondylus can be thorny or spiny, and you have to be very careful when handling it. The spines, whether short or long, can be very sharp. It might be necessary to wear thick leather gloves when handling some of the specimens. 

The following images are of a representative Recent species of Spondylus, namely S. varians Sowerby, 1838 from low-energy, tropical shallow waters of Thailand. Like all bivalves, Spondylus consists of two valves. The right valve (so-called "lower" valve) is attached to rocks, corals, or other hard substrate. As a result, the right valve can have an extremely variable shape because it resembles the shape to which it is attached. The left valve (so-called "upper" valve) is free, and its shape can be determined by the degree of water energy (e.g., quiet water versus rough water). Quiet-water Spondylus species have longer spines that the rough-water ones. As seen below, the right valve (lower valve) of a specimen of S. varians extends 120 mm, when viewed from left to right, and the left valve (upper valve) is 80 mm when viewed from top to bottom.

                  both valves (side views): upper valve flat, lower valve cup shaped

both valves (top views): upper valve (with red "dot"); if you look closely, you can see the hinge line between the two valves along the top edge of the "red dot"

  

                                                    upper valve exterior 

                                                    upper valve interior

lower valve exterior

                                                    lower valve interior

Like all specimens of Spondylus, the right (attached) lower valve of S. varians has a thin dark ligament between two stout teeth (crural teeth). The left (free) valve has two corresponding sockets to accommodate these teeth.

In the images shown below are the presently known four species of fossil Spondylus from the west coast of North America, from Late Cretaceous through Eocene age. The diagram shown here is a summary of their geologic history in this area. As you can see, Spondylus occurred only during warm times.

The oldest Cretaceous species from this region is Spondylus rugosus Packard, 1922 from southern California. Preservation is commonly very poor. Specimens can be large, up to 95 mm height. The specimen illustrated here is the left valve of the holotype [= the specimen found by Packard and used by him to name the species], height 95 mm, width 72 mm, Santa Ana Mountains, southern California.

The next geologically younger Late Cretaceous species is Spondylus subnodosa (Packard, 1922) from northern and southern California and possibly Vancouver Island, British Columbia. Preservation is generally good. Specimens can be quite large, up to 150 mm height. \The specimen illustrated here of S. subnodosa is the right valve of the holotype, height 150 mm height, 140 mm width, Santa Sana Mountains, southern California. 

The next geologically younger species of Spondylus known from the west coast of North America is of early Eocene age (about 52 million years ago) and is Spondylus batequensis Squires and Demetrion, 1990 from Baja California Sur, Mexico. The specimen illustrated here is the right valve of the holotype, height 21 mm. 

NOTE: There has been confusion regarding the name and the morphologic characteristics of subnodusus because Packard (1922) described it as both Lima subnodosa Packard, 1922, p. 421 and as Spondylus striatus Packard, 1922, p. 422. This bivalve actually is a Spondylus and not a Lima. Furthermore, because subnodosa was described one page before striatus was described, subnodosa is the official name of this species. 

During the middle Eocene (about 45 million years ago) on the west coast of North America, Spondylus carlosensis Anderson, 1905) was present in southern and central California. Another possible species is Spondylus cliffensis M.A. Hanna in the San Diego area, but it is likely a synonym of S. carlosensis.

At the end of the middle Eocene and the beginning of the late Eocene, about 37 million years ago, global cooling created a turnover event, which strongly affected the warm-water, shallow-marine bivalves and gastropods of the west coast of North America. They began to decline in numbers and many were eventually replaced by cooler water genera and species. 

HETEROMORPH AMMONITES (SOME CALIFORNIA EXAMPLES)

Ammonites are fossil shells of predatory mollusks (cephalopods) that swam in shallow seas during Devonian through Cretaceous time (an interval of about 340 million years). The previous two posts focused on flat-coiled (like a coiled rope) ammonite shells, which the majority of ammonites had. This post focuses, however,  on irregularly coiled ammonite shells. These can have a helicoidal shape, which can resemble hooks, paper clips; a slightly bent or U-shaped shape (like those of "under-the-sink drain pipes;" or a “Gordian knots.” All of these unusual shaped ammonites are called “heteromorphs,” meaning “different shapes.” 

There are at least approximately 60 known heteromorph genera (way too many to show each in a blog or a series of blogs). In addition, there are about 15 or some other genera that might be synonyms of some of the previously named genera. 

There were pulses of new genera evolving, as well as times of old genera that went extinct throughout the 340 million-year geologic range of heteromorph ammonites. There were only a few that lived during the Triassic and Early Jurassic. The Late Jurassic had only moderate diversity, but during the Early Cretaceous their numbers greatly increased (approximately 35 genera). Heteromorphs declined during the Late Cretaceous (approximately 25 genera), and after the end of the Cretaceous, all ammonites (including heteromorphs) went extinct. 

It is interesting to note that heteromorphs were not confined to a single family nor to a single geographic area or geologic age. Many had widespread dispersal because of their swimming ability; thus, most ammonites are excellent for age correlation of stratigraphic units. It is likely that some of the more complex-shaped (e.g. paper clips) heteromorphs could not swim very well and spent most of their time foraging along shallow-ocean floor.

The first example of an heteromorph ammonite shown below is from a state, other than California, in the USA.

1. Didymoceras stevensoni Whitfield, 1877, 23 cm height, Late Cretaceous: late Campanian; Colorado, Wyoming, Montana, Delaware. Referred to by some researchers as belonging in the genus Cirroceras. [Image credit: Wikimedia Commons].

2. Ancyloceras? sp., 7 cm diameter (estimated) (plaster replica). I have no information about this plaster replica of a heteromorph, but it must have taken painstaking effort to clean this specimen from its rock matrix and then make a replica.

