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Mid Term Exam Review
Major Concepts:
The solid earth can be considered a subsystem of the larger earth system, which also includes the atmosphere,
hydrosphere, and biosphere. Solid earth interacts with the other subsystems by the exchange of matter and energy,
resulting in a dynamically changing earth.
Geology is the study of Earth. Physical geology is concerned with the materials and processes which compose
and operate on the surface of, and within, Earth. Historical geology is concerned with the origin and evolution of
Earth's continents, oceans, atmosphere, and life.
The Solar System formed from a cloud of interstellar matter about 4.6 billion years ago. This cloud, upon
condensing, collapsed under the influence of gravity and flattened into a rotating disk. The Sun, planets, and moons
formed within this disk.
Earth has a layered structure. The outermost layer is divided into continental crust and oceanic crust. The upper
mantle lies below the crust. The crust and upper mantle comprise the lithosphere and overlie the asthenosphere, a
layer which flows slowly. Below the asthenosphere is the solid lower mantle, and then the core. The outer core is
liquid and the inner core is solid.
The lithosphere consists of separate plates that diverge, converge, or slide sideways past one another.
The scientific method involves observation about phenomena, the development of hypotheses to explain these
phenomena, the testing of these hypotheses, and the formulation of theories from those hypotheses which
successfully explain related phenomena. A scientific theory is a testable explanation for some natural phenomenon,
that is supported by a large body of evidence.
Plate tectonic theory unifies the observations and hypotheses which attempt to explain many geologic facts and
events.
The movements of the plates and the interactions of plates along their boundaries cause volcanic eruptions,
earthquakes, the formation of mountain ranges and ocean basins, and recycling of rock material.
The three major groups or families of rocks are igneous, metamorphic and sedimentary rocks. Each group
defines rocks types which have a common origin.
Igneous rocks are formed by the cooling and crystallization of magma beneath or on the Earth's surface or
consolidation of volcanic ejecta.
Sedimentary rocks are those deposited in layers on the Earth's surface as particles of pre-existing rocks and
minerals, particles of biological origin, or crystals precipitated from water.
Metamorphic rocks are those which have been transformed from pre-existing sedimentary, igneous or
metamorphic rocks into new rocks by the actions of chemically active fluids, heat and pressure.
The rock cycle illustrates the interactions and transformations of earth materials of the three major rock
families. The rock cycle is driven by both internal processes related to plate tectonics and external processes,
principally the action and cyclic movement of water from land to atmosphere to sea.
The principle of uniformitarianism is the fundamental tenet of geology. This principle states that the laws of
nature have remained unchanged through time and thus, that the processes observed today have also operated in the
past, though possibly at different rates.
The concept of continental movement was first suggested when it was noticed that Africa and South America
had coastlines which appeared to be counterparts of one another, suggesting they may once have been joined and
drifted apart.
Mid Term Exam Review
The hypothesis of continental drift is generally credited to Alfred Wegener. His treatment and hypothesis were
the most extensive and well developed. Wegener presented paleontological and geological evidence that the
continents were once united into one supercontinent which he named Pangaea.
Wegener could not provide a convincing mechanism to demonstrate how the continents could have moved, and
his ideas were largely ignored.
Paleomagnetic studies during the 1950's revived interest in continental drift because they indicated that either
the magnetic poles had wandered and each continent had its own pole (an impossibility), or the continents had
moved over time. If the continents were moved into different positions relative to each other, the separate poles
could be resolved into one.
Magnetic surveys revealed that oceanic crust exhibited magnetic anomalies —reversals of magnetic polarity—
and that these anomalies were disposed in symmetrical stripes paralleling the oceanic ridges. This indicates that new
oceanic crust must be formed along the spreading ridges.
Sea floor spreading is confirmed by the ages of fossils in sediments overlying oceanic crust of various ages, and
radiometric dating of rocks on oceanic islands. These indicate that oceanic crust is youngest at the spreading ridges
and oldest at the farthest points from the ridges.
Overwhelming evidence in support of plate tectonics led to its rapid acceptance and elaboration since the early
1970's. The theory is widely accepted because it explains so many geologic phenomena, including volcanism,
seismicity, mountain building, climatic changes, animal and plant distributions in the past and present, and the
distributions of natural resources. For these reasons, it is known as a unifying theory.
