Chapter 1: The briefest summary of Geology.
The big picture—
The Earth has been compared to a hard-boiled egg. The eggshell represents the Earth’s Crust, which is hard and brittle and includes all the land and oceans on the surface of the Earth. The crust is as thin as the eggshell; its thickness is about 100 kilometers. Under the crust is the Mantle, the egg white; it is much thicker than the crust. It consists of soft, malleable and even partly molten and easily deformable rock called Magma. The Core of the earth is the yolk, very dense, metallic and hot.
The crust is broken into several Plates, which float on the mantle like closely spaced ice sheets on a spring pond. Heat-driven currents circulate in the relatively fluid-like Mantle. These currents move the crustal plates over the surface of the Earth. The North American Plate includes the North American continent except for western California and the western half of the Atlantic Ocean bed. Because of the Mantle circulation, our plate has been at the South Pole and on the Equator and, for the moment, is in the Northern Hemisphere and moving westward. Western California and Baja are part of the Pacific Plate and are moving north. The earthquakes on the San Andreas Fault system are the movement of these two plates grinding past one another in fits and starts. Someday this chunk of California and Mexico will probably be an island off the coast of Oregon, and later still, perhaps, be re-attached to the mainland in Alaska.
In addition to the sliding, Strike-Slip, type of plate boundary represented by the San Andreas Fault, the other two types of plate boundaries are Divergent or spreading, and Convergent or colliding.
Divergent plate boundaries found in the middle of most of the ocean basins and are the Mid-Ocean Ridges. They are spreading centers where the two sides of the ocean bed move away from each other allowing Magma, molten rock, to rise through the gap to the surface of the ocean floor. This formation of new ocean bed material creates parallel sea bottom mountain chains that circle the Earth. The rate of divergence is about the same as fingernail growth. The Red Sea may someday look like the present Atlantic and the present Atlantic once looked like the Red Sea.
If new crust material is being added at the Mid-Ocean Ridges, someplace else plate material has to be absorbed or compressed since the surface area of the earth can’t get bigger. When plates converge, either one plate is thrust beneath the other, or they collide. The Nazca Plate, the southeastern Pacific Ocean floor, is subducting, sinking, below the western edge of the South American Plate. Subduction causes earthquakes as one plate slides into, against and under the other. As the subducted seabed material is pushed downward, it is heated and melted when it sinks into the mantle. This new molten material then wells up through the crust of, for instance, the South American plate, lifting the plate and, if it reaches the surface, forming volcanoes. The Andes are such volcanoes.
When plates collide, the crust in the impact zone is squeezed in a giant tectonic vice. The colliding plates crush each other, break and fold the rocks of the crust and pile up and crumple the crust to create mountains. The deeper parts of the crust are warmer tend to bend and fold while the colder rocks near the surface are brittle and break. The ongoing collision between the Indian Plate and the southern border of the Asian Plate is why the Himalayas are still growing. The fossils and composition of the rocks atop the Himalayas show that they were originally created on the ocean bottom. The Andes and Himalayas are still rising. The Appalachians, now hundreds of million years old, were once that tall or even taller. All of the big-picture phenomena described above, and more, are elements of the ‘large-scale, architectural deformation’ of the Earth’s crust, or Plate Tectonics.
Geologists can assess the age of a rock with confidence. They measure the relative amounts of elements that undergo radioactive decay in the rock, such as the decay of uranium into lead. By measuring the lead and uranium content of the rock it is possible to compute how much uranium has decayed to lead and how long that took since the time needed for that decay to occur is constant. The oldest rocks show that the Earth is 4.5 billion years old. The fraction of this time span that geologists are most familiar with is the last 570 million years. This interval represents only the most recent 12 percent or so of Earth history (but as short as this may seem, consider that modern humans have only been around for about 100,000 years, or a mere 0.002 percent of Earth history!). The earliest 88 percent of Earth history was a time of life’s emergence and dramatic changes in the global atmosphere and climate. It was a time of wonder, to be sure, but little of its record is found in the immediate region surrounding Dartmouth.
