Igneous Rocks

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Assignments

Chernicoff, Igneous Processes and Igneous Rocks, ch. 3.

Reminders

For the upcoming optional movie night (date to be announced) we may watch any two movies of geological, meteorological, or planetary interest. The following movies have been nominated: Twister, The River, Dante's Peak, Jurassic Park, Contact, and Bringing up Baby (starring Cary Grant as a palaeontologist). Email additional nominations to me this week and then next week we'll take a vote to get the top two.

Reading Focus Questions

Read the entire chapter. Study the illustrations. Focus your attention on the vocabulary terms found in the following questions. Be prepared to answer any of these questions should they occur on a quiz or on the final exam.

Introduction

1. "Ignis" in Latin means _________.
2. ____________ rocks cool and crystallize directly from molten rock.
3. More than _______ percent of the Earth's outer 30 miles consists of igneous rock.
4. True or False? Most igneous rock solidifies when it reaches the surface of the Earth, as it is exposed to water or air.

Melting Rocks and Crystallizing Magma

1. _________ is molten rock that flows within the Earth.
2. True or False? To be called magma, the rock materials must all be in a liquid state.
3. _________ is molten rock that flows above ground.
4. Crystals grow until they touch the edges of adjacent ______________.

Igneous Textures

1. The slower a magma or lava cools, the (circle one: larger smaller) the crystals it contains.
2. With phaneritic texture crystals may be seen with a(n):
   1. unaided eye
   2. magnifying glass
   3. light microscope
   4. electron microscope
3. Magmas that enter preexisting rocks form _____________ rocks which, if found deep below the surface are also known as plutonic rocks.
4. Igneous rock that has exceptionally large crystals (sometimes several meters long) is most likely:
   1. aphanitic
   2. obsidian
   3. pegmatic
   4. phaneritic
5. An unmelted fragment from surrounding rock that is incorporated by a magma flow is call a(n) _____________.
6. Igneous rocks like basalt have _____________ textures because their crystals cannot be seen with the unaided eye.
   • They are called ______________ rocks because they formed from lava that flowed out onto the Earth's surface, solidifying rapidly.
   • They are known as ____________ because of the lava's source.
7. Igneous rocks with ______________ texture have large, often perfect crystals surrounded by regions with much smaller or even invisible grains.
   • They probably formed as a result of (slow, rapid) cooling followed abruptly by (slow, rapid) cooling.
8. The texture of an igneous rock may be phaneritic, pegmatic, aphanitic, porphyritic, or _________ (such as obsidian and pumice).
   • The latter texture results from nearly instantaneous cooling, as a result of which (no, many) crystals may be seen with the unaided eye.
9. ____________, a light-colored volcanic glass, forms when bubbling, highly gaseous, silica-rich magma cools instantaneously. It is sometimes light enough to float on water.
10. ____________, a dark volcanic glass, forms when silica-rich magma cools instantaneously. It breaks by fracture rather than cleavage and was used by prehistoric humans to fashion cutting tools.

Igneous Compositions

1. ______________ igneous rocks are dark in color, rich in magnesium and ferrum (Latin for iron), and are less than 40 percent silica.
   • ____________, the most common rock of this type, has the characteristic coarse-grained texture that results from its slow cyrstallization deep in the Earth's interior. It is over 70 percent olivine.
2. ______________ igneous rocks are the most abundant rocks of the Earth's crust. They are rich in magnesium and ferrum (Latin for iron), and have a silica content between 40 and 50 percent.
   • ____________ is the most common rock of this type. It is aphanitic, and comprises most of the ocean floor.
   • ____________ has the same composition as basalt, but crystallized more slowly deep within the Earth (plutonic) and therefore is coarse-grained.
3. ______________ igneous rocks contain about 60 percent silica and are lighter in color.
   • ____________ is a common aphanitic rock of this type.
   • ____________ is its phaneritic plutonic equivalent.
4. ______________ igneous rocks are rich in feldspar and silica. They contain 70 percent or more silica and are usually lighter in color.
   • ___________ is a common phaneritic plutonic rock of this type.
   • ___________ is its aphanitic volcanic equivalent.

Creation of magma

1. True or False? Felsic minerals melt at a lower temperature than mafic minerals.
2. True or False? Felsic rock can be produced from mafic magma.
3. The most important factors controlling the creation of magma are:
   1. pressure, heat and water
   2. radiation, water and oxygen
   3. water, heat and oxygen
   4. radiation, pressure and water
4. The three primary sources of the Earth's heat are:
   1. residual heat from the Earth's formation, plate friction, and radioactivity
   2. radioactivity, radiation absorbtion, and plate friction
   3. residual heat from the Earth's formation, plate friction, and the geothermal gradient
   4. volcanic eruption, radioactivity, and plate friction
5. Temperatures in the Earth increase with depth at a rate referred to as the ______________ _________________.
6. True or False? The presence of water makes rocks easier to melt.

Intrusive Rock Structures

1. A ____________ is an intrusive igneous body formed from magma that cooled underground.
2. Unmelted rocks carried within magma are called ________________.
3. Country rock is:
   1. rock located on the surface of the Earth
   2. any common rock
   3. preexisting rock into which magma intrudes
   4. all rock that is assimilated into a magma flow
4. Igneous intrusions that lie parallel to country rock layers are _____________ plutons.
5. Discordant intrusions:
   1. lie in a vertical position
   2. lie in a horizontal position
   3. cut across layers of preexisting rock
   4. have completely different minerals than the surrounding rock
6. Igneous intrusions that are broader than they are thick are _____________ plutons.
7. A discordant tabular pluton is a _____________.
8. A ______________ _____________ is a vertical pluton that results from magma that rose into a volcano's central magma pathway and then solidified.
9. A sill is:
   1. a discordant tabular pluton cutting across preexisting rocks
   2. a domed intrusion within other rocks
   3. a concordant tabular pluton lying parallel to layers of preexisting rocks
10. True or False? Sills usually form fairly close to the Earth's surface.
11. True or False? Sills form from faster-flowing mafic magmas and are usually basaltic and relatively flat.
12. How might a geologist in the field distinguish between a sill and a lava flow that has been covered by more recent sedimentary strata?
    1. Lava flows have vesicles, sills do not.
    2. o
13. When thick, viscous magma intrudes between two parallel layers of rock and lifts the overlying one into a dome, it cools to form a large mushroom-shaped concordant pluton, or ______________.
    • These are usually granitic, formed from felsic magma.
14. Saucer-shaped concordant plutons that sag downward are called _______________.
    • These are usually formed from mafic magma.
15. Massive discordant plutons of great size, with surface areas (when exposed) of 40 square miles or more, are called __________________.
16. True or False? Since batholiths are deep-seated and cool slowly, their texture is quite uniform throughout.

Basalts and Gabbros

1. True or False? Gabbros are the most abundant igneous rocks and also the most variable.
2. True or False? Mid-ocean ridge basalts account for 65% of the Earth's surface area.

Igneous Rocks on the Moon

1. True or False? The Moon's surface is formed by plate tectonics much like the surface of the Earth.
2. The dark spots on the Moon, or lunar Maria ("seas"), are _____________ flows.
3. Water in the form of ice has recently been discovered lining craters at both the north and south poles of the Moon. However, the lunar rocks brought back by the Apollo missions contained no water at all.