Geology 1610 online exam 1

What might be the explanation for southwestern B.C. having much less subduction-related volcanism than adjacent Washington and Oregon?

No one is certain why there a lower rate of volcanism in B.C. than in adjacent Washington and Oregon, but one theory is that the northern part of the Juan de Fuca Plate (the Explorer Plate) is not subducting as quickly as the rest of the plate.

How would some familiarity with biology be helpful to a geologist?

Paleontology is an important aspect of geology and requires an understanding of biology, including evolution, the physiology of animals and plants and ecological relationships

Under what other circumstances might a lahar form?

Some lahars form during an eruption when snow and ice melt quickly, while others may form from heavy rain.

Explain how the need for an atom's outer shell to be filled with electrons contributes to bonding.

The element's atomic number will determine the extent to which its outer layers are populated with electrons. If the outer shell is not quite full, the atom may gain electrons to fill them and become an anion (negative charge). If the outer shell has only a few electrons, it may lose them and become a cation (positive charge). Cations and anions attract each other to form molecules with ionic bonding.

What is the main component of Earth's core?

The main component of Earth's core is iron (Fe).

What is the primary mechanism for partial melting at a convergent plate boundary?

The primary mechanism for partial melting at a convergent plate boundary is the addition of water to hot mantle rock. The water reduces the melting temperature of the rock (flux melting).

What processes must take place to transform rocks into sediment?

The rock must be exposed at surface so in many cases uplift and removal of overlying sediments is required. Then chemical and/or physical weathering can take place, which reduces the rock to smaller loose fragments. These fragments are sediments that can be eroded and then transported by a variety of maechanisms.

What are the three main tectonic settings for volcanism on Earth?

The three main tectonic settings for volcanism are (1) subduction zones at convergent plate boundaries, (2) divergent plate boundaries, and (3) mantle plumes (a.k.a. hot spots).

Why do the gases in magma not form gas bubbles when the magma is deep within the crust?

When magma is deep within the crust the pressure is too high for the gases to bubble out of solution.

Describe two ways in which batholiths intrude into existing rock.

. Batholiths (or stocks) intrude into existing rock by (a) melting through the country rock, or (b) causing the country rock to break and fall into the magma (stoping), or (c) pushing the country rock aside.

How can GPS technology be used to help monitor a volcano in the lead-up to an eruption?

. GPS technology is used to determine if there is any slow deformation of the flanks of a volcano related to movement of magma toward the surface.

Dinosaurs first appear in the geological record in rocks from about 215 Ma and then became extinct 65 Ma. For what proportion (%) of geological time did dinosaurs exist?

215 - 65 = 150 Ma. Since the age of the Earth is 4570 Ma, this represents 150/4,570 = 0.033 or 3.3% of geological time.

With respect to tabular intrusive bodies, what is the difference between a concordant body and a discordant body?

A concordant body (a sill) is parallel to any pre-existing layering (bedding or foliation) in the country rock is. A discordant body (a dyke) cuts across any pre-existing layering or is situated at any angle in country rock that has no layering (e.g., granite).

What is the electrical charge on an anion? A cation?

An anion has a negative charge and a cation has a positive charge.

What is the significance of the term reaction in the name of the Bowen reaction series?

As the temperature decreases minerals that formed early (e.g., olivine) may react with the remaining magma to form new minerals (e.g., pyroxene).

Why is it common for plagioclase crystals to be zoned from relatively calcium-rich in the middle to more sodium-rich on the outside?

Calcium-rich plagioclase forms early on in the cooling process of a magma, but as the temperature drops, a more sodium-rich variety forms around the existing crystals.

You have an unknown mineral that can scratch glass but cannot scratch a porcelain streak plate. What is its approximate hardness?

Glass has a Mohs hardness of about 5.5 while porcelain is close to 6.5. The mineral is between these two, so it must be close to 6.

Describe the lattice structure and elemental composition of the mineral halite.

Halite is composed of sodium (Na) and chlorine (Cl) with the Na+ and Cl- ions alternating with one another in all three directions within a cubic structure.

Why are helium and neon non-reactive?

Helium and neon (and the other noble gases) have complete outer shells and therefore no tendency to form ionic bonds.

Explain the difference between aphanitic and phaneritic textures.