Instead, relatively younger rocks are exposed here. A review of some names for geologic time is needed. The Paleozoic (Greek for ‘ancient life’) Era spanned 570-to-225 million years ago (mya) and is divided into seven geologic Periods of about equal duration. The Cambrian Period is the oldest of these intervals and is marked by the first appearance of animals with readily preserved hard parts, saved as fossils, such as calcium containing shells like clams and oysters and chitinous shells like modern lobsters and insects. Next, in sequence, are the Ordovician and the Silurian Periods, during the later of which land plants and fish first appear in the geologic record. The Devonian, Mississippian, Pennsylvanian and Permian Periods make up the mid and late Paleozoic. Evidence of amphibians first appears in rocks of Devonian age, and vast peat swamp teeming with primitive insects arose during Mississippian and Pennsylvanian times. The peat was subsequently buried and altered to become economic deposits of coal now found around the world.
Rocks underlying the Appalachians and their highland counterparts in Europe and northern Africa indicate that, throughout the Paleozoic Era, a slow but progressive collision took place between these continents. At least three episodes of mountain building occurred, the last of which culminated during the Permian Period and resulted in one large continent, Pangaea, comprising bits of what are now North and South America, Europe, Asia, and Africa. Owing perhaps to the loss of shallow ocean habitat and to global climate change associated with the assembly of Pangaea, life on Earth was all but extinguished at the close of the Paleozoic Era.
The names of the Paleozoic periods represent locations from which rocks and fossils of the respective ages were first identified. Cambria and Devon are in the UK and the Permian Basin is in Russia. The Ordovician and Silurian Periods were named after ancient tribes of Wales and western England.
The Mesozoic (‘middle life’) Era began about 225 mya and ended about 65 mya. It is divided into three periods, from earliest to most recent, the Triassic, Jurassic and Cretaceous. These periods were named by early European geologists, respectively, for i) a ‘third’ division, or Trias, following earlier identified periods, ii) rocks in the Jura Alps, and iii) the chalky rocks typical of Cretaceous (from the Latin creta, meaning chalk) age in France.
During the Mesozoic Era, circulation of the mantle underlying Pangaea apparently reversed direction. The processes of tectonic collision and Appalachian mountain building that characterized the Paleozoic era were also reversed, and Pangaea was rent apart. Continental rifting started along what is now the east coast of North America. These rifts were soon invaded by the ocean and continued to widen as the Atlantic Mid-Ocean Ridge divergence, which continues to widen today. By the end of the Mesozoic, the Atlantic Ocean was well established and North America, Europe and Africa were nearly in their present configuration. The Mesozoic was also the time of the dinosaurs as indicated, for example, by tracks seen in Mesozoic-age rocks of northern Massachusetts. The end of the Mesozoic Era is marked by the dramatic extinction of the dinosaurs and other life forms brought on, many suggest, by one or more huge meteorite impacts, the largest crater of which is thought to be on the Yucatan peninsula.
The Cenozoic (Greek for ‘new life’) Era is divided into the Paleogene and Neogene Periods. Although mammals first appeared in the Cretaceous, they only rose to environmental dominance following the extinction of the ‘terrible lizards’ at the end of the Mesozoic. Also arising during the last 65 my, and preserved in the fossil record, were birds, modern corals, clams and oysters. All these comings and goings are indicated and documented in the fossil record. The Neogene covers the last 2 my and is divided into two geological epochs: the Pleistocene (Greek for ‘most new’) or the Ice Age. Spanning only the last 10,000 years is the Holocene (’entirely new’) Epoch. Some have suggested that we are now entering another epoch in which the global environment is measurably affected by human activities, and therefore would be appropriately called the ‘Anthropocene’ by geologists in a far-off future studying their Earth’s ancient past, should their species survive.
Rocks of the Upper Valley are mostly Paleozoic in age. By analyzing the composition and structure of the rocks and their relationship to one another, a history of progressive plate collisions, and the resulting growth of the young Appalachian Mountains during that era can be seen. There are some rocks of Mesozoic age, however, including the granites underlying Ascutney Mountain and the White Mountains. These rocks are thought to be the once-deeply buried, now-exhumed roots of volcanoes that dominated an eroding Appalachian, Mesozoic landscape. Some of the material eroded from that wider landscape traveled west via ancient rivers, and now lies buried in Mesozoic rocks of the American West.
There are no rocks of Paleogene age in the Dartmouth region, as that was a period of continued wasting of the once proud Appalachians. Debris eroded from the Paleogene landscape now lies buried in the Atlantic Coastal Plain, and on the continental shelf lying offshore of eastern North America. There is widespread and dramatic evidence of the Pleistocene ‘Ice Age’ throughout New England.
