March 2012 Rock of the Month

 Septarian Nodules

 Septarian concretions or septarian nodules are concretions containing angular cavities or cracks, which are called "septaria". This word comes from the Latin word septum; meaning “partition", and refers to the cracks or separations in this kind of rock. Septarians formed somewhere between 50 to 70 million years ago as a result of volcanic eruptions. Dead sea life was then chemically attracted to the sediment around them, forming mud balls. As the oceans receded, the balls dried and cracked. Due to their bentonite content they also shrank in size, creating the cracks inside. As decomposed shells seeped down into the cracks in the mud balls, calcite crystals formed. The outer thin walls of calcite then transformed into aragonite. Septarians are composed of Calcite (the yellow centers), Aragonite (the brown lines), and the outer grey rock is Limestone. Septarians can be found all over the world in many different locations.

March 2012 Field Trip Info

RIGHT CLICK ON THE IMAGE ABOVE TO OPEN IN A NEW WINDOW

February 2012 Rock of the Month

Gneiss

Gneiss is a typical rock type formed by regional metamorphism, in which a sedimentary or igneous rock has been deeply buried and subjected to high temperatures and pressures. Nearly all traces of the original structures (including fossils) and fabric (such as layering and ripple marks) are wiped out as the minerals migrate and recrystallize. The streaks are composed of minerals, like hornblende, that do not occur in sedimentary rocks. In gneiss, less than 50 percent of the minerals are aligned in thin, foliated layers. You can see that unlike schist, which is more strongly aligned, gneiss doesn't fracture along the planes of the mineral streaks. Thicker veins of large-grained minerals form in it, unlike the more evenly layered appearance of schist. With still more metamorphism, gneisses can turn to migmatite and then totally recrystallize into granite. Despite its highly altered nature, gneiss can preserve geochemical evidence of its history, especially in minerals like zircon which resist metamorphism. The oldest crustal rocks known are gneisses from western Greenland. Their carbon isotopes show that life existed there at that time, nearly four billion years ago. Gneiss makes up the largest part of the Earth's lower crust. Pretty much everywhere on the continents, you can drill straight down and eventually strike gneiss. Gneiss is an old German word meaning bright or sparkling.

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.