The oldest Precambrian rocks shown here are about 2.6 billion years old, meaning that they formed during the Archean Era. Even older rocks are exposed in Canada; the Superior Upland in Minnesota and Wisconsin is a younger part of this ancient "core" of North America, called the Canadian Shield. Other ancient rocks are exposed in various "windows" into the past across the country, like the Adirondacks in New York and the Llano Uplift in Texas.
Text adapted from Chapter 4 of
"The Geology of New York: A Simplified Account"
by the NYS Geological Survey
and reprinted from
The Adirondack Mountains are young, but these young mountains are made from old rocks. How do we explain this seeming contradiction? First, we try to answer many other questions.
What kinds of rocks do we find in the Adirondacks?
Under what conditions were they formed?
How old are they? How have they been deformed?
The answers to these questions give us clues to the geologic history of the Adirondacks.
The Big Picture - The Adirondack Dome
The Adirondack Mountains make up a roughly circular region about 120 miles in diameter. The region is divided into two sub-regions, the Central Highlands (we call them the High Peaks) and the Northwest Lowlands. They are separated by a narrow belt of intensely deformed rocks geologists call the Carthage-Colton Mylonite Zone,.
The metamorphic bedrock in the Highlands resists erosion well. It was left towering over the rest of the countryside when the sedimentary rocks that once covered it were worn away. The highest elevations are found in the High Peaks area of the Central Highlands; there, numerous summits rise above 4000 feet. The highest peak, Mount Marcy, is 5344 feet. high. Elevations fall off rapidly north and east of the High Peaks and more gradually to the south and west.
The Adirondack region was once flat and was covered by the same sedimentary layers that now surround it. However, in relatively recent geologic time, the Adirondack region was uplifted, forming a dome. During uplift, erosion removed most of the sedimentary layers. This erosion eventually created a "window" (openings) through the sedimentary rocks that permits us to see the much older basement rocks beneath. The Adirondack basement extends well into Canada at the surface along a narrow zone called the Frontenac Arch. The Frontenac Arch crosses the St. Lawrence River at the Thousand Islands.
Seen from space, the Adirondack Highlands look cracked and wrinkled. We can see three prominent types of features on satellite images:
Long, straight valleys. Those that run north-northeast are the most prominent. Throughout the Adirondacks, these valleys contain streams and lakes. Many of the larger Adirondack lakes, such as Lake George, Schroon Lake, Indian Lake, and Long Lake, follow this north-northeast trend. In the High Peaks region, these valleys divide the area into a number of long, straight mountain ranges: Great Range, the Colvin Range, and the MacIntyre Range, are the most obvious of these. Their respective valleys have formed along faults and fracture zones where the broken rocks are less resistant to erosion.
Gently curved ridges and valleys. These ridges and valleys are usually more subtle than the deep, fault-related ones. They are most prominent in the central and southern Adirondacks, where they make an east-west arc. They follow the layering in folded rocks. Harder, more erosion-resistant rocks (such as granitic gneiss) form the ridges, while softer layers (like marble) form the valleys.
Radial drainage patterns. Streams and rivers in general flow out from the central and northeastern parts of the Adirondack dome toward its edge. We can see this pattern most clearly in the outer parts of the dome; elsewhere, the rivers tend to follow the dominant north-northeast valleys.
ADIRONDACK ROCKS AND THEIR METAMORPHISM
"The rocks we now walk on in the Adirondacks once lay
beneath nearly a full thickness of continental crust."
Almost all of the rocks in the Adirondack region are metamorphic rocks. Three general types are present. Meta-sedimentary rocks, as the name suggests, were formed by metamorphism of sedimentary rocks. Meta-volcanic rocks are metamorphosed lavas and volcanic ash. Meta-plutonic rocks were formed by metamorphism of igneous rocks that cooled and crystallized from magma (molten rock) deep in the earth's crust. Each kind of rock is made up of a specific collection of minerals, called a mineral assemblage. Before describing the main rock types that make up the Adirondacks, it will be useful to discuss the conditions under which they were metamorphosed.
Rock becomes metamorphosed when it is subjected to elevated pressures and temperatures. In a continent-to-continent collision the movement of tectonic plates against one another cause mountain-building forces that bury rock many kilometers beneath the earth's surface over millions of years. The weight of the overlying rock subjects the buried rock to enormous pressures. The internal heat of the earth gradually heats the buried rock to extremely high temperatures. Under these conditions, the minerals in the buried rock react chemically with each other to form new mineral assemblages.
