January 2012 Rock of the Month

Granite

Granite is the signature rock of the continents. More than that, granite is the signature rock of the planet Earth itself. The other rocky planets—Mercury, Venus and Mars—are covered with basalt, as is the ocean floor on Earth. But only Earth has this beautiful and interesting rock type in abundance. Three things distinguish granite. First, granite is made of large mineral grains (which is where its name came from) that fit tightly together. Second, granite always consists of the minerals quartz and feldspar, with or without a wide variety of other accessory minerals. The quartz and feldspar generally give granite a light color, ranging from pinkish to white. But that light background color is punctuated by the darker accessory minerals. Thus classic granite has a "salt-and-pepper" look. The most common accessory minerals are the black mica biotite and the black amphibole hornblende. Third, almost all granite is igneous, meaning it solidified from a fluid state and almost all granite is plutonic, meaning it did so in a large, deeply buried body or pluton. The random arrangement of grains in granite is evidence of its plutonic origin. Granite is found in large plutons on the continents, in areas where the Earth's crust has been deeply eroded. This makes sense, because granite must solidify very slowly at deeply buried locations to make such large mineral grains. Plutons smaller than 100 square kilometers are called stocks, and larger ones are called batholiths. On the grandest scale, granite represents the way the continents maintain themselves. The minerals in granitic rocks break down into clay and sand and are carried to the sea. Plate tectonics returns these materials through seafloor spreading and subduction, sweeping them beneath the edges of the continents. There they are rendered back into feldspar and quartz, ready to rise again when and where the conditions are right.

October 2011 Fossil of the Month

Amber

Amber is fossilized tree resin (not sap), which has been appreciated for its color and natural beauty since Neolithic times. Amber is used as an ingredient in perfumes, as a healing agent in folk medicine, and as jewelry. Because it originates as a soft, sticky tree resin, amber sometimes contains animal and plant material as inclusions. Amber occurring in coal seams is also called resinite, and the term ambrite is applied to that found specifically within New Zealand coal seams. Amber is globally distributed, mainly in rocks of Cretaceous age or younger. Historically, the coast around Konigsberg in Prussia was the world's leading source of amber. About 90% of the world's extractable amber is still located in the Kaliningrad Oblast of Russia on the Baltic Sea. Pieces of amber torn from the seafloor are cast up by the waves, and collected by hand, dredging, or diving. Elsewhere, amber is mined, both in open works and underground galleries. Amber occurs in a range of different colors. As well as the usual yellow-orange-brown, amber itself can range from a whitish color through a pale lemon yellow, to brown and almost black. Other more uncommon colors include red amber, sometimes known as "cherry amber", green amber, and even blue amber, which is rare and highly sought after. Much of the most highly-prized amber is transparent, in contrast to the very common cloudy amber and opaque amber. Opaque amber contains numerous minute bubbles. This kind of amber is known as "bony amber". Although all Dominican amber is fluorescent, the rarest Dominican amber is blue amber. It turns blue in natural sunlight and any other partially or wholly ultraviolet light source. In long-wave UV light it has a very strong reflection, almost white. Only about 100 kg is found per year, which makes it valuable and expensive. Sometimes amber retains the form of drops and stalactites. It is thought that, in addition to exuding onto the surface of the tree, amber resin also originally flowed into hollow cavities or cracks within trees, thereby leading to the development of large lumps of amber of irregular form.