Supercontinents like Pangea form, break up, and re-form in a cycle spanning approximately 500 years. The
breakup forms rift valleys within the supercontinent that eventually becomes a long, linear sea as the crust is
depressed below sea level. As the width of the narrow sea continues to expand an open ocean develops.
Plate boundaries are of three types: convergent, divergent and transform. Convergent boundaries exist where
one plate is subducted beneath another and/or two plates with continental crust on their leading edges collide.
Divergent boundaries are where plates move away from each other. Transform boundaries are where the margins of
two plates slide horizontally past one another.
Associated rock assemblages and geologic structures aid in the recognition of ancient plate boundaries. Rift
valleys with thick sedimentary sequences and numerous dikes and sills may indicate divergent boundaries, while
ophiolites and andesitic rocks may be characteristic features of convergent boundaries. Generally, no diagnostic
features are left by transform faults.
The average rate of plate movement can be calculated in several ways. All calculations indicate that plates
move at different average velocities.
Absolute rates of plate movement can be calculated by reference to stationary hot spots which originate from
deep mantle plumes.
Although the mechanisms of plate tectonic movement are not understood, many geologists think that some type
of mantle convection is involved.
Temperature and composition are the main determinants of viscosity in lava flows, with relatively cool and/or
more felsic (silica-rich) flows being more viscous than hotter and/or more mafic (silica-poor) ones.
Volcanism is a constructive process, creating oceanic islands, oceanic crust, island arcs, and volcanic mountain
chains along convergent plate margins as well as critical atmospheric gases.
Volcanism is the eruption of magma, and associated gases at the surface. Some magma erupts explosively as
pyroclastic (fire-broken) rock and other erupts as lava flows.
Mid Term Exam Review
Shield volcanoes have gentle slopes, rounded shapes and are composed largely of basaltic lava flows. Cinder
cones consist of cinder-like pyroclastic materials accumulated as relatively small, steep-sloped cones. Composite
volcanoes (stratovolcanoes) are comprised of lava flows of intermediate composition, and layers of pyroclastic
debris and volcanic mud flows (lahars).
The volcanic explosivity index (VEI) combines volume of material erupted, eruption cloud height, and other
qualitative information to arrive at a measure of the explosive intensity of an eruption.
Volcanoes are not randomly distributed, but occur in well-defined zones or belts. The circum-Pacific and
Mediterranean belts together include more than 80% of all active volcanoes. The majority of the remaining 20% of
active volcanoes are on or near mid-oceanic ridges.
Volcanic activity at or near mid-oceanic ridges is mainly submarine, but in a few places such as Iceland, it
occurs above sea level. The volcanoes that form are mostly mafic shield volcanoes.
The Hawaiian Islands formed as a series of volcanoes originating from a stationary mantle plume as the Pacific
Plate moved over it. Yellowstone in Wyoming may also have formed over a hot spot.
Rocks and minerals are disintegrated and decomposed by the processes of mechanical and chemical weathering.
This breakdown occurs because the parent material reacts with its new physical and chemical environment
transforming it into a new equilibrium state.
The products of weathering include soluble salts, ions in solution, and solid particles which can be eroded and
become sedimentary rock or modified in place to become soils.
Mechanical weathering includes the processes of frost action, pressure release, thermal expansion and
contraction, crystal growth, and the activities of organisms. The products of mechanical weathering are chemically
the same as their parent materials.
Chemical weathering processes include solution, oxidation, and hydrolysis. These processes cause a change in
the chemical composition whereby parent material is transformed into products including ions in solution, soluble
salts and clay minerals.
Hot and wet environments accelerate chemical weathering. Chemical weathering occurs in all environments,
except, possibly, permanently frozen polar regions.
Mechanical weathering enhances chemical weathering by breaking material into smaller pieces, thereby
increasing the surface area for chemical reactions.
Possible Essay Questions:
Why do most scientists think overpopulation is the greatest environmental problem facing the world today?
ANSWER:
Overpopulation taxes geologic resources such as water and energy, has an impact on the environment
through pollution, and puts more people at risk from natural hazards.
What is the Big Bang? What evidence do we have that the universe began approximately 15 billion years ago?
ANSWER: The Big Bang refers to a model for the origin of the universe. The rate at which galaxies are currently
moving away from each other is used to calculate how long ago they were together as a single mass.
Why is Earth considered a dynamic planet? What are its three concentric layers and their characteristics?