The original composition of the rock, together with the temperature and pressure to which it is subjected, determines what kind of metamorphic rock will form. It is difficult to reconstruct what conditions were like during metamorphism in the Adirondacks because metamorphism takes place deep below the surface of the earth. However, we can use laboratory experiments to estimate the pressures and temperatures that produced the rocks we see at the surface today.
When we compare Adirondack rocks with experimental results, we conclude that rocks in the Central Highlands were formed under rather extreme conditions-at temperatures of 750-800'C and at pressures 7000 to 8000 times the pressure of air at sea level. These pressures are equivalent to those at depths of 25 to 30 km below the earth's surface. Conditions affecting the rocks of the Northwest Lowlands were a little less extreme. Temperatures were about 600-750'C, and burial depths were about 20-25 km. When we learn how deeply they were buried, we realize that the rocks we now walk on in the Adirondacks once lay beneath nearly a full thickness of continental crust.
To reconstruct the geologic history of the Adirondack region, we need to figure out what the rocks were like before they were metamorphosed. The first question is: Were they sedimentary or igneous? For some rocks we need only look at the mineral makeup. For example, we know that the metamorphic rock quartzite must have originally been a quartz sandstone (a sedimentary rock), because both rock types are made almost entirely of the mineral quartz and there are no igneous rocks of that composition. Similarly, metanorthosite has the same mineral composition (chiefly plagioclase feldspar) as the igneous rock anorthosite, which is unlike any known sedimentary rocks. Certain sedimentary or igneous features in the original rock may have survived metamorphism. These features are also clues to what the rock was before metamorphism; some examples are shapes of mineral grains or the presence of sedimentary bedding. Some metanorthosites have mineral grain shapes that show the original rock crystallized from magma. For other Adirondack rocks, the nature of the original rock is much less clear. We do not yet know, for instance, whether some granitic gneisses are metaplutonic, metavolcanic, or metasedimentary.
Metasedimentary and Metavolcanic Rocks
Metasedimentary and metavolcanic rocks make up well over 80 percent of the exposed bedrock in the Northwest Lowlands. They are less abundant in the Central Highlands (High Peaks), where most of the rocks exposed at the surface are metaplutonic. They include both marbles (metamorphosed limestones) and quartzite, as well as various kinds of gneisses that are the end products of metamorphism of shales and sandstones.
What was the environment like when the original sedimentary and volcanic rocks were formed? An exciting discovery in recent years gives us some help in finding an answer. In the early 1980s, fossils of dome-like, laminated structures called stromatolites were discovered in the Adirondacks. They were found in marbles near Balmat. This find was very surprising, because the rock containing the stromatolites had been metamorphosed and deformed. Usually, intense deformation and recrystallization destroy any fossils that are present. In fact, stromatolites are the only fossils ever found in the metamorphic rocks of the Adirondacks. Both ancient and modern stromatolites are formed by cyanobacteria (blue-green algae) that live in shallow, well-lit water. We conclude from the presence of stromatolites in Adirondack marbles that these rocks were originally deposited in shallow marine waters.
The metasedimentary and metavolcanic rocks of the Adirondacks record a complex geologic history. These rocks were originally horizontal layers. Now, the layering has been complexly folded and faulted, and in places disrupted by magma.
Three major types of metaplutonic rocks are found in the Adirondacks: granitic gneiss, metanorthosite, and olivine metagabbro.
Granitic gneiss. The most common metaplutonic rock in the Adirondacks is granitic gneiss. Geologists are still arguing about the origin of these rocks. However, much of the granitic gneiss in the Central Highlands appears to be metamorphosed plutonic rock, so we have put it in the metaplutonic category. This rock is composed largely of alkali feldspar and quartz, with lesser amounts of other minerals.
Metanorthosite. Metanorthosite forms several large bodies in the Central Highlands. It is an unusual rock, composed chiefly of a single mineral type, plagioclase feldspar. It is similar to the rock that makes up the highlands (bright areas) of the Moon. The largest metanorthosite mass in the Adirondacks, called the Marcy Massif, underlies roughly 1500 km2, including most of the High Peaks area. Near its southern border, we find ore deposits composed of heavy, black iron and titanium oxides. One such deposit, at Tahawus, has been mined for both titanium and iron. There are also several smaller, dome-shaped masses of metanorthosite in the northeastern and south-central Adirondacks. A number of even smaller bodies are scattered throughout the region.
The metanorthosite originated as anorthosite magma in the earth's mantle and lower crust. The magma rose into shallower levels of the crust, where it cooled and hardened. Later metamorphism converted the anorthosite to metanorthosite.