If the texture is aphanitic the crystals are too small to see without a microscope. In rocks with phaneritic textures the minerals are large enough to see and distinguish from each other with the naked eye. The dividing line is somewhere between 0.1 and 1 mm, depending on the minerals.

What was the likely cause of most of the deaths from the most recent eruption at the Tseax River Cone?

It is likely that carbon dioxide released during the eruption flowed downhill from the volcano to the village on the shore of the Nass River.

Why is weak seismic activity (small earthquakes) typically associated with the early stages of a volcanic eruption?

Weak seismic activity is associated with all stages of a volcanic eruption. In the early stages magma is moving at depth and pushing rock aside, creating small earthquakes. The flow of magma can also produce special type of seismic response known as harmonic tremor.

Name the mineral group for the following minerals: calcite biotite pyrite gypsum galena orthoclase hematite graphite magnetite quartz fluorite olivine

calcite = CaCO3 carbonate biotite = silicate pyrite = FeS2 sulphide gypsum = CaSO4 sulphate galena = PbS sulphide orthoclase = KAlSi3O8 silicate hematite = Fe2O3 oxide graphite = C native magnetite = Fe3O4 oxide quartz = SiO2 silicate fluorite = CaF2 halide olivine = MgSiO4 silicate

Name the following rocks: (a) An extrusive rock with 40% Ca-rich plagioclase and 60% pyroxene (b) An intrusive rock with 65% plagioclase, 25% amphibole, and 10% pyroxene (c) An intrusive rock with 25% quartz, 20% orthoclase, 50% feldspar, and minor amounts of biotite

(a) basalt (b) diorite (c) granite

What is the difference between a batholith and a stock?

A batholith has an exposed area of greater than 100 km2; a stock has an exposed area less than that.

What is a lahar, and why are lahars commonly associated with eruptions of composite volcanoes?

A lahar is a mud flow or debris flow on a volcano. Lahars are common on composite volcanoes because they are steeper than shield volcanoes, they typically have ice and snow, and they are not as strong as shield volcanoes.

In what way is a mineral different from a rock?

A mineral has a specific chemical composition and lattice structure. Rocks are made out of minerals, and most rocks contain several different types of minerals.

Why does a dyke commonly have a fine-grained margin?

A rock has to crack in order for a dyke to intrude into it, and it has to be cool to crack. When the hot magma intrudes into the cold country rock its margins cool quickly (forming small crystals), while its centre cools more slowly (forming larger crystals).

What is the net charge on an unbonded silica tetrahedron?

An unbonded silica tetrahedron has one Si ion (+4 charge) and 4 oxygens (-2 charge each) so the overall charge is 4 - 8 = -4 for SiO4-4

If a typical rate for the accumulation of sediments is 1 mm/year, what thickness (metres) of sedimentary rock could accumulate over a period of 30 million years?

At 1 mm/y 30,000,000 mm would accumulate over that 30 million years. This is equivalent to 30,000 m or 30 km. Few sequences of sedimentary rock are even close to that thickness because most sediments accumulate at much lower rates, more like 0.1 mm/y.

Why is biotite called a ferromagnesian mineral, while muscovite is not?

Biotite includes iron and/or magnesium in its formula, while muscovite does not.

What is the electrical charge on a proton? A neutron? An electron? What are their relative masses?

Charges: proton: +1, neutron: 0, electron: -1, Masses: proton: 1, neutron: 1, electron: almost 0.

What two kinds of rock textures are typically found in a composite volcano?

Composite volcanoes can produce rocks with a wide range of textures, including (1) aphanitic or porphyritic rock from lava flows, (2) pyroclastic rock (with textures ranging from fine ash to coarse fragments) from explosive eruptions, and (3) sedimentary rock from lahars.

Why is compositional layering a common feature of mafic plutons but not of felsic plutons?

Compositional layering forms when early-crystallizing mineral sink toward the bottom of a magma chamber. This can only happen in non-viscous magma, and mafic magma is typically much less viscous than felsic magma

What must happen within a magma chamber for fractional crystallization to take place?

Early-forming minerals, which are typically quite dense (e.g., olivine) may sink to the bottom of the magma chamber (if the magma is not too viscous) and thus become separated from the rest of the magma, resulting in a change to the composition of the remaining magma (it becomes more felsic).

What is the difference in the role of electrons in an ionic bond compared to a covalent bond?

