J. Michael Howard answers questions about Geology, Rock Types, and Earth Science
A. Shale looks like flat rock that splits into thinner and thinner sheets. As important as what it looks like, is to understand what shale is composed of. Shale is a sedimentary rock that may have a variety of colors, but because of its composition typically has partings that are parallel to the bedding of the layers of rock above and below it. The partings are a property called fissility, that is shale is fissile. This property is due to the minerals that compose the rock.
Shale is initially deposited as clay, from either fresh or salt water. Clay consists of very small mineral grains that are platelike in shape or form, in other words, they are generally flat and thin, kind of like a dinner plate or sheet of paper. When they are first deposited, they lay at all angles from horizontal to vertical (standing on edge). Then as more sediment is deposited on top of the clay layer or bed, the water between the clay particles begins to be squeezed out and the clay minerals begin to all lay flat or horizontal. As this happens over a period of time, the resulting rock - shale - develops the property of fissility.
Fissile rocks can be split or weather into flat pieces, kind of sheetlike in form. Shale rarely contains any carbonate minerals, like calcite or dolomite, so it will not fizz if you put a drop of vinegar or weak hydrochloric acid on it. Shale may be black, gray, red, green, or brownish, depending on how much pyrite, iron oxide, or carbonaceous (organic) material was deposited with it or formed after it was deposited. A shale having a red color is evidence that the clay underwent some oxygen-rich process, like weathering, before it was consolidated into a rock.
So a sample of shale may exhibit one or more colors, however, it is commonly black or gray, it splits into sheets (is fissile), is usually soft enough to scratch with your fingernail, but sometimes it takes a nail if the rock has really gotten hard, and is very fine-grained. You may see a few sparkly grains of mica on the broad flat surface of a sample, but mica is really not too common. When you see shale on an outcrop that has been weathered, you may see a lot of platy pieces of rock below the outcrop.
When I was a kid, I always liked to skip some of these thin flat rocks on a pond or creek, if I could find some near the water. Some shale contains no silt (fine-grained quartz) and some does. To tell them apart, you have to use your teeth. It may sound silly, but all geologists who have ever worked in the field doing mapping have used this test. It's easy. Take a clean piece of shale and put a tiny piece of it between your front teeth and grind it. If there is any silt, you will feel some grittyness between your teeth. If no silt, then the piece of rock will be ground up and is not gritty. Then spit it out --- pitooiee. You've finished the test!! I hope this helps you identify shale when you see it.
A. Mica is a widespread mineral in igneous and metamorphic rocks, but is only a minor constituent of sedimentary rocks. I wish you had told me what type or at least the color of the mica you are wanting to know about so I could be more specific. However, I will try my best to answer your question as it is:
In all rocks the type of mica that is present reflects the nature of the materials available when it formed. Water is an essential component because micas are hydrous phases which form under conditions of low to moderate temperature and pressure. In igneous rocks, as mica forms it takes up chemicals from the magma so what elements are present in the magma influence the type of mica which forms. Granites typically have muscovite mica, which is relatively low in iron and high in magnesium. Silica-rich pegmatite rocks, which are extremely coarse-grained rocks, are the source of commercial muscovite mica deposits. Some of these crystals may produce single cleavage sheets over 18 inches across. Intermediate to silica-deficient rocks, graniodiorites to syenites general contain biotite because the magmatic liquid has a higher content of iron relative to magnesium. The biotite may vary from very fine-grained to moderately coarsely crystalline (to 3-4 inches across). Ultramafic rocks, like kimberlites or lamproites, contain phlogopite, a golden colored mica, usually as very fine-grained crystals.
Now, in Arkansas, we have a few areas of igneous rocks all of syenitic or ultramafic affinities so we see no muscovite in our igneous rocks, only biotite and one location with phlogopite (Crater of Diamonds State Park near Murfreesboro in Pike County). Sizable flakes of biotite are somewhat scarce as specimens from Arkansas, but can sometimes be recovered from soils where these rocks have altered to clay. Several localities include Granite Mountain in Pulaski County, near Bauxite in Saline County, Magnet Cove in Hot Spring County, and Potash Sulphur Springs in Garland County. In many creek beds west and south of Little Rock over to Hot Springs to near Malvern contain outcroppings of lamprophyre dikes. These rocks are iron-rich and often contain biotite mica as crystals, which may weather out of the rock.