How do we know that the metanorthosite of the Adirondacks was originally igneous anorthosite? In the less deformed parts of the metanorthosite bodies, we find textures typical of igneous rocks. These textures survived metamorphism. In addition, we find blocks of older rocks in the metanorthosite. These blocks were broken off the surrounding rock and mixed in with the magma as it forced its way up through the crust.
Olivine metagabbro. Olivine metagabbro is less abundant than granitic gneiss and metanorthosite, but numerous masses of this rock are scattered throughout the eastern and southeastern Adirondacks. Like metanorthosite, olivine metagabbro commonly has textures that show its igneous origin. It also contains features called coronas, which show incomplete chemical reactions between minerals. These reactions happened during metamorphism, but so slowly that even in the millions of years before the rock cooled the original minerals were not wholly consumed. Near the edges of some olivine metagabbro bodies, we find spectacular large red garnets that also formed during metamorphism. At the Barton Mine on Gore Mountain near North Creek, garnets up to one meter in diameter have been found.
DEFORMATION OF ADIRONDACK ROCKS
The rocks of the Adirondack region have been complexly deformed. Deformation refers to folding, faulting, and other processes that change the shape of rock bodies.
We find two main kinds of deformation in the Adirondack rocks: ductile deformation and brittle deformation. Brittle deformation occurs in rocks that are at shallow depths or at the surface, where they are cold; here they deform by breaking. Ductile deformation can occur in rocks that are deeply buried and hot enough to bend or flow without breaking.
One of the most obvious kinds of ductile deformation in the Adirondacks is folding. We find folds of all sizes in the rocks of the region. The complex patterns on the geologic map result in part from large, irregular folds. Some of these folds in the southern Adirondacks are tens of kilometers across. Major folds in the northwest Adirondacks generally run northeast. Those in the southern half of the Adirondacks make an east-west arc.
We also see folds in individual rock exposures. We find folded rocks throughout the Adirondacks; some of them appear to have been folded several times, Clearly, great geologic forces were at work to make such folds. In the folded rocks, we often observe layer-like arrangements of minerals called foliation, and parallel streaks of minerals called lineation. Foliation and lineation give us clues about the directions in which the folding forces acted.
Rocks at high temperatures deep within the crust may also deform by ductile shear. Ductile shear happens when one block of rock slides past another; the rock between the blocks deforms and stretches like chewing gum or hot plastic, rather than breaking to form a fault as it would at lower temperatures. This movement creates a ductile shear zone, a relatively narrow, intensely deformed area between the two blocks. The rock in such ductile shear zones is greatly stretched and flattened and commonly shows strong foliation and lineation.
As movement occurs in a ductile shear zone, the minerals in the rock recrystallize. This process reduces the size of the mineral grains, sometimes drastically. The result is a fine-grained rock called a mylonite with strong foliation and lineation. From the shapes of the mineral grains in a mylonite, we can sometimes tell which way the blocks of rock moved along the shear zone.
Mylonites are common throughout the Adirondacks, but are most abundant in the southeastern Adirondacks and along the Carthage-Colton Mylonite Zone, which separates the Central Highlands and the Northwest Lowlands. They range in width from a few centimeters to several kilometers. In the mylonites of the Carthage-Colton Mylonite Zone the shapes of the mineral grains tell us that the Lowlands probably slid along this zone northwestward and down relative to the Central Highlands. We can't tell how far the Lowlands moved, but it may have been a considerable distance. In other parts of the world, blocks of crust have moved tens or even hundreds of kilometers along similar ductile shear zones.
Brittle deformation refers to the breaking of rock, in contrast to the flowing of rock that accompanies ductile deformation. In the Adirondacks, we find the most prominent examples of brittle deformation in the long, straight valleys that run north-northeast across the eastern half of the region.
Some of these valleys, such as those occupied by Lake George and Schroon Lake, have steep faults on either side. The central block has moved down at least 400 m along these faults. Such down-dropped blocks of crust are called grabens. In the southern Adirondacks, we find several grabens that contain flat-lying sedimentary rocks of Cambrian and Ordovician age. The most recent fault movement must have happened after deposition of the Cambrian and Ordovician rocks cut by the faults-that is, sometime after Middle Ordovician time. We think that some of these faults originally formed in the Late Proterozoic and were reactivated in Middle Ordovician time. We can see small faults in many outcrops in the Adirondacks. Some faults contain shattered rocks known as fault breccias.
Other straight valleys are the result of erosion along zones of intensely broken rock called fracture zones. Valleys form along such zones because the broken rock erodes more rapidly than the surrounding rock. Fracture zones differ from faults: the blocks on opposite sides of the zone have not moved relative to each other, but the rock has simply shattered in place. In addition to the faults and fracture zones that run north-northeast, we find many others that run east-northeast, east, and southeast.
