Sedimentary rocks cover much of the Earth’s surface, but they are often hidden by a thin v
A.concealed
B.eroded
C.protected
D.softened
A.concealed
B.eroded
C.protected
D.softened
Sedimentary rocks are composed largely of minute fragments derived from the disintegration of existing rocks and in some instances from the remains of animals. As sediments are transported, individual fragments are assorted according to size. Distinct layers of such sediments as gravels, sand, and clay build up, as they are deposited by water and occasionally wind. These sediments vary in size with the material and the power of the eroding agent. Sedimentary materials are laid clown in layers called strata.
When sediments harden into sedimentary rocks, the names applied to them change to indicate the change in physical state. Thus, small stones and gravel cemented together are known as conglomerates; cemented sand becomes sandstone; and hardened clay becomes shale. In addition to these, other sedimentary rocks such as limestone frequently result from the deposition of dissolved material. The ingredient parts are normally precipitated by organic substances, such as shells of clams or hard skeletons of other marine life.
Both igneous and sedimentary rocks may be changed by pressure, heat, solution, or cementing action. When individual grains from existing rocks tend to deform. and interlock, they are called metamorphic rocks. For example, granite, an igneous rock, may be metamorphosed into a gneiss or a schist. Limestone, a sedimentary rock, when subjected to heat and pressure may become marble, a metamorphic rock. Shale under pressure becomes slate.
Which one of the following is a metamorphic rock?
A.Granite.
B.Shale.
C.Slate.
D.Limestone.
Hornfelsis a metamorphic rock formed when magma(molten rock) heats sedimentary rocks on Earth’s sur-face. According to Figure 1, hornfels is most likely amember of which of the following facies?
A.Facies A
B.Facies C
C.Facies E
D.Facies G
A.increases in the carbon dioxide content of sedimentary rocks
B.the formation of bodies of water
C.decreases in the level of nitrogen
D.the formation of clouds
For example, an atom of radioactive rubidium decays to form. an atom of strontium (another element) by converting a neutron in its nucleus to a proton and releasing an electron, generating energy in the process. The radiogenic daughter products of the decay-in this case strontium atoms--diffuse away and are lost above a certain very high temperature. So by measuring the exact proportions of rubidium and strontium atoms that are present in a mineral, researchers can work out how long it has been since the mineral cooled below that critical "blocking" temperature. The main problems with this dating method are the difficulty in finding minerals containing rubidium, the accuracy with which the proportions of rubidium and strontium are measured, and the fact that the method gives only the date when the mineral last cooled below the blocking temperature. Because the blocking temperature is very high, the method is used, mainly for recrystallized (igneous or metamorphic) rocks, not for sediments--rubidium-bearing minerals in sediments simply record the age of cooling of the rocks which were eroded to form. the sediments, not the age of deposition of the sediments themselves.
Potassium decays to form. (a gas) which is sometimes lost from its host mineral by escaping through pores. Although potassium-argon dating is therefore rather unreliable, it can sometimes be useful in dating sedimentary rocks because potassium is common in some minerals which form. in sediments at low temperatures. Assuming no argon has escaped, the potassium-argon date records the age of the sediments themselves.
Carbon dating is mainly used in archaeology. Most carbon atoms (carbon-12) are stable and do not change over time. However, cosmic radiation bombarding the upper atmospheres constantly interacting with nitrogen in the atmosphere to create an unstable form. of carbon, carbon-14.
What is the common feature of rubidium, potassium and carbon?
A.They can be made into clocks.
B.They are rich in content.
C.Their decay is slow but regular.
D.The products of their decay are the same.
Milankovitch proposed in the early twentieth century
that the ice ages were caused by variations in the Earth’s
orbit around the Sun. For sometime this theory was
considered untestable, largely because there was no suffi-
(5) ciently precise chronology of the ice ages with which
the orbital variations could be matched.
To establish such a chronology it is necessary to
determine the relative amounts of land ice that existed
at various times in the Earth’s past. A recent discovery
(10) makes such a determination possible: relative land-ice
volume for a given period can be deduced from the ratio
of two oxygen isotopes, 16 and 18, found in ocean sedi-
ments. Almost all the oxygen in water is oxygen 16, but
a few molecules out of every thousand incorporate the
(15) heavier isotope 18. When an ice age begins, the conti-
nental ice sheets grow, steadily reducing the amount of
water evaporated from the ocean that will eventually
return to it. Because heavier isotopes tend to be left
behid when water evaporates from the ocean surfaces,
(20) the remaining ocean water becomes progressively
enriched in oxygen 18. The degree of enrichment can
be determined by analyzing ocean sediments of the
period, because these sediments are composed of calcium
carbonate shells of marine organisms, shells that were
(25) constructed with oxygen atoms drawn from the sur-
rounding ocean. The higher the ratio of oxygen 18 to
oxygen 16 in a sedimentary specimen, the more land ice
there was when the sediment was laid down.
