Diamond Head Crater, Oahu, Hawaii. This image was the Earth Science Picture of the day on Sept. 15, 2011. It was ranked #4 in the Viewer's Choice for top pictures for the month. Thanks to all who voted!
Geology Field Photo: Isoclinal folds in high-grade gneiss, Southern Appalachians, Eastern Blue Ridge, South Carolina-Georgia.
Trace the thin layers of white minerals as they are folded back & forth within the rock. On left near center, a thin layer of white material is folded all the way back on itself so that the limbs of the fold are parallel to one another (isoclinal, meaning "same angle"). Smaller parasitic "Z" and "S" folds are present on the limbs of the larger fold. Location is at Woodall Shoals, on the South Carolina side of the Chattooga River. Hammer head shows sense of scale. #geopic
En echelon veins in the Van Hise rock, Wisconsin
Geology Field Photo: Galena Crystals in Dolomite.
Galena is pictured as the silver-colored, cube-shaped crystals in the center of the photograph. These crystals and others are visible along a road cut along Hwy. 72 near Fredericktown, MO. Galena is a lead-ore mineral, sought after as a valuable commodity for mining when present in high enough concentrations. Lead is commonly used in the batteries of automobiles, as well as other applications. Lead is highly toxic when in elemental form, and its affect on the environment can be significant. The surrounding area in southeast MO contains a large number of both active and abandoned mines that have produced lead and other metal commodities. Vertical height of the image is about 1.25 inches.
Geology Field Photo: Folds in Marble.
These folds occur in a carbonate-rich metamorphic rock along a road cut on Hwy 28 in Ontario, CA. The rocks are within the Grenville Province, a large region of rocks metamorphosed ~1.0-1.1 billion years ago. The layers show differences in their ability to accommodate the strain, with some of them, visible in the lower right corner, pinching out & separating in the hinge of the fold. #geopic
Geology Field Photo: Liesegang Rings at Garden of the Gods, IL.
Most of the state of Illinois was covered by glaciers in the last ice age, but the glaciers stopped just north of the area of this photograph. Here in southern Illinois, at Garden of the Gods in Shawnee National Forest, high bluffs of sandstone mark a landscape that is unusual in the state. The rocks seen here are sandstones, but a secondary feature, which formed in the rock after the sand was deposited and hardened into sedimentary rock, captures the eye. These swirling bands within the rock stick out as ridges, and are called Liesegang rings or Liesegang bands. These that are on display along the Observation Trail are a spectacular example of this feature. These bands are due to the presence of iron-oxide minerals. Within the original sandstone, small pore spaces existed in between the sand grains, and within those spaces the iron oxide minerals formed when these rocks were deep enough in the Earth that ground water was flowing through them. The presence of the iron oxide minerals causes the bands to be more resistant to weathering and stick out relative to the rest of the rock. #geopic
Geology Field Photo: Chilled Margin in Granite
Granite is one of the most common kinds of igneous rock on the Earth, forming from the cooling of magma deep in the Earth's crust. In this photo, the blackish rock on the right side is a volcanic rock, an old ash-flow long since hardened into rock. The granite on the left, called the Knob Lick Granite, is distinctly coarser in grain size, and individual mineral grains can be clearly seen as black, white, and pink colors. Next to the rhyolite, however, the granite clearly shows a finer grain size. Geologists call this a chilled-margin. This results from the granite magma cooling quickly when coming into contact with the surrounding rock, in this case the rhyolite. Faster cooling means that the minerals in the granite melt don't have as much time to grow, and consequently do not grow as large. This type of contact between these two rocks also indicates that the granite is younger than the rhyolite. These rocks are found in an old abandoned rock quarry at the top of Knob Lick Mountain, just off of U.S. 67 and a few miles south of Farmington, MO. #geopic
Geology Field Photo: Deformed Mudcracks in East Tennessee