Joints, another type of brittle deformation, are found in every Adirondack rock exposure. These breaks look like neat slices through the rock. A joint is different from a fault because there has not been any movement along a joint.
How Adirondack Deformation Happened
What caused the deformation of the Adirondack rocks? Immense tectonic forces compressed the entire region now known as the Grenville Province. This compression, or squeezing, of the crust was accompanied by folding of the rock layers. As the crust was squeezed, it thickened and shortened in the same way that a cube of soft caramel candy shortens and thickens when you push on its sides. In addition to the folding, large blocks of crust moved along ductile shear zones and were stacked one on top of the other. As the crust thickened, the lower parts were buried deeper beneath the surface. There, they were subjected to high pressures created by the weight of the overlying rock. These pressures, along with heat rising from the mantle and additional heat from intrusions of magma, thoroughly metamorphosed the rocks.
Where did these forces come from? Our best guess is that they resulted from a collision between two continents. This collision began the complicated sequence of events we call the Grenville Orogeny.
A SUMMARY OF THE GEOLOGIC HISTORY
We know enough about the geology of the Adirondack region to begin to piece together a history of the Middle and Late Proterozoic there. But there are many things we still don't know. We have to make some educated guesses at nearly every stage of our reconstruction.
We find the age of igneous rocks by radiometric dating. However, this task is not simple. Sometimes intense metamorphism, like that which occurred in the Adirondacks, can "reset" some or all of the radioactive "clocks" in the rock. If this resetting happens, radiometric dating will tell us when the rock was metamorphosed. It will not give us the age of the original igneous rock. Radiometric dating has been done on many Adirondack rocks, but we have to be very careful in interpreting the results.
We have found that almost all rocks in the Adirondacks are of Middle Proterozoic age. Radiometric dating of the meta-volcanic rocks suggests that the oldest ones may be as much as 1.3 billion years old. We think the meta-sedimentary rocks were deposited as sedimentary rocks beginning at about the same time.
The original sedimentary rocks of the Adirondack basement---sandstone, limestone, dolostone, and shale were probably deposited in a shallow inland sea. Although they were deposited most likely no more than 1.3 billion years ago, some contain grains of the mineral zircon that are about 2.7 billion years old. This fact tells us that the sediments that formed these rocks were eroded from a much older landmass. This landmass was probably the Superior Province, located to the west and north of the Grenville Province. Meta-volcanic rocks that occur with the meta-sedimentary rocks indicate that volcanoes were present in the region at that time.
Most of the meta-plutonic rocks of the Adirondack Highlands are probably between 1.15 and 1.1 billion years old. Shortly before the Grenville Orogeny, large volumes of magma may have risen from the mantle into the crust. Heat from the magma partially melted the surrounding crust, producing molten rock of different compositions. The various kinds of molten rock, such as anorthosite and granite, tended to rise through the crust because they were less dense than the surrounding rocks. Some continued to rise even after they partly cooled and solidified, eventually forming balloon-like domes or spreading out as thick sheets within the crust.
At some point during the Middle Proterozoic, the rocks we now find at the surface in the Adirondack region were as much as 30 km below the surface. Remember that some of these rocks began their existence as sedimentary rocks at the surface, which means that they must have been pushed down that far. For them to be buried so deeply, the continental crust in the region had to be nearly twice as thick as normal continental crust. A modern example of double-thick crust is the Tibetan Plateau just north of the Himalayan Mountains. As India continues to collide with Asia, the collision is creating the Himalayas, the world's highest, mountains, along the collision zone, and a double thickness of continental crust under them and to the north. This double-thick crust makes Tibet the world's highest plateau region, with an average elevation of 5 km above sea level. Far below the surface, the rocks are subjected to very high temperatures and pressures.
The Grenville Orogeny, which may have been caused by a similar collision, buried the Adirondack rocks. It is difficult to say when the orogeny began. It was underway at least 1.1 billion years ago. The deformation and metamorphism appear to have peaked between 1.1 and 1.05 billion years ago. Some additional plutonic rocks may have been formed at the time, either by partial melting of the crust or by injection of new magma from below. By about 900 million years ago, the rocks had cooled again. We still don't know the details of these complex events.
Like the collision of India and Asia, the Grenville Orogeny built huge mountain ranges along the collision zone and a high plateau behind it. Over the next several hundred million years, erosion coupled with uplift levelled the mountains and stripped more than 25 km of rock from the plateau. Between 650 and 600 million years ago, the crust of eastern proto-North America was stretched and was broken by major faults. These faults are the ones that run north-northeast throughout the eastern Adirondacks. There are also many smaller faults running east-northeast, east, and southeast. Igneous rocks called diabase dikes show that molten rock was injected and hardened in narrow vertical zones, often along faults. Radiometric dating tells us that these dikes were formed about 600 million years ago.
