Download Background Information: Mountain Building

GEOLOGY 12
STUDY GUIDE / NOTES
CHAPTER 12 STRUCTURAL GEOLOGY
Name __________________
Major Concepts
Mountain Building
1. Continental Margins
Continental margins are the boundaries between the oceanic crust and the continental
crust. There are:
a) Active types like the West coast of South America which caused the Andes
Mountains. Volcanoes and earthquakes are common here.
b) Passive types like the east coast of North America. The margin is a buildup of
sediments. The wedges of sediment is 250 km wide and up to 10 km deep. These
will become the mountains of the distant future.
Mountains have been built by the subduction of ocean plates under continent plates.
Sometimes pieces of the oceanic plates don't subduct but rather are scraped off and
go over the top to make terranes. When 2 continents collide in a converging plate
boundary, mountains result because the earlier subduction changes to converging
because the continents are too light to subduct and thus run into each other pushing
each up into mountain ranges. This is accompanied by earthquakes but not usually
volcanoes.
2. Faults
a) Normal fault - when the higher side drops in relation to the other side. This is not
common in mountain building.
b) Reverse fault - when the higher side is raised in relation to the other side. Also
called thin-skin thrusting. This helps build mountains.
c) Strike-Slip fault (transcurrent fault) - The blocks slip horizontally past each other as
in the San Andreas Fault.
3. Fractures
Fractures are cracks in bedrock along which no movement has occurred (as
compared to faults). These usually occur in a plane. They always occur in parallel sets.
Sometimes they intersect.
4. Folds
Anticline is an upfold in rock layers. A syncline is a downfold.
The dip is the steepness of a fold while the strike is the direction of the fold. Most
famous folded mountains are Valley and Ridge Province of the Appalachian Mountains.
A dome is a nearly circular folded mountain. They don't form
mountain chains but are individual. They vary from 10 km to 180 km
across. There are two kinds:
a) Plutonic domes are formed by magma pushing the surface up, or
b) Tectonic domes are formed by an uplifting of the surface.
A basin is a fold which is equivalent to a radial syncline. The layers
are shaped like an inverted bowl.
5. Volcanoes
Volcanoes can add to the mountain material and form batholiths like the Cordillera
Blanca of the Andes, the Sierra Nevada Batholith in California and Coast Mountains in
BC. Material is also added by volcanoes such as those in the Cascade Mountains.
6. Uplifting and Tilting
Sedimentary rocks also are uplifted by tectonic action. This is part of mountain
building. Evidence includes sea fossils found at higher elevations, and former beaches
raised much above sea level. There has also been evidence using modern surveying
techniques.
Since sedimentary rocks are formed more or less horizontally, when you find sediment
lines sharply tilted, this is evidence that tilting has taken place. The east side of the
Rocky Mountains has many steep and almost vertical rock layers. This was caused by
the uplifting of the Rocky Mountains.
Fault block mountains are uplifted blocks of crust noticeably higher than the
surrounding areas. These usually have a steep side on one side, but a gently sloping
side on the other. There are some in Nevada and western Utah. Some of the rock
layers may be tilted so far that they are turned over. Some evidence of this is
overturned ripple marks, mud cracks, shells and cross-bedding.
Chapter 12
Study Guide
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Types of Faults
Strike-Slip Faults:
b
b
Dip-Slip Faults:
Types of Folds
Types of Mountains
A. Fold & Thrust Mountains
B. Dome Mountains
C. Fault Block Mountains
D. Volcanic Mountains
Chapter 12
Study Guide
Page 3
The Geometry of Folds
Plunging Syncline
Plunging Anticline
Outcrop pattern of plunging folds.
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Chapter 12
Study Guide
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Geological Map Symbols
Examples
Basin
(youngest = center)
Dome
(oldest = center)
A SET OF SIMPLE RULES FOR INTERPRETING GEOLOGIC MAPS
1. Anticlines have their oldest beds in the center.
2. Synclines have their youngest beds in the center.
3. Anticlines plunge toward the nose (closed end) of the structure.
4. Synclines plunge toward the open end of the structure.
“My AUNT is OLD and very CLOSED –minded.”
Chapter 12
Study Guide
Page 5
Isostasy
What will happen to the mattress when she gets up?
In simple terms, ISOSTASY is the principle observed by Archimedes in his bathtub when
he saw that an object displaced water equal in volume to that of the object. (Trying to
drown a rubber ducky perhaps?) On a geological scale, isostasy can be observed where
the Earth's strong lithosphere exerts pressure or stress on the weaker asthenosphere
which, over geological time, adjusts to accommodate.
Burden decreases
Asthenosphere moves underneath
Burden increases
Asthenosphere moves outwards
Chapter 12
Study Guide
Page 6
Assignments:
PART A
♦ Read pages 242 to 249.
♦ Use your text book and lab manual (Exercise 14) to complete these questions.
1. Distinguish between the plastic and brittle behavior of rocks.
2. Provide definitions and diagrams/sketches to distinguish the following:
faults and joints (fractures)
hanging wall and footwall
dip-slip faults (normal, reverse, thrust)
strike-slip faults (left lateral, right lateral)
transform faults
effects of compressional, tensional, and shear forces
anticlines, synclines, plunging folds, domes, basins, and over-turned folds
unconformity and angular unconformity
3. Define and diagram the dip and strike of an outcrop. (see Fig. 12.3)
PART B
♦ You will need pencil crayons, a pencil and a ruler to complete this exercise.
♦ Complete Exercise 14 in the lab manual using the models and worksheet provided.
Follow all instructions carefully.
PART C
1. Complete the Chapter 12 Worksheet and take-home quiz.
2. Read page 250 and answer question #6 on page 265. Attach to your take-home quiz
Chapter 12
Study Guide
Page 7