Homework 1 Solutions
1. What was Wegener’s evidence for continental drift?
Wegener’s evidence included the puzzle-like fit of the continental coastlines,
matching of geologic features (rock types, mountain ranges, etc.) across
continental boundaries, and the occurrence of identical fossils on widely
separated continents. In addition, Wegener noted that the distribution of
glacial markings and deposits is consistent with drifting continents, as well
as the evidence for polar wander (actually wandering continents).
3. What is the evidence that South America and Africa were once joined?
Evidence that South America and Africa were once joined includes matching rock
types across these now-separated continents, similar fossils on both
continents, and matching coastlines as well as matching continental shelves
(submerged continental edges).
5. What is Pangea?
Pangea was a giant super continent that existed until approximately 200
million years ago, when it began to break up.
7. How does seafloor spreading account for the age of the sea floor.
Seafloor spreading implies that the youngest rocks should be found at the
crest of the mid-ocean ridge. Away from the ridges, the seafloor becomes
progressively older as it is carried away from the ridge via seafloor
spreading, similar to the motion of a conveyor belt. Seafloor age is greatest
at large distances from the mid-ocean ridge, as implied by seafloor spreading.
8. What is a plate in the concept of plate tectonics?
In plate tectonic theory, Earth’s outer, brittle shell (the lithosphere) is
broken up into a series of mobile fragments called plates.
9. Define lithosphere and asthenosphere.
• Lithosphere—Earth’s outer, brittle shell; consists of the crust and
uppermost mantle and extends to approx. 100 km depth.
• Asthenosphere—A mobile, highly plastic region within Earth’s mantle, that
occurs directly below the plates. The asthenosphere is the weak substrate upon
which the plates slide.
10. What is the origin of marine magnetic anomalies according to Vine and
According to Vine and Matthews, marine magnetic anomalies (“zebra stripes”)
form as a result of two processes, including seafloor spreading and reversal
of Earth’s magnetic field. As new lava erupts from the mid-ocean ridge and
cools, its magnetism aligns itself with that of Earth’s magnetic field, and a
magnetic stripe is created. With additional seafloor spreading, this magnetic
stripe is broken into two stripes that are symmetrical about the ridge. At
some point Earth’s magnetic field reverses itself, and new lava that erupts at
the mid-ocean ridge after the reversal also acquires a reversed magnetic
character, creating a newer set of symmetrical (but reversed) magnetic
16. What is transform fault?
A transform fault is a zone of fractured rock that typically develops between
two offset portions of the ridge crest. The relative motion across a transform
fault is sideways.
17. Discuss possible driving mechanisms for plate tectonics?
Driving mechanisms include slab pull (where the gravitational sinking of
subducting plates pulls on the rest of the plate), ridge push (where opposite
sides of the mid-ocean ridge slide away from one another down the slope of the
ridge), and trench suction (where mantle flow currents set up by a subducting
plate pull on the bottom of the non-subducting plate, “sucking” it toward the
trench). Mantle convection (the slow overturning of the Earth’s hot, plastic
interior) may also play a role in driving plate motions.
18. Describe the various type of plate boundaries and the geologic features
associated with them.
Plate boundary types include divergent (where plates move away from each
other), transform, (where one plate slides horizontally past another plate),
and convergent (where two plates move toward each other). Divergent plate
boundaries are associated with active volcanism at mid-ocean ridges; transform
boundaries are associated with seafloor fracture zones between offset portions
of the mid-ocean ridge crest and also with active faults on the continents
such as the San Andreas Fault; convergent plate boundaries are associated with
mobile mountain belts, volcanic arc volcanism, and deep marine trenches.
Convergent plate boundaries include ocean-continent convergence (subduction),
ocean-ocean convergence (subduction), and continent-continent convergence
1. Compare feldspar and quartz.
• How do they differ chemically?
Quartz is made up of oxygen and silicon atoms; formula is SiO2. (silica)
Feldspar has aluminum that is a substitute for some of the silicon in the
linked tetrahedrons. It also contains sodium, calcium, or potassium.
• What type of silicate structure does each have?
They both have framework silicate
• How would you distinguish between them on the basis of cleavage?
Quartz has no cleavage because it is strong in all directions and bonds are
equally as strong. Feldspar has two directions of cleavage: right angles, 90
2. How do the crystal structures of pyroxenes and amphiboles differ from one
The pyroxenes are single-chain silicates, whereas the amphiboles are
4. Distinguish the following terms:
• Silica – a term for oxygen combined with silicon.
• Silicate – substances that contain silica as part of their chemical formula.
• Silicon – an element enriched in Earth’s crust.
• Silicon-Oxygen Tetrahedron – four-sided, pyramidal object that visually
represents the four oxygen atoms surrounding a silicon atom; the basic
building block of silicate minerals. Also called silicate tetrahedron.
9. What are the three most common elements (by number approximate percentage)
in the Earth’s crust?
Oxygen, 46.6%, Silicon 27.7, and Aluminum 8.1% (by weight)
10. What are the next five most common elements?
Iron 5%, Calcium 3.6, Sodium 2.8%, Potassium 2.6%, Magnesium 2.1% (by weight)
1. Why do mafic magmas tend to reach the surface much more often than silicic
Mafic magmas are typically much less viscous than silicic magmas, so they can
more easily reach Earth’s surface before cooling crystallizing.