Rocks can be distinguished not only by their age, but also by their Mineral Composition. Minerals each have a specific chemical make-up and a specific crystalline structure. That is, rocks are readily categorized by the relative amounts of their constituent minerals, such as pure silicate quartz, silicates rich in aluminum and alkali metals, or feldspars, biotite and muscovite micas, and alumino-silicates rich in iron and magnesium such as hornblende and amphibole. Quartz appears glassy or whitish to the eye, the feldspars light colored (whitish or pinkish), micas translucent dark gray (biotite) to brown or silver (muscovite), and hornblende and amphiboles dark to almost black. Rocks rich in quartz and feldspars are described as felsic; rocks rich in magnesium- and iron-rich alumino-silicates are described as mafic. Continental crust is typically comprised of felsic rocks. In contrast, the mafic composition of oceanic crust reflects its origins of mantle upwelling at mid-ocean ridges.
Rocks are also distinguished by their mode of formation. Sedimentary rocks form from deposits of the eroded products of any other kind of rock, or of biologic fragments such as shells or bits of coral. Eroded debris is laid down by a river on a floodplain, or on a delta in a lake or in a coastal sea, or by open-ocean currents farther from land. Accumulation and re-accumulation of sand, mud, or shell fragments in such settings, over geologic time, results in their progressive burial by later sediment deposits, compaction and cementation of the grains to each other by the heat deeper in the crust and the weight of newer sediments on top of the older. Sedimentary rocks are further distinguished by the size and composition of the original grains, and called, for example, sandstone, mudstone or limestone. It is in sedimentary rocks formed at the Earth’s surface that the record of life is most commonly found in the form of fossils and tracks.
Igneous (or, ‘fire-born’) rocks that solidify from molten rock, or Magma, at depth below the Earth’s surface are referred to as Intrusive igneous rocks. An example of an intrusive igneous rock common in the Dartmouth Region is Granite, rich in quartz and feldspar and therefore typically light in color but with characteristic dark flecks of mica, hornblende and amphibole. Granite is an example of a felsic rock, and its composition is typical of continental crust. Igneous rocks that solidify on the Earth’s surface are Extrusive rocks, or lava rock. The Hawaiian Islands are composed almost entirely of mafic lava rock, or Basalt. The composition of such rocks is representative of oceanic crust, and as a rule such rocks are denser than those of continental crust.
The cooling and solidification of an igneous rock from a molten state involves the growth and interlocking of its mineral grains. The more time a rock takes to cool and solidify, the more time the crystals have to grow larger, and thus the larger grains are found in slowly cooling rocks. Intrusive rocks forming underground are well insulated and thus cool slowly; as a result, large mineral grains develop which are typically visible to the eye. In contrast, extrusive rocks exposed at the Earth’s surface cool and solidify relatively quickly, providing time only for very small component grains to form. Such grains are barely visible to the eye, if at all.
Any existing rock changed in the either composition or arrangement of its mineral grains as the result of exposure to re-heating and/or re-compression without completely melting is called a Metamorphic (‘change in form’) rock. Exposure to metamorphosing conditions is usually associated with mountain building, deep burial, or contact with intrusive magma. Limestone subjected to intense heat and pressure, for example, will change to marble. Sandstone fuses to become a quartzite or, if intensely fused, almost becomes dense glass, metaquartzite. Mudstone or shale is easily parted, or cleaved, into parallel plates. Either one may metamorphose to slate familiar to everyone as tiles or roofing shingles. If shale or slate is subjected to progressively greater temperature and pressure, it will alter to phyllite characterized by a fine surface sheen, and then, with more change, schist, usually characterized by, shiny grains of mica and by having foliations, cleavage planes, that let the rock break into sheets and blocks. Much of the bedrock of the Upper Valley is schist. Ultimately schist may be further changed into gneiss (pronounced ‘nice’) and characterized by high density and hardness as well as by a coarsely granular texture arranged in alternating bands of light (quartz- and feldspar-rich) minerals and dark (hornblende and mica) minerals. Where a rock shows a gradation of metamorphic textures, it is perfectly reasonable to refer to it with a hybrid term. Getting one’s point across is the aim, not unwarranted precision. The expression ‘phyllitic schist’, for example, refers to a rock with a general texture of schist, but which, in places, exhibits a fine sheen typical of a phyllite.