In sedimentary rocks, the most common mica present is sericite, a fine-grained variety of muscovite. It is not uncommon in sandstones from the Arkansas Valley region and some portions of the Ouachita Mountains. It is an authigenic mineral, meaning it formed in the rock by alteration of previously existing minerals. In the case of sericite, it forms from the clays which were originally deposited with the quartz sand. During the process of lithification as the sediment slowly turns to stone, the clay slowly changes to fine-grained mica. Many people find sparkly samples of sandstone and bring them in for identification, thinking they have found gold or some precious metal. But its just mica. Mica is easily identified by its perfect single cleavage.
In southeast Missouri there are large outcroppings of granite, some of which is very coarse-grained. This would be the closest source of any sizable muscovite. In western Oklahoma are also some major outcroppings of granite, as well as the Llano region of Texas. I think the granite from Llano, Texas, however, contains mostly biotite. The nearest source of coarse muscovite to the east are the pegmatites of North Carolina and the nearest source to the west are the pegmatites of Colorado.
Q. Recently the construction company I work for completed a job about half way between Fayetteville and Siloam springs. We encountered a condition underground that consisted of a layer of red clay and a layer of rock. The rock was fractured. One person I talked to called this "TERRA ROSA" or weathered limestone. What can you tell me? The depth of the rock was from on top of the ground down to 20' 0' (this is a deep as we went). Can you shed some light on this. Thank you.
A. The term "terra rosa" (literally red ground) is a term used by soil scientists, but not often by geologists. Terra rosa soils are predominantly clay, yet they have surprisingly good drainage characteristics.
When limestone weathers, the clay contained in the rocks is left behind, along with any other non-soluble rock material, like chert. Under oxidizing conditions when the soils are above the water table iron oxide or rust colors the clay, giving it a characteristic red or orangish color. There are some major areas of terra rosa soils in north Arkansas. Terra rosa soils may be quite thick. One drilling project I was on in Randolph County encountered a 125 foot thick bed of this material! You might wish to contact either the Soil Conservationist (Federal) or County Sanitarian (State Health Department) in the county or district you are interested in for detailed information for any specific locality.
A. Geodes form under special conditions. There must be a hole in the rock and then minerals dissolved in fluids must be able to reach that hole to allow crystals to be able to form. In geodes the minerals form from the outside in, therefore the crystals point inwards. Certain types of rock are more susceptible to the formation of geodes.
The most common rocks containing geodes are lavas that contain gas pockets and carbonate rocks which contain holes where other minerals dissolved away, sometimes during the formation of the rock from the original sediment (the process is called lithification and means "turn to stone"). These rocks are common in certain areas of the country.
Keokuk, Iowa, is well known amongst collectors for the quartz-lined geodes which abound in the area. These geodes are present in carbonate rocks. Mexico has "coconut" geodes which are very round and typically the size of coconuts. Some state out west like Colorado or Oregon have lava flows where geodes are recovered. Hall's Gap in Kentucky is famous for a site that produces millerite-containing geodes. There are many locations around the world. Brazil produces huge amethyst-filled geodes from lava flows. Florida produces some beautiful silica-replaced corals from some of the bays that could certainly be considered geodes.
However, in Arkansas, there are few localities. One general area to examine in north Arkansas is around Greer's Ferry Lake. I have seen a few "snowball" type small geodes weathering from the carbonate rocks and residuum along the lake shore. Most contain drusy quartz and rarely other minerals like dolomite or calcite may be present. They don't look like much when compared to the larger fancy geodes from other places, but they do come from Arkansas. If you are not from Arkansas, check with your state geological survey to find out if there are any potential geode-collecting sites where you live.
Q. Mikey, I am a flintknapper and have purchased some noviculite (mispelled) from a store in Mo. I have family in Rogers, Ark., that I visit often. I would be very interested in locating a flint mine or source where I could purchase or pickup some of this stone. Can you help me.
A. If you want novaculite you need to go to the source -- Hot Springs. Once there you can look in the yellow pages of the telephone directory under "Sharpening Equipment & Stones" and you will find many whetstone companies listed.
Most of these places are open during the week days. Just tell them what you want to use the novaculite for and they will know if they have the type of stone you need. They get inquiries from flintknappers all the time. You might ask if they have a reject pile of stone you may pick through or if they will provide you with some different types of novaculite so that you can learn what may work best for your purposes. Good luck!
Ed. wife's note: How much do you want? We have a pile of flintknapping rock out by the barn Mikey could sell reasonably....
