The logo sure does sound official, doesn't it? Well, given the diaries posted thus far, all of which explain some essentials of geology, including the larger tri-series on the Colorado Plateau, I thought today we would take a field trip together and I can learn the community about geology and some of you can teach me something about correct grammar. Deal?
And following up on rather singular diary posted this past week (on Ethnobotany). . .as the saying goes, "And now for something completely different!" That being said I think this Geology 101 class underway is unavoidable. What I mean is in the sense of going into more detail given the various geologic settings thus far discussed in this tour. Thus a more proper geologic explanation entirely focused on this academic discipline. On the other hand, what follows is a rather informal overview of the essentials of geology, and once gleaned, will help make it easier to tell one basic rock type or group from another (including making better sense of conglomerate formations defining the overall geologic setting).
Additionally, this diary's broad subject matter adds to a data base and will be helpful for all the diaries still to come. For those of you, like me, who sort of have rocks in your head, now you can at least shake 'em out and identify what lands on the ground and at your feet.
(Continues after the fold.)
The Colorado Plateau is rock heaven and a haven for those who especially love geology and the various rock formations on display. Rocks are also divided into three basic families. Each is based on how they form. Geologists classify all rocks on the planets in these three categories:
Let's start with the planet's original building block material, igneous rocks. If you found this sample lying on the ground. . .
Which is obsidian (or volcanic glass), then you would surmise it came from this hotter-than-Hades source. . .
The sample is also just one of many different types of igneous rocks.
Igneous rocks form from a melt, or magma, deep within the planet. Such rocks can be extrusive (volcanic) in origin, or else intrusive (still volcanic). Geologists place intrusive rocks in two categories: hypabyssal (meaning, igneous rocks derived from magma that has solidified at shallow depths in the form of dykes and sills of intrusions) and plutonic (meaning, material that is formed deep in mountain-building zones). Plutonic rocks are formed by partial fusion of lower continental crust and some from magma rising from the mantle. Slow cooling yields large mineral crystals, which are coarse-textured rocks (granite, diorite, gabbro, and peridotite). Granite, mainly that which is made of quartz, feldspar, and mica, is the main igneous rock of continental crust.
By contrast, hypabyssal rocks are relatively smaller masses, often strips or sheets. They cool at a lesser depth and much faster than plutonic rock. They are therrefore comprised of smaller crystals (microgranite, microdiorite, and diabase).
All intrusive rocks produce the following features:
1) Batholith (a huge deep-seated, dome-shaped intrusion, usually of acid igneous rock).
2) Stock (similar to a batholith but smaller; also have an irregular surface area under forty square miles or thereabouts).
3) Sill (a sheet of usually basic igneous rock intruded horizontally between rock layers).
4) Laccolith (a lens-shaped usually acidic igneous intrusion that domes overlying strata).
5) Lopolith (a saucer-shaped intrusion between rock strata; up to hundreds of miles across).
With respect to extrusive igneous rocks, these occur mainly at volcanic vents along the active margins of lithospheric plates. Magma erupts as lava, which cools and hardens quickly on the surface as fine-grained or glassy rock (i.e., obsidian). When the magma hits cold water or land, it cools and crystallizes very quickly. Consequently, such rocks leave little or no time for the mineral crystals in the rock to grow to a very large size. Sometimes they cool so quickly that gases from the magma are trapped in the rock, forming gas bubbles, also known as vesicles. These small holes are also plainly visible when the final product is through. Examples of the most common volcanic rock types are: basalt, andesite and rhyolite.
(FYI: All basic lavas are rich in metallic elements but poor in silica. These lavas flow easily and erupt relatively gently. Basalt is the most famous kind of magma, which accounts for more than ninety per cent of all volcanic rock. Basalt is also formed by partial melting of peridotite, the chief rock of the upper mantle. Welling up from oceanic spreading ridges, the basalt builds new ocean floor material. There are also acid (i.e., silica-rich) extrusive lava flows. These appear at destructive plate margins. They probably comprise selected substances from basic lava of the upper mantle or reprocessed crust. Acid lavas are therefore explosive and slow-flowing. They produce such rocks as dacite, rhyolite, and obsidian.)
