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- Glossary
- accretion
- A theory of continental growth by the addition of successive geosynclines
to the craton.
- allocthon
- A mass of rock that has been moved a long distance from its place of
origin, commonly by a tectonic process such as overthrusting.
- Archean
- Said of the rocks of the Archaistic.
- Archeozoic
- The earlier part of two great divisions of the Precambrian
time.
- autochthon
- A body of rocks that remains at its site of origin, where it is rooted to
its basement. Rocks of an autochthon may be mildly to considerably deformed.
- basement rock
- Also called basement complex, the assemblage
of undifferentiated rock that underlies the oldest stratified rock in any
region. It is Precambrian in age, crystalline and metamorphosed.
- craton
- A part of the earth's crust that has attained stability and has been
little deformed for a long time. The term is restricted to continents.
- cross-section
- A diagram showing the features transected by a vertical plane, e.g. a
vertical section through an orebody, an anticline, or a fossil.
- diorite
- A group of plutonic rocks intermediate in
composition between acidic and basic, characteristically composed of
hornblende, oligoclase, or andesine, pyroxene, and sometimes a little
quartz.
- footwall
- The mass of rock beneath a fault.
- ductile
- Said of a rock that is able to sustain, under a given set of condition,
5-10% deformation before fracturing or faulting.
- foreland
- A stable area marginal to an orogenic belt, toward which the rocks of the
belt were thrust or overfolded. Generally the foreland is a continental part
of the crust, and is the edge of the craton or
platform area.
- geochronology
- Study of time in relationship to the history of the Earth, esp. by the
absolute age determination and relative dating systems developed for this
purpose.
- geophone
- A seismic detector, placed on or in the ground,
that responds to ground motion at its point of location.
- granite
- Broadly applied, any quartz-bearing plutonic rock
composed entirely of crystals, i.e. having no glassy part.
- greenstone
- A field term for any compact dark-green altered or metamorphosed basic
igneous rock that owes its color to chlorite, actinolite, or epidote.
- Grenville
Orogen
- A name that is widely used for a major plutonic, metamorphic, and
deformational event during the Precambrian which affected a broad province
along the southeastern border of the Canadian Shield.
- Grenville province
- A series of the Precambrian of Canada and the
United States.
- Igneous
Rock
- Igneous rocks played an important role in the
formation of the Adirondacks. Along with sedimentary and metamorphic
rock they comprise the three great rock
classes that encompass all rock originating on earth. Igneous rocks solidified directly
from molten rock. "Rock melt", called magma, originates at
enormous depths beneath the earth's surface and eventually rises through
the earth's crust to cool and become igneous rock. There are two types of igneous rocks, differentiated by
origin, as either volcanic rocks or plutonic rocks.
- Volcanic rock arrives
at the earth's surface in its molten state, which we know as lava.
Thus it arrives, often spectacularly, from the mouths of
volcanoes.
- Plutonic rock also comes from molten rock, but it is molten rock
that solidifies before surfacing. Thus plutonic igneous rock
mostly remains underground relatively near the surface, or it emerges
later mostly through geologic action such as earthquake, shift of
tectonic plates, etc.
- impricate
- Overlapping, as shingles or tiles on a roof.
- klippe
- An isolated rock unit that is an erosional remnant.
- lithosphere
- A layer of strength relative to the underlying asthenosphere. It includes
the crust and part of the upper mantle and is of the order of 100 km in
thickness.
- mafic
- Said of an igneous rock composed chiefly of one or more ferromagnesian,
dark-colored minerals in its mode; also said of those minerals.
- metamorphic rock
- Metamorphic rocks comprise the most complex
group of rocks. Virtually the entire Adirondacks is metamorphic.
As the name suggests,
metamorphic rocks are rocks that have "metamorphosed" or changed
from one kind of rock into another. This change process occurs when preexisting igneous, sedimentary, or earlier metamorphic rocks
are subjected to high pressures and temperatures as well as chemical activity deep
within the earth's crust.
- metamorphism
- The mineralogical, chemical, and structural adjustment of solid rocks to
physical and chemical conditions imposed at depth below the surface zones of
weathering and cementation, whcih differs from the conditions under which
the rocks originated.
- metamorphic grade
- The intensity of metamorphism, measured by the
degree of difference between the parent rock and the metamorphic rock.
- metasedimentary rock
- Metamorphosed sedimentary rock.
- metaplutonic rock
- Metamorphosed igneous plutonic rock (see "plutonic"
below)
- metavolcanic rock
- Metamorphosed igneous volcanic rock (see "volcanic rock" below)
- meteorite
- Any solid object from interplanetary space that has fallen to the earth's
surface without being vaporized during its passage through the atmosphere.
- Mohorovicic discontinuity
- The boundary surface or sharp seismic-velocity discontinuity that
separates the Earth's crust from the subjacent mantle. It marks the level in
the Earth at which the P-wave velocities change abruptly from 6.7-7.2 km/sec
to 7.6-8.6 km/sec on average.
- orogen
- an extensive belt of rocks deformed by orogeny, associated in places with
plutonic and metamorphic rocks such as the Adirondack Mts in northern New
York state.
- orogenic belt
- A linear region that has been subjected to folding and other
deformation during an orogenic cycle. Orogenic belts are mobile belts during
their formative stages, and most of them later became mountain belts by
postorogenic processes.