Q. Hi my family and I go rock hunting for shark teeth in a local creek bed and this weekend we found a "tooth" that is 1 inch square and has ridges in the middle like a ripple and it is grey and very obvious square base to the tooth...we think maybe it was a prehistoric shell crunching shark... you have any idea...it is awesome but we'd like to know what it actually is//thanks
A. Well, I can't be exactly certain without seeing it, but from your description it may be a manta ray tooth. Manta rays are shell-crackers. It would be helpful to know the age of the rocks that the tooth came from. Do you know the age of the rocks that the shark's teeth are weathering out of? Shallow water sharks and mantas are not uncommonly found in similar environments.
A. Please tell me what type of mineral we are talking about because there are many different cleaning methods, depending upon what minerals compose your specimen. If the specimen consists of delicate needles or thin fragile crystals, then the cleaning techniques will be different that if the specimen consists of blocky stout crystals of a mineral with little or no cleavage. The force exerted to remove debris is dependent not on the debris itself, but on the mineral you wish to have remaining. So, what is the specimen? Quartz? Fluorite? Calcite? or something else?
Q. My name is Jim and I would like to tell you how informative your rockhounding in Ark. has been for me. I plan on vacationing in your area and was hoping you could suggest a time to come when crowds are at there least. I plan to stay about a week and dont really know how to get the necessary information I need to accuratly plan the trip. If you could answer some of my questions or you know someone who can It would be greatly appreciated.I would very much like to thank everyone who spent the time putting together the vast amount of information I found in your web site.
A. Jim, may I suggest you contact the cities where you will be visiting. Some crowds are event-dependent, like during the Bass Fishing tournament it may be impossible to get a campsite on Lake Ouachita. The Ark State Parks website is www.1800natural.com and we also have a few and growing number of area links on our linklist.
Q. My 3rd grade son recently read that about 2/3rds of the earth is covered with sedimentary rock. We would like to see a chart or a map that shows roughly where this rock is not located. We've looked on the internet, but really haven't found anything yet. Can you help us? Carla and Noah; Benton, Arkansas
A. (Quote from W. K. Hamblin, 1989, The Earth's Dynamic Systems) "Sedimentary rocks are probably more familar to most people than the other major rock types because they cover approximately 75 % of the surface of the earth's continents and therefore form most of the landscape."
You need to read the book your son was using to see exactly what it states, for if it states "that about 2/3rds of the earth is covered with sedimentary rock", it is incorrect. What you need to find is a geologic map of the world to show your son those major exposures on the continents of igneous and metamorphic rocks. Most of the oceans are underlain by basalt, a type of volcanic igneous rock. Some 75 % of the continent's crust is underlain by metamorphic rocks. Therefore, as dominant as sedimentary rocks are at the surface, they are only a thin skin over the continent. This is because sedimentary rocks are derived by weathering processes acting on pre-existing rocks. Only those rocks exposed at or very near the surface undergo weathering, so those are the only ones available to generate sedimentary rocks. Some sedimentary rocks are composed of materials formed from sea water, like gypsum or calcium carbonate, but like the other types (sandstone, siltstone, and shale), they are formed at or very near the surface.
I suggest you follow up on this question by visiting the US Geological Survey's website and ask one of there geologists about the availability of a world map displaying the distribution of the 3 major rock types on the continents and in the ocean basins. The website address is: http://geology.usgs.gov/inquiries.html
Q. Hello, about 8 years ago a friend and I travelled through Murfreesboro, Ark. on our way back to GA. While there we saw a "mineral" that I can only describe as "natural glass", not obsidian, but a clear (and opaque) substance that came in every color of the rainbow (both clear and opaque). We obtained a specimen as a souvenir without gaining any background information as to what we had. Ever since then I have had to field endless questions as to exactaly what this substance was, the only answer I have been able to give is "natural glass" from Ark. As of yet I have not run across any example in gem and mineral texts and have had little luck on the net. At your convenience, could you provide me with some background info? If memory serves me correct this "natural glass" was very prevalant and cheap on the many roadside stands located in Murfreesboro. As I will be travelling through Ark. in Nov. I would like to obtain more without going on a wild goose chase. Any help you could provide would be greatly appreacited. Curious, tom mcmahon
A. Tom, You are the proud owner of some steel foundary slag glass! You are absolutely right, it is very prevalent and available at many Arkansas rockshops, from Murfreesboro to Mount Ida and the Hot Springs area. The original material came from a foundary at Fort Smith in Sebastian County. Some now comes from some foundaries on the east coast. It is used as an "eye-catcher" at many of the shops and is prominently displayed on tables at the front of many shops. It is not natural, but instead is the product of the removal of silica from iron ores, so naturally you will not find it listed in any books. The most common color is a light blue-green, and yellow, red and orange are fairly scarce. Most sells for from $1 to $3 per lb. It is very popular for fish tanks and is quite inert in that application.