The igneous rock contribution to the planet's geophysical transformation is, of course, magma that has to go somewhere, that is, it occurs where heat melts parts of the Earth's upper mantle and lower crust. Most magma that has cooled and solidified escaped up through the crust from oceanic spreading ridges. Smaller quantities came from destructive plate boundaries, colliding continents, and hot spots.
In addition to the main magma release there are intermediate lavas in the group, which contain plagioclase, feldspar, and amphibole. Alkali feldspar and quartz are also found in this group. The intermediate lavas stem from partial melting of certain minerals in subducted oceanic crust. Amazingly, some 860 known volcanoes have erupted in the last 2000 years. Those that emit continuously or periodically are considered active. Volcanoes that donât erupt in recent times are labeled dormant. Long-inactive volcanoes are said to be extinct.
One more thing to mention here, and that is how the cooling process of both extrusive and intrusive rocks is important to note for this specific reason: as the magma cools the chemical elements in the material begin to join together in crystalline forms or minerals. As magma cools, it also crystallizes and turns into solid rock. The time it takes for some of these rocks to cool may take minutes, years, or even hundreds of thousands of years.
Perhaps with this above explanation this chart may make it easier to understand the nature of igneous rocks:
Contrasting igneous fire rocks with sedimentary materials, the latter are the easiest to understand.
Sedimentary rocks also form at the Earth's surface. Unlike igneous or some metamorphic rocks, their geologic clocks are not reset. This means they are not changed or altered from their original constituent properties as are igneous and metamorphic rock types. Primary sedimentary rock examples are limestone, sandstone, shale, and conglomerate. These softer rocks, as compared to the harder metamorphic rocks, are mostly what makes up the formations of the Grand Canyonâs upper walls, the horizontally-placed strata. Limestones are rich in calcium and magnesium carbonates. They make up about eight per cent of all sedimentary rock; only shale and sandstone are more plentiful. Organic limestones also contain calcium carbonate extracted from seawater by plants and animals that used this compound for their protective shells. Thus limestone rocks include reef limestones built up from the stony skeletons of billions of coral polyps and algae inhabiting the beds of shallow seas. Coquina is a cemented mass of shelly debris. Chalk is a white, powdery, porous limestone comprising tiny shells of fossil microorganisms, drifting in the surface waters before they perished, then rained down on the bottom of the sea.
By contrast, sandstone is a common sedimentary rock composed primarily of particles of sand, with minor amounts of silt and clay. These particles are cemented together, mainly by trace amounts of calcite or silica. Sandstone accounts for eleven per cent of the sedimentary rocks on the planet.
Add to this list the smaller fragments. For example, mudstone, like siltstone, and shale, which are the very soft kind of sedimentary rocks. They are made of clay minerals of less than 0.004 mm diameter. Siltstone are rocks formed of particles 0.004 to 0.06 mm in diameter. Shale accounts for more than eighty percent of all sedimentary rock. Shale also indicates a low energy environment, where silt and mud settles. Thus the accumulation of silt and clay equals siltstone and mudstone, whichl ends up as shale in one form or another. Rivers, floodplains, coastal tidal flats, lagoons, even deeper water offshore environments are where you will find these sedimentary materials in the makeup. All three of these rock types, including similar fine-grained rocks of silt and clay, are easily split along their bedding planes. This is why these much softer sedimentary rocks are fragile and easy to break. All sedimentary rocks form in three ways:
CLASTIC formed from pieces of preexisting rocks (i.e., Coconino Sandstone).
ORGANIC formed from the accumulated shells or body parts of once living creatures (i.e., Kaibab Limestone).
CHEMICAL formed when minerals precipitate directly out of water and later form rocks (i.e., Travertine, which comes from calcium carbonate).
The last class of rocks are metamorphic. These rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form." The original rock (called a "protolith") is subjected to heat (temperatures greater than 302 to 392 degrees Fahrenheit) and pressure (1500 bars), causing profound physical and/or chemical change. The protolith may be sedimentary rock, igneous rock or another older metamorphic rock.