3. How do batholiths form?
They are formed of numerous, coalesced plutons, typically at subduction zones,
as the plutons migrate upward through the lower crust and collect below the
surface, where the magma cools and crystallizes.
4. How would you distinguish, on the basis of minerals present, among granite,
gabbro, and diorite?
Grabbo is formed of coarse-grained ferromagnesian minerals and gray,
plagioclase feldspar. You can identify it on the basis of cleavage and verify
that no quartz is present. Granite and rhyolite are composed predominantly of
feldspar, usually white or pink, and quartz. Granite is coarse-grained, and
contains abundant quartz. Diorite and andesite are composed of feldspar and
significant amounts of ferromagnesian minerals but little to no quartz.
Diorite is coarse-grained and andesite is fine-grained.
5. How would you distinguish andesite from a diorite?
Andesite is fine-grained, can usually be identified by its medium-gray or
medium-green color. Its appearance is intermediate between light-colored
rhyolite and dark basalt. Diorite is coarse-grained and often characterized by
a salt and pepper appearance due to a roughly equal abundance of
ferromagnesian and non-ferromagnesian silicates.
6. What rock would probably form if magma that was feeding volcanoes above
subduction zones solidified at considerable depth?
Diorite (intermediate in composition)
9. What is the difference between a dike and a sill?
A dike is a tabular, discordant, intrusive structure. Dikes may form at
shallow depths and be fine-grained, or at greater depths and be
coarser-grained. A sill is also a tabular intrusive structure, but it is
concordant. That is, sills, unlike dikes, are parallel to any planes or
layering in the country rock.
10. Describe the differences between the continuous and the discontinuous
branches of Bowen’s reaction series.
The discontinuous branch, which contains only ferromagnesian minerals. Also,
earlier crystals react with the remaining melt to create new crystal
structures. Olivine crystallizes before pyroxene; pyroxene crystallizes before
amphibole; and amphibole crystallizes before biotite. The continuous branch
contains only plagioclase feldspar. A complication is that the early crystals
react continuously with the remaining melt so that the composition of the
plagioclase is continuously changing as crystallization proceeds.
2. What roles do gases play in volcanoes?
During an eruption, expanding, hot gases may propel pyroclasts high into the
atmosphere as a column rising from a volcano. At high altitudes, the
pyroclasts often spread out into the dark, mushroom cloud.
3. What do pillow structures indicate about the environment of volcanism?
Pillow structures indicate underwater volcanism.
5. What property or characteristic of obsidian makes it an exception to the
usual geological definition of rock?
Obsidian is volcanic glass, is one of the few rocks that is not composed of
6. What determines the viscosity of a lava?
Viscosity, or resistance to flow, is determined by silica content (the higher
the silica, the higher the viscosity), and temperature (the lower the
temperature, the higher the viscosity), and dissolved gas (the higher the gas
content, the lower the viscosity).
7. What determines whether a series of volcanic eruptions builds a shield
volcano, a composite volcano, or a cinder cone? Describe each.
A shield volcano is a broad, gently sloping volcano constructed of solidified
lava flows. The low viscosity of its lava allows the flows to spread out into
wide, thin layers, producing a flattened dome shape or shield. Cinder cone is
a volcano constructed of pyroclastic fragments ejected from a central vent.
Cinder cones are much smaller than shield volcanoes and are associated with
various magmas types. Composite volcanoes are constructed of alternating
layers of pyroclastic fragments and solidified lava flows. Composite volcanoes
are built over long spans of time and are typically intermediate in size
between cinder cones (which are smaller) and shield volcanoes (which are
larger). Composite volcanoes are typically associated with
8. Explain how a vesicular porphyritic andesite might have formed.
Before erupting, a magma of intermediate composition slowly crystallizes,
producing early-formed, large crystals within the magma called phenocrysts.
Later, the magma erupts as lava, and cools rapidly, creating a second
generation of small crystals. Together, the two generations of crystals create
an intermediate volcanic rock (andesite) with two distinct sizes of crystals—a
texture known as porphyritic.
9. Why are extrusive igneous rocks fine-grained?
When lava cools rapidly, atoms only have time to move for a short distance
before linking up into crystals, so the resulting crystals don’t have the
opportunity to grow large.
10. Why don’t flood basalts build volcanic cones?
Because not all extrusive rocks are associated with volcanoes. The mafic lava
associated with flood basalts is very non-viscous and therefore flows easily,
and does not build a cone around its vents.
4. Why do stone buildings tend to weather more rapidly in cities than in rural
In cities, industrial atmospheric discharges are associated with the burning
of coal, oil, and natural gas. Such discharges also add carbon dioxide to the
atmosphere, which creates acidic rain water that chemically weathers such
5. Describe at least three processes that mechanically weather rock.
(1) frost wedging--the expansion of freezing water pries rock apart; (2)
pressure release--the reduction of pressure on a brittle rock mass, which can
cause it to crack as it expands; (3) plant growth--as roots grow in cracks,
the rock mass breaks up.
6. How can mechanical weathering speed up chemical weathering?
Mechanical weathering can increase the surface area of a rock, accelerating
the rate of chemical weathering by allowing water to percolate into the rock
7. Name at least three natural sources of acid in solution. Which one is
important for chemical weathering?
Natural sources of acid in solution include volcanic eruptions, hot springs,
and atmospheric carbon dioxide, which can combine with water to create
The most important natural source of acid rock weathering is dissolved carbon
dioxide in water (carbonic acid).