Geologic Structure and Mountain Building

What do current mountain belts coincide with from the past?

present/past tectonic boundaries

What is the Law of Superposition?

younger rock layers sit atop older layers

What is the Law of Original Horizontality?

layers of sedimentary rock are originally deposited flat

What is the Law of Crosscutting Relationships?

rock layers A and B must be older than the intrusion in them

What is the Law of Lateral Continuity?

layers of rock are continuous until acted on by other solid that blocks them or other forces that degrade them

What does deformity change?

geometry of the rocks

What do rock formations look like when undeformed?

horizontal beds, spherical mineral grains, no faults or folds

What do rock formations look like when deformed?

tilted beds, often metamorphized rocks, elliptical mineral grains, folding and faulting

What is displacement?

change in location of rock unit

What does displacement look like?

bed slipped apart, does not line up anymore

What is rotation?

change in orientation of rock unit

What is distortion?

change in shape of rock unit (buckles)

What is stress?

force applied across a unit area

What causes greater deformation in relation to stress?

force applied to small area creates more deformation than same force on large area

What are the 3 kinds of stress?

compressive, extensional, shearing

When does compressive stress take place?

when an object is squeezed

What does horizontal compression result in?

mountain belts

What does compressive stress do other than make mountain belts?

shortens and thickens crustal material

When does extension stress occur?

when the ends of an object are pulled apart

What does horizontal extensional stress drive?

continental rifting

What does extensional stress cause/do other than create rift zones?

stretches and thins crust, tilts/rotates rock blocks in thinned area

When does shear stress occur?

when surfaces slide past each other

What does shear stress do?

moves crustal material from its original location

What is strain?

change in shape caused by stress

What are the three types of strain?

shortening/contraction, stretching/extensional/elongation, shear

What are the two types of rock deformation?

brittle and ductile

Where does brittle formations break?

along fractures

What do ductile formations have that brittle formations don’t?

large scale folds

What does brittle vs. ductile depend on?

depth, temperature, pressure, deformation rate, rock composition

At what depth are formations brittle?

<10-15km

At what depth are formation ductile?

>10-15km

What is brittle deformation?

rocks break by fracturing due to stress

What is ductile deformation?

rocks behave like fluids, flow and fold in response to stress

What are fractures or joints?

planar fractures where there is no movement/offset of layers

What do fractures or joints develop from?

tectonic stresses in shallow rocks

What are mineral veins?

fractures or joints filled with minerals

Where do minerals come from in mineral veins?

dissolved minerals in groundwater

What are faults?

planar fractures showing movement/displacement/offset of layers

What is the hanging wall?

block above the fault plane

What is the footwall?

block below the fault plane

What is a normal fault?

hanging wall moves down relative to footwall, results from extensional stress

What is a reverse fault?

hanging wall moves up relative to footwall, results from compressive stress, slope of fault plane >45 degrees

What is a thrust fault?

slope <45 degrees, hanging wall moves up, compressive stress, common in mountain belts

What is a strike-slip fault?

one block slides past, fault plane almost vertical, results from shearing

What is a left-lateral strike slip fault?

block on other side of fault plane moves left

What is a right lateral slip strike fault?

block on the other side of the fault moves right

What is Case 1 of mountain building processes?

Basin and Range Province, hundreds of normal faults

What is a thrust fault?

large-scale, older rocks on top, seen in mountain belts

What is Case 2?

Brevard Fault, AL-VA, Appalachian Mountains

What is Case 3?

thrust faults in Smoky Mountains

What is Case 4?

Cades Cove, part of Great Smoky Fault

What is Case 5?

San Andreas Fault, laterally offset streams and creeks

What does brittle faulting cause/result in?

shattered and crushed rock

What are slicken lines?

striations on fault plane indicating slip direction

What are scarps?

part of fault plane exposed on surface

What do folds consist of?

anticlines and synclines

Which way does an anticline close?

closes up (upside down v)

Which way does a syncline close?

closes down (normal v)

What is an axial plane?

plane that divides fold symmetrically, connects hinge lines in successive layers

What is the fold axis/hinge line?

line of intersection of axial plane with fold

What is a limb?

two arms of fold across axial plane

What are block diagrams?

help us visualize folded structures that have been eroded

In a map view, what do anticlines expose?

older rocks on hinge

In a map view, what do synclines expose?

younger rocks on hinge

What is a dome?

fold pattern that looks like overturned bowl

What is a basin?

a fold shaped like a bowl

What do the rock layers look like in a dome?

dip radially outwards, oldest rocks at center

What do the rocks look like in a basin?

radially dip inwards, youngest rocks at center

What else can you call mountains made by continent-continent collision?

fold-thrust belts