Metamorphic rocks are readily generated under the conditions associated with plate collision, and it is the widespread occurrence of such rocks of Paleozoic age in the Upper Valley, and New England in general, that are one indication of the history of regional upheaval during that era. Much of the fortunate economic history of the region reflects the abundance of slate, marble, granite and gneiss for building materials used around the world. On the other hand, all those rocks make agriculture difficult, but nice walls around the fields.
On the left is a piece of gneiss from a wall in Thetford, VT with red Garnets. Below and right are Kyanite crystals [blue] in schist in situ in Thetford.
Sedimentary rocks are deposited in layers, which are almost completely horizontal, and spread laterally, with younger layers superimposed on older layers. However, the same tectonic conditions leading to metamorphism can also result in the structural rearrangement of those layers. The extent and complexity of the altered rock reflects the intensity and number of deforming episodes. As result, rock layers in metamorphic terrain such as the Dartmouth Region are likely to have been folded and refolded, twisted, overturned, cleaved and fractured. Teasing out the exact sequence of deformational events in such tortured rocks, let alone figuring out which layer was on top when the rock was created, can be daunting indeed.
One more term in rock nomenclature is helpful, and is related to an invaluable tool used by geologists—the geologic map. A geologic map depicts the distribution, orientation, location and type of different rocks in a region on a topographic map. A body of bedrock of the same type with the same origin and spread continuously over a wide region is referred to as a Geologic Formation. A formation is the basic operational unit recognized by geologists in the construction and interpretation of geologic maps. A geologic formation is by convention named for the location or landmark where it was first described, or where it bests exhibits its distinguishing properties in outcrop.
The Gile Mountain Formation, for instance, is widely distributed in eastern Vermont and is readily seen in exposures along the hike to the Gile Mountain Fire Tower in Norwich as well as many of the road cuts on I-95. This phyllitic schist, by virtue of its mineral composition, extremely scanty fossil content and position relative to other rocks formations located above and below it, is thought to have been derived from mudstones and sandstones which were originally deposited in tropical seawaters approximately 500 mya. Those sedimentary deposits were subsequently caught up and deeply buried in the various Paleozoic episodes of plate collision, mountain building and associated metamorphism. Since the late Paleozoic, processes of weathering and erosion have progressively exhumed the Gile Mountain Formation, resulting in the exposures we see today.
Weathering, Erosion and Isostasy—
The Appalachians were once like the Himalayas, and the Himalayas will someday be like the Appalachians are now. Rocks are weathered or broken down in place into progressively smaller fragments and ultimately into soil, by natural processes, which are chemical (e.g., incorporation of water into the mineral structure, or hydration), mechanical (e.g., ice wedging), and bio-mechanical (e.g., root growth and animal burrowing). This weathered debris and soil are eroded from hill slopes by wind or the gravity-driven runoff of rain and snowmelt into creeks and streams, and ultimately, by rivers to the sea. Rocks and minerals weather at different rates. Quartz is the mineral most resistant to weathering. In the rocks of the Gile Mountain Formation the quartz stands out in higher relief than the gray mica and feldspar material that are weathered and eroded more easily. Among rocks, limestone is easily eroded and granite and gneiss are quite resistant.
Walking along a stream or ocean beach, we are attracted to the smooth round stones we see there and we wonder how they got that way. The more angular a rock’s surface is, the more surface area it presents for erosion. The corner of a square has three sides and three edges exposed to the eroding media while a sphere shows the least surface area for any given mass of material. The viscosity of the eroding medium is also important. Wind and wind blown sand are the most effective at rounding rocks. Water and water borne sand are next most. Rocks carried in glacial ice or mudflows are less rounded.
Geologists reckon that, on average, about 2x1010 metric tons of sediment are delivered by the world’s rivers to the ocean each year. This amounts to the removal, on average, of about 5 cm (2 in) of rock every 1000 years from the Earth’s landmasses to the ocean bed. Rates of erosion are notably faster in tectonically active mountain belts, such as the Himalayas, and notably slower in tectonically inactive regions like the modern, subdued Appalachians. Nevertheless, carrying out the math to a logical conclusion is instructive: 5 cm per 1000 years is equivalent to 50 m per million years, or 5 km per 100 million years. Based on this average rate of erosion, the rocks we see in the Dartmouth Region today were likely to have been exhumed from depths of 10 km, or more, since the opening of the Atlantic Ocean 200 mya. Thus, the ancestral Appalachian Mountains at the end of the Paleozoic were undoubtedly Himalayan in scale, with elevations of perhaps 10 km (6 miles) or more. How does rock exhumation take place, and why, with the loss of all this rock to erosion, aren’t we living on a vast plain at sea level?