No, this isn't a fossilized dragon skin, unfortunately! The rocks below show old mudcracks, now hardened into stone. Mudcracks indicate that the sediment was deposited in a near-shore tidal environment, where the muddy water would at times retreat, leaving the mud exposed to the sun & wind, which allowed the roughly hexagonal-shaped cracks to form. These mudcracks, however, are deformed. The are shorter in one direction than another. This shortening is called a strain marker, and geologists use strain markers to get an idea of how much deformation occurred in the rocks. In the Valley & Ridge of east Tennessee, a large package of sedimentary rocks has been deformed, mainly through thrust faults & folding of layers. These rocks collectively demonstrate the progressive squeezing of the crust as Africa collided with North America. This collision was the final stage of the assembly of the supercontinent Pangea, about 300 million years ago, an event that shaped the Appalachian Mountains into what we see today. The rocks pictured below are part of the Chickamauga Group (Luther Fm.), which were deposited as sediments in the Middle Ordovician about 470 million years ago. #geopic
Geology Field Photo: Delta Clast in Gneiss
The grain pictured below is a classic, textbook1 example of a delta clast. Delta clasts are a type of shear-sense indicator, structures in rocks that indicate the directions that material in rocks moved when it was being deformed. Deep within the Earth's crust, rocks eventually become too hot to break. Instead, they flow as the minerals, under stress, recrystallize. Geologists refer to these processes as ductile, rather than brittle. The large, pinkish, round grain pictured below is the mineral potassium feldspar. Note the tails trailing off in both directions. This rock is (was) found within the Parry Sound Shear Zone in the Grenville Province of Ontario, CA. Shear zones are deep-earth, ductile equivalents of brittle fault zones. A delta clast is produced when material is "rolled-up" on itself during rotation. This grain indicates a clockwise sense of rotation - material at the top moved to the right, while material below moved to the left. Imagine grabbing the clast by its center, and rotating your hand in clock-wise fashion, and you'll get the idea. The tails on each side were being slowly rolled up into the center of the grain, preserving a record of deforming rocks that have long since stopped recrystallizing. Sadly, this delta clast was lost when these rocks were removed due to an expanding roadway (I saw the rubble in the fall of 2010. and cried a little.). As of this posting, the old roadcut can still be seen in Streetview in Google Maps. #geopic
1Seriously, this grain has been pictured in structural geology textbooks. See Van der Pluijm & Marshak, Earth Structure, Fig. 12.5b, p. 300.
Geology Field Photo: Shattercones at Sudbury, ON
Shattercones are structures that develop in rocks during meteorite impacts. They are simply a cone-shaped fracture, with a "feathered" texture on the fracture surface. The "point" of the cone points toward the direction of the impact, and so the orientations of a large number of shattercones can be used to determine the center of an impact zone. Shattercones are typically a few inches up to a few feet in length. They are often found with impact breccias (broken up rocks), high-pressure minerals, and glass formed as a result of melting rock during an impact. These shattercones are found around the Sudbury Impact, located in Sudbury, Ontario, CA, an area also known for related Ni mines. This impact is thought to have occurred approximately 1800 million years ago. The foliage gives a rough sense of scale. #geopic
Adult Iguanodon footprint at Dinosaur Ridge, CO. Photo 2 of 2.
Geology Field Photos: Akaka Falls, HI
Welcome to a slew of new followers I've gained today! I'm a geologist and an educator, and if you want to more about those things you can check out my about page. I post a photo about once a week or so related to geoscience, with a short description to show people a bit of the wonder of our planet. This one is from the Big Island of Hawaii, about ~11 miles north of Hilo, at a Hawaii state park. These falls are a bit over 400' high on the Kolekole stream. #geopic
In addition to the pictures, I also post links to interesting videos, pictures, or news items about geoscience, and I also post my blog links periodically about issues related to planet Earth, our natural resources & environment.