September 2011 Mineral of the Month

Petrified Wood

Petrified wood comes from the Greek root petro meaning "rock" or "stone"; literally translating to "wood turned into stone". It is the name given to a special type of fossilized remains of terrestrial vegetation. It is the result of a tree having turned completely into stone by the process of permineralization. All the organic materials have been replaced with minerals, mostly silica such as quartz while retaining the original structure of the wood. Unlike other types of fossils which are typically impressions or compressions, petrified wood is a three dimensional representation of the original organic material. The petrifaction process occurs underground, when wood becomes buried under sediment and is initially preserved due to a lack of oxygen. This inhibits aerobic decomposition. Mineral-laden water flowing through the sediment deposits minerals in the plant's cells and as the plant's lignin and cellulose decay, a stone mold forms in its place. In general, wood takes less than 100 years to petrify. The organic matter needs to become petrified before it decomposes completely. A forest where the wood has petrified becomes known as a Petrified Forest. Elements such as manganese, iron and copper that are present in the water or mud during the petrification process give petrified wood a variety of color ranges. Pure quartz crystals are colorless, but when contaminants are added to the process the crystals can take on a yellow, red, or other tint. Petrified wood can preserve the original structure of the wood in all its detail, down to the microscopic level. Structures such as tree rings and the various tissues are often observed features. Petrified wood has a Mohs hardness of 7, the same as quartz. Petrified wood is the provincial stone of Alberta, Canada and also the state gem of Washington.

July 2011 Fossil of the Month

Ammonites

Ammonites or ammonoids (both terms are used interchangeably) are an extinct group of marine invertebrate animals in the subclass Ammonoidea of the class Cephalopoda. These mollusks are more closely related to living coleoids (i.e. octopuses, squid, and cuttlefish) than they are to shelled nautiloids such as the living Nautilus species. Ammonites are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geological time periods. Their fossil shells usually take the form of planispirals, although there were some helically-spiraled and non-spiraled forms as well. The name ammonite, from which the scientific term is derived, was inspired by the spiral shape of their fossilized shells, which somewhat resemble tightly coiled rams' horns. Pliny the Elder (circa. 79 AD. near Pompeii) called fossils of these animals’ ammonis cornua ("horns of Ammon") because the Egyptian god Ammon was typically depicted wearing ram's horns. Ammonites first appeared in the Devonian Era (circa 400 MYA) and became extinct at the close of the Cretaceous around 65 MYA along with the dinosaurs. Few of the ammonites occurring in the lower and middle part of the Jurassic period reach a size exceeding 9 inches in diameter. Much larger forms are found in the later rocks of the upper part of the Jurassic and the lower part of the Cretaceous, such as Titanites from Jurassic of southern England, which are often 2 feet in diameter and another species of the Cretaceous period of Germany, which is one of the largest known ammonites, have been known to reach 6.5 feet in diameter. The largest documented North American ammonite is Parapuzosia bradyi from the Cretaceous with specimens measuring 4.5 feet in diameter, although a newly discovered 7.5-foot British Columbian specimen, if authentic, would appear to trump even the European champion

June 2011 Fossil of the Month

Trilobites

Trilobites, meaning "three lobes”, are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record dates back to the early Cambrian period approximately 526 million years ago. They flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Devonian, all trilobite orders with the sole exception of Proetida, died out. Trilobites finally disappeared in the mass extinction at the end of the Permian about 250 million years ago. The trilobites were among the most successful of all early animals, roaming the oceans for over 270 million years.

When trilobites first appeared in the fossil record they were already highly diverse and geographically dispersed. Because trilobites had wide diversity and an easily fossilized exoskeleton an extensive fossil record was left, with some 17,000 known species spanning the Paleozoic era. The study of these fossils has facilitated important contributions to biostratigraphy, paleontology, evolutionary biology and plate tectonics.

Trilobites had many life styles; some moved over the sea-bed as predators, scavengers or filter feeders and some swam, feeding on plankton. Most life styles expected of modern marine arthropods are seen in trilobites, with the possible exception of parasitism where there is still some scientific debate. Some trilobites are even thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food. Trilobites range in length from .04 inch to 28 inches, with a typical size range of 1-4 inches. The world's largest trilobite, Isotelus rex, was found in 1998 by Canadian scientists in Ordovician rocks on the shores of Hudson Bay. It measured 28 inches long by 16 inches wide by 3 inches high.