A.You are right in assuming that amber looks much different in its natural state than as a finished jewelry product! First, it is usually dull lustered and has a somewhat brown or tan cast or color. The Arkansas amber may be yellowish, brown, or reddish. It may look somewhat like a piece of weathered wood (having that texture). All amber is very brittle so if you step on it, it will break to a powder. I look for something small, of the general description given above, and then use my fingernail to chip off a small corner. Because it is brittle, it will easily break. A fresh break will have a glassy luster, show its translucent to transparent nature, and have a conchoidal fracture, like glass. If you find something like this, use a match or butane cigarette lighter to heat it on a knife blade. It will smolder, may even catch fire, and give off a heavy rosin odor. To clean amber, first rinse with water and gently scrub with a soft toothbrush, then pat dry. Next, drop a small piece in acetone (you can get a small can in the paint section of your local hardware store). Just a rinse really is what you want as the amber will begin to dissolve, leaving a shiny surface luster as it dries.
When geologists talk about time, we need a way to discuss billions and billions of years! So, over the past 150 years, we have set up a time scale. The time scale outlines the total history of the earth since its formation as a planet. As you might suspect, the older items on the geologic time scale are less understood than the events that happened more recently.
Rocks are dated by two principle methods:
The first is a RELATIVE DATING method used for most sedimentary rocks -- Sedimentary rocks are generally deposited in layers. If undisturbed by other processes (like those that build mountains), then an outcrop of sedimentary rock will have the oldest layers on the bottom and younger layers as you go up the outcrop. This is a type of stratigraphic dating. We also use fossils found in sedimentary rocks and compare these from outcrop to outcrop. Ancient plants and animals lived, died, and some were preserved in the rocks. Now, think about this. In the past 150 years, geologists have examined millions of outcrops all over the world (We've been very busy with our little rock hammers and hand lenses!). We have compared (correlated) these rocks worldwide. And we have also examined millions of feet of samples drilled from the ground during oil, mineral, and engineering studies of rock.
So we now have a pretty good handle on the ages of the sedimentary rocks by comparison to other outcrops in many parts of the world. Again, using fossils and layering gives rise to this relative age scale.
But how do we know really how old a given rock is? By some methods developed to determine ABSOLUTE AGE, which is the second and most direct method. Chemists and physicists early in the 1900's discovered many radioactive elements and isotopes (varieties of the same element). They discovered that these elements decay by nuclear reaction at set natural rates. Now if you lock these elements into a rock, say an igneous rock by solidifying it from a magma, then the decay process begins. If you want to know the absolute age of a given igneous or metamorphic rock, then you must run an isotopic analysis (this may be for certain isotopes, like those of the uranium or thorium series of the chemist's periodic table of elements). We know the ratio of the isotopes by this analysis and can back calculate it to the original value using the constant decay rate for whatever isotope we use. The decay rate is expressed in half lives. As an example, Uranium 238 has a half life of 4,460,000,000 years! This is close to the actual age of the earth. So, in nearly 4.5 billion years, half of U238 that existed has changed to lead.
So, we can directly calculate the age of an igneous or metamorphic rock from an isotopic analysis. This age is called the absolute age. With sedimentary rocks, we want to find some widespread event, like an ash fall from a large volcano that may have extended across a large area. Ashfalls are rapid events geologically, and they create time lines in the geologic section and across geologic outcrops. Remember when considering sedimentary rocks that different sediments are being deposited in different places, all at the same time. Think about the Gulf of Mexico. Near the shore sands and limestones are being deposited and out in the deep water, miles from shore, muds and clays are being deposited. Now if we had a big volcano in Mexico that erupted and the winds were blowing to the east, then the ash would be carried for several hundred miles into the Gulf of Mexico. This ash will fall out of the air in a few hours or days and is deposited on the ocean floor, then it may get covered with other sediments. So it becomes a good time line to use for isotopic dating.
Now, geologists have made 10's of thousands of isotopic dates in the past 50 years. Using these dates, we have determined how they fit into the earlier geological time scale we had built using indirect methods. Now when you look at a geologic time scale in your school book, it should have some actual dates on the major breaks in time. When I went to school, over 30 years ago, we had a time scale, but it was only the relative one, not the absolute one. I think I remember my science teacher telling me that the earth was over a billion years old, but she did not say how old, like we can today.