Isostasy is a useful concept to understand the consequences of erosion, and other Earth surface processes. Consider the following: all crustal plates float on the mantle like ships in the sea. When a container ship, a sea-going mountain, reaches port and its containers are unloaded, it rides higher in the water. Similarly, landmasses bearing the tallest mountains ride lowest in the mantle and have the deepest crustal roots. As material is removed from the land surface by weathering and erosion, the underlying crust rises like a partially unloaded ship and rides higher in the mantle.
So, as erosion removes material from the surface of a landmass, that landmass floats higher bringing deeply buried rocks closer to the surface. Underlying rock is uplifted, uncovered by more erosion and gradually exposed to view. Different rates of weathering and erosion are determined by differences in rock and mineral resistance to erosion and account for the carving of river valleys and persistence of hills and mountains.
Consider another example of Isostasy. When a continent-covering ice sheet melts and recedes, as happened at the end of the Pleistocene in North America and northern Eurasia, the underlying land mass rises with the loss of the weight of the ice. This rebound is not instantaneous, however, and proceeds only as fast as slow-flowing mantle can move in beneath the rising crust. Scandinavia, for example, is still rebounding by a few millimeters per year from the retreat of the Fenno-Scandinavian ice sheet some 10,000 years ago. In New England, the rebound from Pleistocene glacial retreat is now all but complete. Initially, however, rebound here was so slow that it was outstripped by rapid sea level rise. The seas were replenished by the return of vast volumes of meltwater shed by retreating ice sheets. Coastal areas of Maine, the St Lawrence River and Lake Champlain were inundated by seawater, becoming arms of the ocean. They re-emerged and filled with fresh water when to land rose high enough above sea level to dump out the seawater. Marine fossils are present in deposits of sand and mud found in these areas.
In this light, also consider the following: The difference in density between continental crust and oceanic crust, noted in an earlier section, also explains in part why oceans are oceans, and continents are continents. Denser oceanic crust rides relatively lower in the mantle and thus collects the ocean water. Less dense, and thus relatively higher-riding continents stand proudly above sea level.
So make no mistake--the rock and the landscape we see around us today is not static. It is being slowly exhumed, weathered and eroded at a rate of a few cm per 1000 years. This is but one leg of the Rock Cycle: weathering, erosion, transport, deposition and burial in a floodplain, delta, or at sea, then creation of new sedimentary rock by cementation and fusion, followed by tectonic transport to a colliding plate boundary and there involved in yet another episode of metamorphosis and mountain building or, alternatively, to be subducted into the mantle, melted, and returned to the crust as igneous rock, only again to be exhumed, and eroded.
As noted in the late 1700’s by essayist John Playfair while recounting the observations and ideas of his friend James Hutton, medical doctor, gentleman farmer, highly-regarded member of the Scottish Enlightenment, founder of modern geology and all around hero to many…in this cycle, there is no vestige of a beginning, no prospect of an end. There is only the breathtaking abyss of geologic time. Time enough, perhaps, to go for a walk, and to wonder.
General References [1-4] and Further Reading [5-6]:
1. Sam Boggs, Jr. Principles of Sedimentology and Stratigraphy, Fourth Edition. Pearson Prentice Hall, Upper Saddle River, New Jersey 07458, 2006.
2. Jon P. Davidson, Walter E. Reed, Paul M. Davis. Exploring Earth. Prentice Hall, Upper Saddle River, New Jersey 07458, 1997.
3. George H. Davis, Stephen J. Reynolds. Structural Geology, Second Edition. John Wiley & Sons, New York, NY, 1996.
4. Donald R. Prothero, Robert H. Dott, Jr. Evolution of the Earth, Seventh Edition. McGraw-Hill, New York, NY 10020, 2004.
5. Bradford B. Van Diver. Roadside Geology of Vermont and New Hampshire. Mountain Press Publishing Company, Missoula, MT, 1987.
6. Chet Raymo, Maureen E. Raymo. Written in Stone, a Geologic History of the Northeastern United States. The Globe Pequot Press, Chester, Connecticut, 1989.