Geology Field Photo: Conjugate Joints in Sandstone
Joints are commonly defined in geology as breaks in the rock on which there has been no movement. They are usually interpreted as tensile fractures in the rock, forming perpendicular to the direction of the least principle stress. Joints are very likely the most common geologic structure on the Earth, occurring in nearly every rock type and all geologic settings. Below, two sets of joints can be seen, intersecting each other at roughly 60/120°, forming a conjugate joint system. Joints are important to the study of the Earth for their impact on hydrology, the land surface, interpretation of tectonic stresses, and on geologic engineering. Joints often are part of the cause of rockfalls and landslides. #geopic
Geology Field Photo: Ripple Marks in Baraboo Quartzite
The Baraboo Quartzite is a beautiful pink rock exposed in the hills both north and south of Baraboo, WI. It is a metamorphic rock, yet contains clear evidence of its prior history as a sedimentary rock before metamorphism occurred. The roadcut pictured below, on Hwy 12, runs N-S just to the west of Devil's Lake State Park. This cut shows ripple marks on the bedding planes, which indicate that the sand was originally deposited by moving water. The metamorphism was low-grade, allowing the ripple marks to survive. The bedding plane dips to the North (toward the viewer) because the Baraboo Quartzite is folded into a large syncline (U-shape fold).
Geology Field Photo: Folded Baraboo Quartzite
This fantastic little folded layer is visible in Devil's Lake State Park, near Baraboo, WI. Folds that point downward like a "U" shape are termed synclines, while those that point upward are termed anticlines. The pink rock is the Baraboo Quartzite, which here is interlayered with a phyllite unit, and is folded into an anticline/syncline pair. Interestingly, this fold does not mimic the larger Baraboo syncline, which has a very steep northern limb (left side) & a shallowly dipping southern limb (right). Small scale folds often mimic large scale structures, and are termed "parasitic folds". This is the same rock unit & general area of two previous field photos: http://bit.ly/xkF9bQ & http://bit.ly/oyzerd . Knife tool bottom center is ~4" long. #geopic
Geology Field Photo: Mt LeConte
This panorama is stitched from 3 photos I took of Mount LeConte from a cabin we stayed in in 2011 in Gatlinburg, TN. The main mountain visible here is Mt. LeConte, which has several main high points in the center of the photograph. Mt. LeConte is one of the highest mountains in Great Smoky Mtn. Natl. Park, and the tallest from immediate base to top in East Tennessee. The rocks that make up the Smokies & most of the Blue Ridge of TN-NC-VA are metamorphic rocks. In this area, they consist primarily of the Ocoee SuperGroup, a package of rocks that were originally deposited as sedimentary rocks in the Precambrian and subsequently metamorphosed & deformed in the Ordovician Period. This package of rocks was later thrust several hundred kilometers along the Blue Ridge Thrust Fault during the Pennsylvanian Period. #geopic
Geology Field Photo: Black Hills Fold
Here's a fantastic fold in the north-central Black Hills, SD. The fold is upright, meaning that the axial plane of the fold (the imaginary plane bisecting the fold) is vertical. The fold is also isoclinal, meaning that the two limbs of the fold are at about the same orientation and the interlimb angle is very low. The rocks are Precambrian metamorphic rocks.
Geology Field Photo: Angular Nonconformity, Black Hills
This exposure reveals what is at times referred to as the fundamental discontinuity of geology, the break between Precambrian rocks below and Cambrian rocks above. Here the rocks above are Cambrian sandstones, and the rocks below are tilted metasedimentary Precambrian rocks. The unconformity can therefore be thought of both in terms of an angular unconformity and in terms of a nonconformity.
Geologists define the concept of an unconformity as a break in the rock record where erosion has taken place between packages of rocks. There are generally three kinds: Disconformities, Angular Unconformities, and Nonconformities. The first kind consists of sedimentary rocks deposited on top of horizontal sedimentary rocks, and is the least complex. Angular unconformities consist of sedimentary rocks lying on top of other sedimentary rocks that are at a significant angle. Nonconformities consist of sedimentary rocks deposited on top of igneous or metamorphic rocks.
Panorama of Niagara Falls.
Cross-cutting granite dikes in Harney Peak granite above Lincoln's left eye.