Metamorphic rocks make up a large part of the Earth's crust and are classified by texture and by chemical and mineral assemblage (known as "metamorphic facies"). They may be formed simply by being deep beneath the planet's surface and subjected to extreme high temperatures, as well as great pressure of the rock layers above it. They can form from tectonic processes such as continental collisions, which cause horizontal pressure, friction and distortion. They are also formed when rock is heated up by the intrusion of hot molten rock called magma from the planet's interior. The study of metamorphic rocks exposed at the surface following erosion and uplift provides geologists with key information about the temperatures and pressures that occur at great depths within the planet's crust. Some examples of metamorphic rocks are gneiss, slate, marble, schist, and quartzite, including this other wide assortment:
There is much more information available on the Web concerning all three types of rock. However, what was just explained provides you with the gist of each rock type. Now see if you can identify what type of rock given each of these three samples:
If you came up with igneous, sedimentary and metamorphic, I'd say you just made my day and you done good! So. . .Amen and W-women, too!
Next, let's consider hold old rocks and formations are, that is, how geologists figure the various geologic ages. Although some of this information has previously been mentioned, here is a more in depth analysis, though still staying with the Geology 101 theme (you know, the K. I. S. concept by keeping it simple).
The Geologic Ages: Next thing to do is take a gander at the names and dates of formations, specifically how the geologic chapters are divided from large to smaller units: Eons, Eras, Periods, Epochs, and Ages. As an example in my other office where I've worked for many years, the Grand Canyon dates from around the Proterozoic Eon (2500 to 570 million years), which is a significant slice of time taken from the Precambrian Era. The canyon's next chapter covers the Paleozoic Era (roughly, 540 to 250 million years) and completes the existing rock record of the Grand Canyon. The Mesozoic and Cenozoic completes the rock chapter eras.
This geologic time chart keeps track of the aging process of the Earth, where the geologic designation, "my," refers to millions of years. A plus or minus factor is common in geological dating of most formations. For example, if the oldest Paleozoic period (i.e., the Cambrian) is dated roughly 540 my, though it might turn out the formation from this period happened anywhere from 535 or 565 my. So, don't be alarmed if other texts state a different age for this period. To a geologist, what's a few million years here and there when assigning dates to rocks and formations?
CENOZOIC Era (Chapter V: Modern Age):
Quaternary Period 2 my
Tertiary Period 65- 2 my
MESOZOIC Era (Chapter IV: Middle Age):
Cretaceous Period 145 - 65 my
Jurassic Period 210 - 145 my
Triassic Period 245 - 210 my
PALEOZOIC Era (Chapter III: Early Age):
Permian Period 285 - 245 my
Pennsylvanian Period 320 - 285 my
Mississippian (or Carboniferous) Period 360 - 320 my
Devonian Period 410 - 360 my
Silurian Period 440 - 410 my
Ordovician Period 505 - 440 my
Cambrian Period 550 - 505 my
PROTEROZOIC Eon (Chapter II; Precambrian): 2,500 to 570 my
ARCHEAN Eon (Chapter I; "In the beginning"): 4,000 to 2,500 my
HADEAN Eon (the time of "Hades"): 4,600 to 4000 my
Geology 101 Rounds house, kindly note: the Paleozoic, Mesozoic, and Cenozoic Eras are also collectively referred to as the Phanerozoic Eon, which entails the much shorter segment of Earthâs recorded geologic record.
Now that you know something about the date of the formations you might as well learn more about the rock materials. This information is especially useful if you plan to hike into the canyon and impress your friends, or even yourself, with the kind of geologic canyon knowledge that most people donât bother to learn.
Dating The Rocks: Sounds like that kind of a date for romance and such but actually it's how rocks are dated. This information I think the community will find interesting, simply from the standpoint geologists really aren't guessing about the extreme dates assigned to various eons, eras and periods. Rather, it's more like a very close approximation. In fact, the science of dating rocks is improving all the time. Pretty soon a geologist will be able to tell you the day of the week the rock was made. (Well, there's always room for showmanship and boasting, right?)