Shown below are nine examples of Late Cretaceous heteromorphs from California. They are arranged by geologic age, from oldest to youngest.  I have included also a geologic time diagram to show the various subdivisions (stages) of the Late Cretaceous and their associated geologic ages (in millions of years before present).

1. Turrilites dilleri Murphy and Rodda, 1960, 3 cm height (partial), Early Cretaceous/Late Cretaceous boundary: (Albian to early Cenomanian), Ono area, northern California.

2. Turrilites sp., 5 cm height (estimated) (plaster replica). Notice that the aperture is on the left (sinistrial) side of the shell. Age and locality unknown.

3. Hamites sp., 3.3 cm height (a fragment). Late Cretaceous: Late Turonian; Santa Ana Mountains, southern California. 

4. Hamites? sp. (plaster replica) I have no information about this heteromorph.

5. Genus and family unknown (two views). The specimen is a fragment, 5.8 cm length. Late Cretaceous: late Turonian; Santa Ana Mountains, southern California.

6. scaphitid, 3 cm height (not a complete specimen). Late Cretaceous: Late Turonian to early Coniacian; Santa Monica Mountains, southern California. Specimens from this locale are very poor preserved as mashed, flimsy specimens on bedding planes in a micaceous siltstone. Preservation is too poor to positively determine genera or species.

7. Baculites sp. (two specimens). The larger specimen (8.4 cm length) has most of its shell missing. Late Cretaceous: middle Campanian; Pentz area, northern California. Near one end of the largest specimen, its sutures (internal walls of the chambers) are visible (these are part of the internal structure of the shell). The smaller specimen (4.2 cm length) has no external shell left, but its complex suture pattern is visible. If you are interested in reading more about ammonite sutures and how they changed through time, please read my blog post of Oct. 14, 2014. Use the Search Box at the top of my posts.

8. Eubaculites sp. (two views). The specimen is a large fragment, approximately 63 cm length. Late Cretaceous: Campanian or  Maastrichtian (i.e., near the end of the Cretaceous Period; San Diego area, southern California. The thin external shell is missing, but the suture pattern is very evident, especially on the closeup.

9. Nostroceras sp., 12 cm height (estimated) (plaster replica). Late Cretaceous: Maastrichtian; Santa Catarina area, Baja California Sur, Mexico. I am not sure if the coils at the top of this specimen are part of the same large U-shaped coil just below it. 

A GIANT AMMONITE FROM CALIFORNIA AND BAJA CALIFORNIA, MEXICO

Ammonites are cephalopods (e.g., octopus, squid, and pearly Nautilus shells) that range in geologic age from the Devonian through Late Cretaceous periods, an interval of about 360 million years. For more information, please see my last posting. You can also search for this blog for previous posts concerning ammonites.

This post concerns a few specimens of a giant species of Late Cretaceous ammonite that lived in shallow-marine waters in south-central and southern California and northern Baja California, Mexico. Complete specimens like the ones shown are not common.

The next three images are of the holotype of this giant ammonite, which was first recognized by Anderson and Hanna, who, in 1928, named this species. The first image is a side view of the holotype (which is the "name bearer" of any species and theoretically the most complete of all the specimens initially used to define that species). There can be only one holotype. It has to be stored in an officially recognized museum. This specimen is 50.3 cm [503 mm] in diameter and is from Baja California, Mexico. 

The second image (shown below) is of a paratype (diameter 19 cm [190 mm]) of this ammonite. It too was picked and used by Anderson and Hanna, the original namers of this species, in 1928.  Paratypes are used primarily to show morphologic feature(s) not observable on the holotype.There is no fixed number as to how many paratypes there can be. They are of  auxiliary (or secondary importance) in the naming of a species. 

The third image (shown below) is of the preceding paratype (19 cm [190 mm]) shown immediately above. It shows an end view.

The next two images are additional (non-type specimens) of this large ammonite. Both are large and heavy! It takes at least two strong people to transport these specimens by hand. This large ammonite represents the last of the “giant” ammonites in the fossil record in this region. It can be nearly 20 inches (50.8 cm [508 mm]) in diameter (not including the “living” chamber which enclosed the actual animal), and the weight of a single specimen can be well over a hundred pounds. 

Parapachydiscus catarinae was the first officially recognized name of this giant ammonite, whose shells were first found in the 1920’s in northern Baja California, Mexico. There have been several changes in its genus name in the last 100 years. For example, Parapachydiscus is no longer used because it is a synonym of Pachydiscus, which was named earlier and therefore has priority in its usage.

Pachydiscus is differentiated by having noticeable ribs on its shell, but catarinae has a smooth shell. Some researchers, therefore, have chosen to identify this ammonite as Pachydiscus (Neodesmoceras) sp., aff. catarinae because Neodesmoceras resembles Pachydiscus but is smooth-shelled. Furthermore, in the above abbreviation “aff.” means “having affinity to,” implying that more study is needed in order to determine if the species really is catarinae or if it is a different species (even possibly a new species). 

Other paleontologic problems are 1) possibly the ribs might not be preserved, 2) the ribs change in strength from juveniles to adults, and 3) the rib strength might just be a variable feature. I should also add that some researchers believe catarinae also lived in Colorado. So, the “last word” on the actual identification of this large ammonite, as well as its geologic age and stratigraphic/geographic distribution, still need be figured out. Such matters are beyond the scope of this blog post. 

This geologic time range chart shows the geologic range of the family, genus, subgenus, and the questionable species: Pachydiscus (Neodesmoceras) catarinae?

The geologic age of this ammonite species is inexactly known, but it is either near the Campanian/Maastrichtian boundary (72 million years ago) of the Late Cretaceous or is early Maastrichtian age (72 to about 70 m.y. ago).