- orogenic cycle
- A period of geologic activity resulting in the formation of mountains
- orogeny
- The process of mountain making or upheaval.
- para-autochthon
- Said of a rock unit that is intermediate in tectonic
character between autochthon and allocthon.
- parautochthonous
- Said of a rock unit that is intermediate in tectonic character between
autochthonous and allochthonous.
- plutonic
- Pertaining to igneous rocks formed by any process at great depth.
- plate tectonics
- A theory in which the surface of the Earth is made up of a number of rigid
plates moving relative to each other, separating at mid-oceanic ridges where
new ocean-floor is produced and converging at subduction zones, where one
plate descends beneath another.
- platform
- A vast area of undisturbed sedimentary rocks that, together with a shield,
constitutes a craton.
- Proterozoic
- The more recent of two great divisions of the Precambrian.
- Precambrian
- A major division of geologic time, and its corresponding rocks, before the
beginning of the Paleozoic; it is equivalent to about 90% of geologic time.
(see Geologic
Timescale, table), from c.5 billion to 570 million years ago. It is
often divided into the Archeozoic and Proterozoic.
Precambrian rocks are mostly covered over by rock systems of more recent origin, but where
visible they commonly display evidence of having been altered by intense
metamorphism.
This is almost universally the case in the Adirondacks.
Precambrian rocks often occur in shields, which are large areas of
relatively low elevation that form parts of continental masses. One of the
largest exposed areas of Early Precambrian rocks is the Canadian Shield. It
covers most of Greenland, extends over more than half of Canada, and reaches
into the United States as the Superior Highlands and the Adirondack Mts.
The rocks of this region, and of the Early Precambrian as a whole, are
generally granite,
schist,
or gneiss.
The most notable formations are the Keewatin and Coutchiching of Minnesota
and the adjoining part of Canada; the Grenville of Ontario, which, however,
may be Late Precambrian; and the widely distributed Laurentian. The Keewatin
series of rocks is composed chiefly of metamorphosed lava, with some
sediments; the Coutchiching series is chiefly of sedimentary gneisses and
schists. The Grenville limestone, marble, gneiss, and quartzite are
predominantly metamorphosed sediments; the Laurentian gneiss and granite are
probably younger than the other series, having been forced up through the
Grenville as igneous rock. After the appearance of the Laurentian, the
Temiskaming, or Sudburian, sediments were deposited, and a second series of
gneisses and granites, the Algoman, was formed.
By the latter Precambrian, heat dissipated enough to allow the continental
crust to form; crustal rifting, mountain building, and volcanic activity
then dominated, as did sedimentation. The life of the Late Precambrian is
poorly represented by fossils, but a few invertebrates including creatures
resembling jellyfish and worms have been discovered. The best evidence that
there probably were numerous forms of life is the variety and complexity
which suddenly appears in Cambrian fauna. Mineral deposits associated with
Precambrian rocks have yielded most of the world's gold and nickel in
addition to large quantities of copper, silver, radium, and uranium.
- radiometric dating
- Calculating an age in years for geologic materials by measuring the
presence of a radioactive element; based on nuclear decay of naturally
occurring radioactive isotopes.
- Sedimentary Rock
- Sedimentary rock played a primary role in the
formation of the Adirondacks, and is one of the three great
rock classes that encompass all rocks. Sedimentary rocks are
formed at or near the earth's surface by the accumulation of particles in
size varying from submicroscopic to boulder size. Sediments
accumulate, often to a thickness measured in miles. The weight of
the upper beds exerts enormous pressure on the lower beds causing heating,
compaction, and cementing of the beds into rock layers. Over time
and in this manner, mud becomes mudstone and then shale, sand is
consolidated into sandstone, etc. Sedimentary rocks are easy to recognize
due to their layered structure.
- seismic
- Pertaining to an earth vibration, including those that are artificially
induced.
- seismic record
- In geophysical exploration, a photographic or magnetic record of reflected
or refracted seismic waves.
- shear zone
- A tabular zone of rock that as been crushed and brecciated by many
parallel fractures due to shear strain. Such an area is often mineralized by
ore-forming solutions.
- shield
- A vast area of ancient crustal rocks which, together with a platform,
constitutes a craton. The Canadian Shield which constitutes most of
central and eastern Canada, also includes the Adirondacks.
- tectonics
- Simple Explanation: Not too long ago geologists figured out
that the continents are moving... really, really s-l-o-w-l-y. At one
time all or most of them fit nice and tightly together, but now it's a long
swim from North America to Europe. When they separate or crash into
each other something's got to budge! Some of them slid over
others. Even smaller parts of continents are crashing into other
parts, but most of this action, while it is going on today, is not visible
to us. It's taking millions of years to do its work. Anyway, all
that rock creates the heat and pressure that turns igneous and sedimentary
rocks into metamorphic rock. It even changes metamorphic rock into new
kinds of metamorphic rock.
Formal Explanation: A branch of geology dealing with the broad architecture of the outer part
of the earth, that is, the major structural or deformational features and
their relations, origin, and historical evolution. It is closely related to
structural geology, but tectonics generally deals with larger features.
- tonalite
- A group of plutonic rocks having the composition of diorite
but with an appreciable amount of quartz, i.e. between 5 and 20 percent.
- underthrust
- A thrust fault in which the footwall was the
active element.
- volcanic rock
- A type of igneous rock formed of magma that reaches the surface of the
earth before solidifying into rock.
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