Dendrochronology: This science is the study of dating tree rings that tells us how old the trees are, including valuable information about the climate and environment they grew in. Rocks can also be dated, though less accurately than the science of dendrochronology. Geologists, especially, view the planet's age in billions of years. In fact, the present day figures for the life of the universe is something on the order of 15 billion years old, and the age of the Earth is somewhere to 4.4 billion years. Closer to home here on the Colorado Plateau, the Grand Canyonâs materials are around 2 billion years old. With respect to the metamorphic rocks down in the inner canyon gorge the clock of time begins somewhere between 1.7 and 1.8 billion years ago.
You can expect geologists to imply a give or take clause (i.e., a plus or minus factor of, say, between 25 to 50 million years either way) whenever dates are given for any given rock or formation. The posited dates for eras and periods of rock formations are, nonetheless, fairly reliable in the sense the established evidence used for the method of dating rocks is credible.
Geology relies on several means to determine the age of rocks, formations, and geologic events. The scientific procedures used determine the various chapters, eras, and periods of the Earth's geologic history. There are a few procedures that denote both the main chapters and their secondary or tertiary units. Nevertheless, whatâs ultimately determined is not explicit.
The subsequent information is about the dating techniques geologists are comfortable working with, starting with this proviso. Geochronology applies relative and absolute dating means to determine the age of rocks. Relative dating allows geologists to know if a rock is older or younger, but does not denote an actual age in years. Absolute dating, however, tells us an age in years.
When it comes to geochronology, these are the four relative dating techniques used today:
The Principle of Superposition refers to any undisturbed sequence of sedimentary strata, where the oldest layers will always be at the bottom.
The Principle of Cross-Cutting Relationships applies whenever some geologic feature cuts across or into a rock. Whatever is cut must be younger than what is being cut. Igneous intrusions and faults are two classic examples of this principle.
The strong>Principle of Inclusions applies where one rock has inclusions of an adjacent rock in its makeup. The inclusions must therefore be older than the rock they are embedded inside.
The Principle of Faunal (or Biologic) Succession is where life forms that correlate to the age of the Earth are unique to that, and only that, particular period of time. Thus fossils will be found in the rocks that correlate to a particular age.
With respect to absolute dating techniques its science was discovered in the early twentieth century. Therefore, it's the youngest method used to dates rocks. When radioactivity was discovered, science jumped forward in leaps and bounds. Suddenly, there was an extraordinary revolution in the earth sciences. Radiometric analysis bases its findings on, what is called, the half life of radioactive isotopes. Radioactive isotopes are unstable forms of particular chemical elements, whereby the isotopes are prone to decay, or else turn into something else more stable. To think of it another way, there are clocks in the rocks and radiometric analysis is a reliable means to determine when the clock was set, that is, the age factor.
These are the three common radiometric dating techniques that apply to this science and based on a process called nuclear fission:
1) Potassium-argon dating exploits the decay of potassium-40 isotope into another isotope called argon-40. This application is mainly used for igneous and metamorphic rocks of any age greater than about 1 million years, also sedimentary rocks containing the mineral glauconite (i.e., Bright Angel shale).
2) Rubidium-strontium dating uses the decay of rubidium-87 to strontium-87. This application is used for igneous and metamorphic rocks, but not basic types. Also, sedimentary rocks that contain the mineral illite (i.e., found in the mineral classification of silicates). This method is compatible for rocks more than 30 million years old.
3) Uranium-thorium-lead methods involve radioactive isotopes in uranium. Uranium-235 decays to lead-207 and thorium-232 decays to lead-208. This application is used for igneous intrusions, metamorphic rocks, and sediments containing Zircon. This method is best used for rocks over 100 million years old.
There is also a radiometric dating process used for fission-track dating. This application involves counting fission tracks produced in rocks by splitting nuclei of uranium-238, whose nuclei split at a known and constant rate. The older the rock, the more fission tracks there are. This method is compatible with many igneous and metamorphic rock types.