Electrons are transferred from one atom to another to form an ionic bond. Electrons are shared between atoms to form a covalent bond.

List three ways in which geologists can contribute to society.

Geologists provide information to reduce the risk of harm from hazards such as earthquakes, volcanoes, and slope failures; they play a critical role in the discovery of important resources; they contribute to our understanding of life and its evolution through paleontological studies; and they play a leading role in the investigation of climate change, past and present and its implications.

In what way is geology different from the other sciences, such as chemistry and physics?

Geology involves integration of various different sciences (chemistry, physics, and biology for example), but also requires an understanding of the importance of billions of years of geological time.

What are some of the processes that take place at a divergent plate boundary?

Hot mantle rock moving toward the surface partially melts because the pressure is reduced. The magma produced moves upward into cracks in the crust and is extruded onto the sea floor.

Explain the difference between porphyritic and pegmatitic textures.

In porphyritic rocks there are two distinct crystal sizes that are indicative of two stages of cooling (slow then fast). The fine material can range from glass to several mm, as long as the coarse crystals are distinctively larger. In pegmatitic rocks the crystals are consistently coarser than 1 cm, and can be much larger. Pegmatites form the slow cooling of water-rich magmas.

Why does quartz have no additional cations (other than Si+4)?

In quartz each silica tetrahedron is bonded to four other tetrahedra, and since oxygens are shared at each bond the overall ratio is silicon (+4) to two oxygens (2 x -2 = -4), which is balanced.

What allows magnesium to substitute freely for iron in olivine?

Magnesium can substitute freely for iron in olivine and several other minerals because they have similar charges (+2) and similar ionic radii.

How does mantle convection contribute to plate tectonics?

Mantle convection creates the traction that can force plates to move around on the surface.

What chemical feature is used in the classification of minerals into groups?

Minerals are classified into groups based on their anion or anion group

Where do pillow lavas form? Why do they form and from what type of magma?

Pillow lavas form where mafic lava erupts in water. When the magma oozes out into the water the outside cools first forming a hard skin that maintains the pillow shape.

How are the silica tetrahedra structured differently in pyroxene and amphibole?

Pyroxene is made up of single chains of tetrahedra while amphibole is made up of double chains.

What are the processes that lead to the formation of a metamorphic rock?

Rock is buried within the crust and heated because of the geothermal gradient. At temperatures over 200˚C some of the existing minerals may become unstable and will be converted to new minerals, or recrystallized into larger crystals.

What processes normally take place in the transformation of sediments to sedimentary rock?

Sediments are buried beneath other sediments where, because of the increased pressure, they become compacted and dewatered. With additional burial they are warmed to the point where cementing minerals can form between the grains (less than 200˚C).

In very general terms, what is the lifespan difference between a composite volcano and a shield volcano?

Shield volcanoes tend to have much longer lives than composite volcanoes. Most of the Hawaiian shields, for example lasted 1 million years, while most composite volcanoes are younger than 100,000 years.

Why is colour not necessarily a useful guide to mineral identification?

Some minerals have distinctive colours, but many have a wide range of colours due to differing impurities.

What type of eruption at Mt. St. Helens might have produced columnar basalts?

The Mt. St. Helens columnar basalts were formed by a flow of mafic lava.

What is the likely geological origin of the Nazko Cone?

The Nazko Cone is thought to be related to a mantle plume.

Why are the viscosity and gas content of a magma important in determining the type of volcanic rocks that will be formed when that magma is extruded?

The explosiveness of a volcanic eruption depends on the pressure of the magma. Gases create that pressure, and if the magma is viscous those gases cannot escape easily. Felsic and intermediate magmas tend to have more gas than mafic magmas, and are also more viscous, trapping the gas in.

Explain why shield volcanoes have such gentle slopes.

The magma at shield volcanoes is typically non-viscous. It can flow easily and also tends to form lava tubes, and thus is able to extend a long way from the vent, forming a low broad shield.

What are the names and compositions of the two end-members of the plagioclase series?

The two end-members of the plagioclase series are Albite (NaAlSi3O8) and Anorthite (CaAl2Si2O8)

What process leads to convection in the mantle?

Transfer of heat from the core to the mantle leads to heating of lower mantle rock. When heated, the rock expands and its density is reduced. Because the mantle is plastic, this lower-density material tends to rise toward the surface, and cooler denser mantle material moves in to take its place.