ANSWER: Earth is considered dynamic because its size, shape, and geographic distribution of continents and ocean
basins have changed over geologic time. Earth is composed of the core, mantle, and crust. The crust
Mid Term Exam Review
and upper part of the mantle are termed the lithosphere and comprise the brittle tectonic plates that
move on the lower, more plastic part of the mantle called the asthenosphere.
Explain how the principle of uniformitarianism allows for catastrophic events.
ANSWER: Catastrophic events such as earthquakes, volcanic eruptions, and floods are infrequent and short-lived.
They are part of the normal distribution of observable Earth events or processes on which the principle
of uniformitarianism is based.
What evidence convinced Wegener and others that continents must have moved in the past and at one time formed a
supercontinent?
ANSWER: Similar rock sequences and mountain ranges of the same age and similarities of many extinct plant and
animal groups between continents whose shoreline shapes fit together and the matching of glacial
deposits and paleoclimatic zones of continents were used as evidence to support the existence of a
supercontinent.
Explain why such natural disasters as volcanic eruptions and earthquakes are associated with divergent and
convergent plate boundaries.
ANSWER: Stress resulting from the interaction of tectonic plates at either the divergent or convergent boundary
result in concentrated earthquake activity. Volcanic eruptions are also associated with these boundaries
because of magma generated through crusting thinning (divergent boundary) and subduction and
assimilation of lithosphere in the asthenosphere (convergent boundary).
What can hot spots tell us about the absolute direction of plate movement?
ANSWER: Hot spots are present in the asthenosphere and are stationary. The abosolute direction of plate
movement is recorded as volcanoes form on the overriding lithospheric plate.
Where does most basalt form, and how does it originate?
ANSWER: Basalt forms from basaltic magmas which result from melting of mantle. Most occur along divergent
plate margins as a result of mantle upwelling.
Why is it so much more difficult for felsic magma to erupt than mafic magma?
ANSWER: Felsic magma tends to be cooler and more viscous than the hotter, less viscous mafic magma.
What geologic events would have to take place for a chain of composite volcanoes to form along the east coasts of
the United States and Canada?
ANSWER: Composite volcanoes occur where oceanic crust is subducted beneath continental crust. Found along
the east coasts of the United States and Canada, they would indicate the presence of a convergent plate
margin.
Explain why eruptions of mafic lava are nonexplosive but eruptions of felsic magma are commonly explosive.
ANSWER: In viscous, felsic magmas, the gas has more difficulty expanding as pressure decrease towards Earth’s
surface. The result is more explosive eruptions.
What does volcanic tremor indicate, and how does it differ from the shaking caused by most earthquakes?
ANSWER: Volcanic tremors are longer lasting than the instantaneous shaking caused by most earthquakes. They
indicate that magma is moving in the subsurface.
Mid Term Exam Review
Why is thermal expansion and contraction not a particularly effective type of mechanical weathering on Earth?
ANSWER: Thermal expansion and contraction may not occur due to the lack of extreme temperatures (hot and
cold) occurring with enough frequency on Earth to be an effective type of weathering. In addition, the
forces produced may not be as significant as a result.
Why is rainfall naturally acidic?
Rainfall (H20) falls through an atmosphere that is contains carbon dioxide (CO2). The reaction of water and
carbon dioxide results in an acid called carbonic acid (H 2CO3)
Briefly explain why water is such an important agent in weathering.
ANSWER: For chemical weathering, water is necessary for hydrolysis and the formation of carbonic acid. Water
serves to hold and transport the dissolved products. The expansion of water upon freezing, salts dissolved
in water, its support of organic growth all facilitate the mechanical weathering processes. For both
chemical and mechanical weathering, it also serves to transport/erode the weathering product.
How does mechanical weathering differ from chemical weathering?
ANSWER: In mechanical weathering physical forces break the rock materials into smaller pieces which retain
their chemical composition; in chemical weathering the composition is altered.
Name an important physical property about water that makes it unique among liquids.
ANSWER: It increases in volume when it freezes (changes from a liquid to a solid state).
Why is this unique property of water important in the physical weathering of rocks?
ANSWER: The expanding volume creates pressures which may break rocks and minerals into smaller pieces.
Global warming is predicted to occur from increased atmospheric carbon dioxide due to the burning of fossil fuels.
How might this increased level of carbon dioxide affect rates of weathering?
ANSWER: Increased amounts of atmospheric carbon dioxide should increase rates of weathering by carbonic
acid and hydrolysis if rainfall remains constant or increases.
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when you arrive in class on Thursday.
Mid Term Exam Review