One other fairly recent method used to determine the age of rocks is called paleomagnetic dating, which uses the Earth's magnetic field in prehistoric times to help date certain rocks. Scientists discovered the Earth is less a huge bar magnet, as was previously thought or assumed, than it is a self-exciting dynamo. Inside the planet radioactive heat keeps streams of molten metal flowing through the outer core. This is the process that generates electric currents, which then produce strong magnetic fields. As the Earth spins around its axis, it naturally directs currents and creates the magnetic poles. Powerful eddies in the currents most likely account for the magnetic pole's slight shift (i.e., the position) from year to year. Hundreds of reversals of polarity have occurred throughout the Earth's history, which are still baffling to scientists to determine just how this happens. Nevertheless, some rocks retain a record of the Earth's polarity at the time those rocks were formed. Hence, the inherent value in paleomagnetism.
See if this chart makes sense to you:
Although determining the age of the rocks is fairly reliable using any of these means at our disposal, no procedure is accurate to the point geologists are able to determine the exact age of rocks. Nevertheless, geologists are able to piece together a sequence of rock strata anywhere on the planet and determine a uniform geologic time scale. In short, geologists have a reliable calendar at this disposal. This calendar therefore divides Earth's history into increments based on appearance, or in some cases, the disappearance of certain fossils, also the existence of particular rock strata.
Before specifying the age of the Grand Canyon's formations, here are two other methods used to date such materials:
1) Carbon-14 dating uses organic material and will only date back to about 50,000 years. This method is less useful to geology, although highly relied on when it comes to archeology and paleo-lithic history. Carbon-14 also decays rather rapidly and is useful for sedimentary rocks laid down (i.e., within the last 50,000 years or so), including dating archeological remains such as bone fragments and charcoal from campfires found in paleo-Indian sites.
2) Biostratigraphy is often used to date sedimentary rock formations, because this rock type is not compatible with radiometric dating. Biostratigraphy uses fossils to recognize the age of a sedimentary unit. Fossils offer valuable aids to relatively dating sedimentary rocks and are often juxtaposed with igneous intrusions and layers that can be dated. These dates are then applied to the fossils found in the rocks, which are then compared to other fossils around the world that are found in similar aged rocks. This is the kind of cross-study diagram that brings life to this science (and, of course, makes sense for those who focus on this particular discipline):
Biostratigraphy therefore involves identifying faunal zones, which are rock strata containing unique assemblages of fossils. Geologists also name each faunal zone after a distinctive species called a 'zone fossil.
However, such fossil correlation should first meet four basic requirements to determine a more accurate date:
• the species was extremely plentiful;
• it spread far and fast (i.e., planktonic organisms);
• it left readily preserved remains;
• then soon died out, which limited the fossil to a few rock layers.
Most of these organisms also lived in the sea and ranged from sizable animals (i.e., macrofossil) and plants to tiny forms (i.e., microfossil). Examples are: trilobites (i.e., three-lobed marine distant relatives of woodlice), ammonoids (i.e., cephalopod mollusks), bivalves (i.e., headless mollusks), and foraminiferans (i.e., small one-celled protozoan organisms).
Bear in mind there are always limits to our knowledge, especially where fossil creatures are concerned. Besides, most soft-bodied organisms left no fossil record. Relatively few land plants and animals were therefore ever fossilized. As it turned out, billions, perhaps trillions, of fossils completely vanished when erosion wore away the rocks that once imprisoned them, or else the fossils were baked or crushed by metamorphic changes. Consider, also, countless fossils are simply too inaccessible to reach and identify.
Well, I'm thinking it's about time for lunch or recess or maybe some of you folks headed down to the mull and do some shopping or people-watching. In other words, let's just say this diary's Geology 101 course has implanted something academic-related in your heads, which for future reference you can always refer back to this overview and background information (i.e., as a continuing data base series stored in my profile). Drop by any time for a review.
Tomorrow's diary will focus entirely on sedimentary rock formations, because we'll be headed for Zion National Park. Given the layout of this setting, it's difficult for some people to comprehend how a seemingly looking gentle stream (actually, the Virgin River) was able to cut down through some 2,000 feet of rock by which all else has followed. It's also the deepest chasm of its kind in North America when considering only one major geologic formation, the Navajo Sandstone, was exposed. Of course, natural and human history will also be part of the tour, not just geology.
As always, thoughtful commentary welcomed. (And, yes, sometimes I think learning science can be fun, at least interesting.)