As an indicator of shifts in the Earth’s climate, the
(30) isotope record has two advantages. First, it is a global
record: there is remarkably little variation in isotope
ratios in sedimentary specimens taken from different
continental locations. Second, it is a more continuous
record than that taken from rocks on land. Because of
(35) these advantages, sedimentary evidence can be dated
with sufficient accuracy by radiometric methods to
establish a precise chronology of the ice ages. The dated
isotope record shows that the fluctuations in global
ice volume over the past several hundred thousand years
(40) have a pattern: an ice age occurs roughly once every
100,000 years. These data have established a strong
connection between variations in the Earth’s orbit and
the periodicity of the ice ages.
However, it is important to note that other factors,
(45) such as volcanic particulates or variations in the amount
of sunlight received by the Earth, could potentially have
affected the climate. The advantage of the Milankovitch
theory is that it is testable: changes in the Earth’s orbit
can be calculated and dated by applying Newton’s laws
(50) of gravity to progressively earlier configurations of the
bodies in the solar system. Yet the lack of information
about other possible factors affecting global climate does
not make them unimportant.
In the passage, the author is primarily interested in______
A.suggesting an alternative to an outdated research method
B.introducing a new research method that calls an accepted theory into question
C.emphasizing the instability of data gathered from the application of a new scientific method
D.presenting a theory and describing a new method to test that theory
E.initiating a debate about a widely accepted theory
The atmosphere that originally surrounded Earth was probably much different from
the air we breathe today. Earth's first atmosphere (some 4.6 billion years ago) was most
likely hydrogen and helium--the two most abundant gasses found in the universe--as
well as hydrogen compounds, such as methane and ammonia, Most scientists feel that
5 this early atmosphere escaped into space from the Earth's hot surface.
A second, more dense atmosphere, however, gradually enveloped Earth as gasses
from molten rocks within its hot interior escaped through volcanoes and steam vents.
We assume that volcanoes spewed out the same gasses then as they do today: mostly
water vapor (about 80 percent), carbon dioxide (about ten percent), and up to a few
10 percent nitrogen. These same gasses probably created Earth's second atmosphere.
As millions of years passed, the constant outpouring of gasses from the hot
interior--known as outgassing--provided a rich supply of water vapor, which formed
into clouds. Rain fell upon Earth for many thousands or years, forming the rivers,
lakes, and oceans of the world. During this Lime, large amounts of carbon dioxide were
15 dissolved in the oceans. Through chemical and biological processes, much of the carbon
dioxide became locked up in carbon sedimentary rocks, such as limestone. With much
of the water vapor already condensed into water and the concentration of carbon dioxide
dwindling, the atmosphere gradually became rich nitrogen.
It appears that oxygen, the second most abundant gas in today's atmosphere, probably
20 began an extremely slow increase in concentration as energetic rays from the sun split
water vapor into hydrogen and oxygen during a process called photodissociation. The
hydrogen, being lighter, probably rose and escaped into space, while the oxygen remained
in the atmosphere.
This slow increase in oxygen may have provided enough of this gas for primitive
25 plants to evolve, perhaps two to three billion years ago. Or the plants may have evolved
in an almost oxygen-free (anaerobic) environment. At any rate, plant growth greatly
enriched our atmosphere with oxygen. The reason for this enrichment is that plants, in
the presence of sunlight, process carbon dioxide and water to produce oxygen.
What is the main idea of the passage?
A.The original atmosphere on Earth was unstable.
B.The atmosphere on Earth has changed over time.
C.Hot underground gasses created clouds, which formed the Earth's atmosphere.
D.Plant growth depended on oxygen in the Earth's atmosphere.
According to a recent theory, Archean-age gold-quartz
vein systems were formed over two billion years ago from
magmatic fluids that originated from molten granitelike
bodies deep beneath the surface of the Earth. This theory is
(5) contrary to the widely held view that the systems were
deposited from metamorphic fluids, that is, from fluids that
formed during the dehydration of wet sedimentary rocks.
he recently developed theory has considerable practical
importance. Most of the gold deposits discovered during
(10)the original gold rushes were exposed at the Earth’s surface
and were found because they had shed trails of alluvial
gold that were easily traced by simple prospecting methods.
Although these same methods still lead to an occasional
discovery, most deposits not yet discovered have gone
(15) undetected because they are buried and have no surface
expression.
The challenge in exploration is therefore to unravel the
subsurface geology of an area and pinpoint the position of
buried minerals. Methods widely used today include
(20) analysis of aerial images that yield a broad geological
overview; geophysical techniques that provide data on the
magnetic, electrical, and mineralogical properties of the
rocks being investigated; and sensitive chemical tests that
are able to detect the subtle chemical halos that often
(25) envelop mineralization. However, none of these high-
technology methods are of any value if the sites to which
they are applied have never mineralized, and to maximize
the chances of discovery the explorer must therefore pay
particular attention to selecting the ground formations most
(30) likely to be mineralized. Such ground selection relies to
varying degrees on conceptual models, which take into
account theoretical studies of relevant factors.
These models are constructed primarily from empirical
observations of known mineral deposits and from theories
35) of ore-forming processes. The explorer uses the models to
identify those geological features that are critical to the
formation of the mineralization being modeled, and then
tries to select areas for exploration that exhibit as many of
the critical features as possible.
The author is primarily concerned with______
A.advocating a return to an older methodology
B.explaining the importance of a recent theory
C.enumerating differences between two widely used methods
D.describing events leading to a discovery
E.challenging the assumptions on which a theory is based
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