Beginning in the Late Cambrian, the Adirondack region was gradually submerged beneath shallow seas. Sandstones with trilobite fossils were deposited over much of the region. The contact between these younger rocks. and the underlying basement is visible in several places near the outer edge of the present Adirondack dome. Sediments continued to accumulate across much of the eastern United States (with some interruptions) through the Pennsylvanian Period, but no rocks younger than Middle Ordovician remain in northeastern New York.
Later erosion in the Adirondack region stripped off nearly all of the Paleozoic sedimentary rocks. However, there are still traces of Cambrian and Ordovician rocks within the Adirondacks; this fact proves that they once covered the region. In the southern Adirondacks, we find grabens that contain Cambrian and Ordovician rocks formed in these seas. Because these blocks dropped down lower than the surrounding landscape, they were saved from erosion when the other Paleozoic layers were worn off during regional uplift. The Lower Paleozoic rocks that originally covered the region still encircle the Adirondack dome.
From the Middle Ordovician into the Tertiary Period, there is no evidence of any tectonic activity in the Adirondacks, despite three more mountain-building events that affected New England and southeastern New York. The region that is now the Adirondack Mountains was flat, just like the rest of the region west of the Appalachian Mountains. In Jurassic or Cretaceous time, some small dikes intruded in the eastern Adirondacks and Vermont.
Sometime in the Tertiary Period, the Adirondacks began to rise. Why? Our best guess is that a hot spot formed under the region near the base of the crust. This hot spot heated the surrounding material at depth, causing it to expand. This expansion raised the crust above, causing the present dome-shaped uplift. In the early 1980s, re-measurement of the elevations of old surveyors' bench marks showed that the Adirondacks may be rising at the astonishing (to a geologist!) rate of 2 to 3 mm per year. The mountains are growing about 30 times as fast as erosion is wearing them away. We suspect, however, that the present rapid uplift is a temporary spurt, and the average rate may be much less.
After the Adirondack dome began to rise, stream erosion (and much later glacial erosion) started wearing away the softer rocks and the fractured zones. Eventually, erosion carved the region into the separate mountain ranges we see today. Glacial ice entered the region about 1.6 million years ago.
Ever wonder how the Adirondacks came to be?
Plainly stated, today's Adirondack Mountains are an arrangement of rock, soils, animal life, and vegetation that have resulted from more than a billion years of geologic activity and weather coupled with the evolution of plant and animal life.
Imagine this. It begins with a tectonic plate, a continent-sized plate of rock cooled from magma that, despite its size, is mobile. It forms the great land mass of North America during the Precambrian age. Half a billion years later, a hot spot deep down in this plate begins to thrust upwards like a boil and forms a dome that is the raw material of what will become the mountains themselves. Later, but still a thousand millennia before the Vikings ever spy Greenland, the processes of weather and further geologic activity begin to create distinct mountains. Sand and gravel are created by rain pounding on rock. Much later a deer dies, his carcass adding organic matter which helps to create soil made up of minerals and organic matter in some unnamed valley between unnamed mountains.
Such unconnected incidents make up the fabric of the Adirondack's continuous evolution. This story extends backwards through all time and it is still going on. The news is, a unique combination of forces have been at work that have overturned past thinking on the Adirondacks. In the 1950's they were thought to be the oldest mountains on earth;, now they are known to be among the newest. But forces still in motion seem to defy logic and are not yet fully understood. One thing we do know, it all started in the Precambrian Age more than a billion years ago. The chart below shows geologic time. The story which follows starts at the bottom of this chart in the Precambrian Era, which is just called "the Precambrian" by geologists. While geology keeps "happening", it is at the beginning of the Precambrian 600 million years ago that this story begins.
Really, Really Old Rocks!
You'll see on this map that the oldest rocks in the US cover only a small part of the nation. The Adirondacks are among these. As the colors show, Adirondack rocks were created in the Archaean and Proterozoic Eras. Some of the rock in the Lake Champlain basin are even older dating back to Hadean Time. The word "Hadean" comes from Hades or Hell which signifies that, as is suggested in the bottom line on the table just above this paragraph, they were formed when the Earth began. That's old!
[Map of U.S. Precambrian rocks - locations]
The rocks highlighted here are the oldest in the United States. They formed in Precambrian time, between 4.5 billion and 560 million years ago. Precambrian time is divided into Eras: