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Rocks
Chapter 6
3 Types of Rock
• Igneous – forms when magma cools
• http://geology.com/rocks/igneous-rocks.shtml
• Sedimentary – forms when other rocks are
eroded and the sediments are deposited and
cemented together
• http://geology.com/rocks/metamorphicrocks.shtml
• Metamorphic - forms when rock undergoes
intense heat and pressure
• http://geology.com/rocks/metamorphicrocks.shtml
Rock Cycle
• The types of rock can be changed into any
other type
• Igneous
– Can become sedimentary if it goes through
EROSION, DEPOSITION, AND
CEMENTATION
– Can become metamorphic if it goes through
INTENSE HEAT AND PRESSURE
Rock Cycle
• Sedimentary
– Can become igneous if it MELTS AND
COOLS
– Can become metamorphic if it goes through
INTENSE HEAT AND PRESSURE
• Metamorphic
– Can become igneous if it MELTS AND
COOLS
– Can become sedimentary if it goes through
EROSION, DEPOSITION, AND
CEMENTATION
Bowen’s Reaction Series
Continental Drift
Section 10.1
Alfred Wegener
• 1912
• Hypothesis of continental drift said that
landmass was once a supercontinent,
Pangaea
• Began breaking up 250 mya
• May be reason for mountain ranges
Evidence for Continental Drift
• Similarities in coastlines
• Fossils
– Ex: mesosaurus found in South America and
western Africa
• Rock formations
– Ages and types of rocks in western Africa and
eastern S. America
– Continuing mountain chains
Evidence for Continental Drift
• Climatic evidence
– Glacial debris found in southern Africa and
South America
– Coal found in Siberia
Why the Hypothesis was Opposed
• Lacked a mechanism
• Died in 1930 before he could find it
Redeeming Evidence
• Mid-Ocean Ridges
– 1947, mapping of Mid-Atlantic Ridge
– Found that sediment nearest the ridge is
thinner so sediment settled on outer regions
for a longer time
– Fossils closer to the ridge are younger
– Oceanic rocks only 175 myo while continental
rocks are 3.8 byo
Sea-Floor Spreading
• 1950s, Harry Hess
– Valley that center of ridge is a rift in Earth’s
crust and magma rises to fill the crack
– Process named by Robert Dietz
– Since ocean floor moving, continents must be
moving, too
– This could be the missing mechanism for
continental drift
Even More Evidence
• Paleomagnetism
– As magma cools, iron-rich sediment aligns
with magnetic field of Earth
– It has been found that the magnetic field of
the Earth has changed through time
– Pattern of alternating normal and reversed
polarity in rocks
– Striped magnetic pattern was mirror image on
both sides of rift
Wegener Redeemed
• Magnetic reversals in rocks found on
continental rocks, too.
• Now, evidence was present and so was
the mechanism.
The Theory of Plate
Tectonics
Chapter 10.2
Plate Tectonics
• Theory that explains why and how
continents move
• Study of formation of features on Earth’s
crust
How Continents Move
• Lithosphere broken into tectonic plates
that ride on the asthenosphere
• Oceanic crust – dense, rich in Fe and Mg
• Continental crust – low density, rich in
silica
Tectonic Plates
• 15 major plates
• Knowledge from studying earthquakes
• Location of volcanoes
Types of Boundaries
May be in the middle of ocean floor, edges
of continents, or within continents
3 major types: divergent, convergent,
transform fault
Divergent Boundaries
• 2 plates move away
from each other
• Magma rises to
surface, cools, and
forms new oceanic
lithosphere
• Forms mid-ocean
ridges and rift valleys
Convergent Boundaries
• 2 plates collide
• 3 types:
– Oceanic-continental
where oceanic
subducts
• Deep ocean trenches
and volcanic mountains
Convergent Boundaries
– Continent-continent
where neither plate
subducts
• Forms large mountain
ranges
Convergent Boundaries
• Oceanic-oceanic
where one plate
subducts under the
other
– Forms deep-ocean
trenches and island
arcs (chain of volcanic
islands)
Transform Fault Boundaries
• 2 plates slide past
each other
horizontally
• Do not produce
magma
• Fracture zones at
mid-ocean ridges are
transform fault zones
Causes of Plate Motion
• Convection
Causes of Plate Motion
• Ridge Push
Causes of Plate Motion
• Slab Pull
– As lithosphere pulls from the mid-ocean ridge,
it cools and sinks
– The leading edge pulls the rest of the plate