Geology Field Photo: Papakolea Green Sand Beach
Digging through some old shots and wanted to share this one of the green sand beach near South Point, Hawaii. This beach has truly remarkable green sand and is quite unique. Certainly a must-see experience for any geoscientist visiting the Big Island. At ~18.9 degrees N latitude, this area is as far south geographically you can get in the United States. The sand is made largely of olivine grains, which are left behind after the less resistant materials (ash fragments, volcanic glass, etc.) are eroded away. Most of the time geologists think of olivine as being one of the more easily eroded minerals, but here the rest of the volcanic rock is less resistant and the olivine is also more dense. The beach is found in a small cove facing the south Pacific Ocean. Truly a breathtaking location as the waves come crashing in to the cove! The beach is the former area of some sort of eruptive activity, but the exact details are debated. Some call it a littoral cone, which is formed when flowing lava interacts with sea water, and the eruptive activity is all produced by the expansion of the water to steam on contact with the lava. Others claim this is a cinder cone, the smallest type of volcano. Zoom in on this one to see people on the beach. Hope to go back again one day!
Geology "Field" Photo: Double Refraction in Calcite
Another shot from my recent visit to +The Field Museum , this one is of a sample of optical calcite placed over a piece of paper with the words "Double Refraction" on it. You see two copies of the paper and the words through the crystal, due to the phenomenon. So how does this work?
Most materials, like air, water, and glass, are isotropic , which means that light passes through them in the same manner no matter how the material is oriented. Many crystals, however, are anisotropic , which means that light behaves differently as it passes through the material. Anisotropic crystals cause light to split into two separate rays as it passes through. Those two rays have slightly different propagation directions, and therefore end up making the object behind the crystal look doubled. One of those rays, called the ordinary ray , obeys Snell's Law, which a lot of people learn about in a physics class. This is the law that governs the way that light appears to bend as it passes from air to water, i.e., refraction. The light bends because air has an index of refraction of ~1.0, while water has an index of refraction ~1.3. Light therefore travels about 30% slower through water than it does through air. The other ray, however, does not obey Snell's Law, and is called the extraordinary ray . Since there are two rays propagating in two different directions, light has been refracted through the crystal twice.
Those two rays are also polarized, meaning that the light can only vibrate in a single direction. The vibration directions of the two rays are at 90 degrees to one another. This can be easily demonstrated by placing a piece of polarizing material over the calcite and rotating it. So if you bring a set of polarizing sunglasses with you to the museum, you can show this effect to your friends, or random strangers also enjoying the minerals exhibit.
The two rays also travel through the crystal at different velocities - there is a fast ray and a slow ray . In the case of calcite, the ordinary ray is the slow ray, and the extraordinary ray is the fast ray. The velocity of the ordinary ray is constant through a given sample of calcite, no matter how the crystal is oriented. The velocity of the extraordinary ray, however, actually depends on the orientation of the sample. The refractive index of the ordinary ray is ~1.66, while the extraordinary ray RI reaches its lowest value at ~1.49 when the crystal is held a certain way. The difference between these two values of RI is called the birefringence , which for calcite reaches a maximum of ~0.17.
Why calcite? Calcite is typically the only mineral used to demonstrate double refraction, even though most crystals also produce this effect. There are two main reasons for this. For one, it has one of the highest values of birefringence, allowing it to be very effective at separating the two rays. For example, another common mineral quartz has a maximum birefringence of only 0.009. So while quartz also causes light rays to doubly refract, the difference between the two rays is so slight that you would not be able to see it very easily unless the sample was ~20-25x thicker! Secondly, calcite can be very clear like the sample in this picture, which is also obviously necessary for this demonstration to work. Many minerals simply aren't transparent enough at this thickness to allow light to travel all the way through.
#mineralogy #crystallography #scienceeducation
ptygmatic folds, Llano uplift
Granite dike cutting through gneiss, Llano Uplift
Gneiss xenolith within granite dike cutting through gneiss.