Chapter 9: Plate Tectonics - LPSisite.lps.org/mgeist/N-GE-U1-2.pdf · in places, producing mountain ... –A surface feature in the seafloor produced by the descending plates

Chapter 9: Plate Tectonics

• Continental Drift• Sea-Floor Spreading • Theory of Plate Tectonics


Section 9.1Continental Drift

• Continental drift– Hypothesis stating that the continents once formed a single

landmass, broke up, and drifted to their present locations• Pangaea

– The single landmass thought to have been the origin of all continents

– “All lands”• Panthalassa

– The giant ocean surround Pangaea– “All seas”

– Hypothesis developed by German scientist Alfred Wegener in 1915

• Believed Pangaea began breaking up into smaller continents about 200 million years ago

Chapter 9: Plate Tectonics-- Section 9.1: Continental Drift --

• Thought that this movement may have crumpled the crust in places, producing mountain ranges


– Evidence of continental drift• Fossil remains

– Identical fossil remains of Mesosaurus, a small, extinct, land reptile from 270 million years ago, were found in both eastern South America and western Africa.

– Fossils of seeds of a small plant called Glossopteriswere found in South America, southern Africa, and India.

– Fossils of the land reptile Lystrosaurus show a pattern of distribution similar to the Glossopteris fossils.

– There were no land bridges for reptiles to traverse and reptiles could not swim across the ocean, meaning South America and Africa were joined at one time.


• Geologic evidence– The age and type of rocks in the coastal regions of

western Africa and eastern South America in Brazil matched closely.

– The Appalachians that end along the eastern United States are of similar age and structure of those found in the British Isles and Scandinavia.

– If the three landmasses are put together in a model of Pangaea, the mountains fit together in one continuous chain.

• Changes in climatic patterns– Layers of debris from glacial deposits in southern

Africa and South America, both of which now have warmer climates. These indicated ice sheets covering those areas 220 million and 300 million years ago.


– Fossil evidence such as the coal deposits in the eastern United States, Europe, and Siberia indicate that tropical or subtropical swamps covered much of the land area in the Northern Hemisphere.

– Continents were once joined and positioned over the South Pole.

• Opposition to continental drift– The force causing continental drift could not be

explained.– Evidence for continental drift was later revealed by

seafloor spreading and the theory of plate tectonics.



Section 9.2Sea-Floor Spreading

• Seafloor spreading– Movement of the ocean floor away from either side of a mid-

ocean ridge– Began with mapping of the ocean floor utilizing sonar (sound

navigation and ranging) to discover two components of the ocean floor

• Deep-ocean trenches– A surface feature in the seafloor produced by the

descending plates– Most occur around the edges of the Pacific Ocean with

some also in the Indian and Atlantic oceans.• Mid-ocean ridges

– System of undersea mountain ranges that wind around Earth

– 70,000 km long

Chapter 9: Plate Tectonics-- Section 9.2: Sea-Floor Spreading --

Mid-ocean Ridges


Seafloor Spreading


– Mid-Atlantic Ridge• Undersea mountain range with a steep, narrow valley

along its center• Part of the mid-ocean ridges

– Discovered that the ocean floor was very young compared with the age of continental rocks

• None of the oceanic rocks found were more than 180 million years old.

• The oldest continental rocks are about 4 billion years old.– Renewal of the ocean floor

• Hypothesis by Harry Hess, American geologist• The valley at the center of the Mid-Atlantic Ridge was

actually a break, or rift, in Earth’s crust and that magma was welling up through the rift because the ocean floor was moving away from both sides of the ridge.


• As the ocean floor moved away from the ridge, it was replaced by rising magma that cooled and solidified into new rock.

– Hypothesis by Robert Dietz, American geologist• Named the movement of the ocean floor “seafloor

spreading”• Suggested that if the ocean floor was moving, the

continents might also be moving• Proof through paleomagnetism and earthquake patterns


Continental Drift


– Paleomagnetism• Study of the past magnetic properties of rocks• When magma cools and solidifies, iron in the rock

becomes magnetized and aligns when the rock hardens to point north.

• Many rocks from previous centuries have pointed south, suggesting Earth’s magnetic field must have reversed itself several times.

– Verified by dating rocks with different magnetic orientations

– Found on the ocean floor by the Mid-Atlantic Ridge as patterns of alternating bands of normal and reverse polarity of magnetic orientations, suggesting spreading on the ocean floor that has occurred for a long time.


– Dates of reversals on land and in the ocean were exactly the same, proving seafloor spreading and continental drift.


– Earthquake patterns• Work of scientists Kiyoo Wadati and Hugo Benioff

– Found a pattern when plotting the depth of earthquakes in relation to their distance from deep-ocean trenches

– Occur in a belt about 50 kilometers thick– No earthquakes detected below about 700 kilometers

• Result convinced scientists that major sections of ocean floor return to the mantle in subduction zones.



Section 9.3Theory of Plate Tectonics

• Plate tectonics– Theory that the lithosphere is made up of plates that float on

the asthenosphere and that the plates possibly are moved by convection currents

– Not only describes continental movement but also provides a possible explanation of how and why the continents move

– Tectonics: study of the formation of features in Earth’s crust– Two types of crust on Earth

• Oceanic crust: material that makes up the ocean floor• Continental crust: material that makes up landmasses

– Movement of plates• Lithosphere: thin outer shell of Earth consisting of the

continental and oceanic crusts and the rigid upper mantle

Chapter 9: Plate Tectonics-- Section 9.3: The Theory of Plate Tectonics --

– 30 lithospheric plates have been identified to date.– Movement of lithospheric plates

» Moving away from each other» Moving toward each other» Sliding past each other

• Asthenosphere: zone of mantle beneath the lithosphere that consists of slowly flowing solid rock, like putty when under pressure


Plate Tectonics


• Lithospheric plate boundaries– Divergent boundaries

• Boundaries formed by two lithospheric plates that are moving apart

• As plates move apart, magma from the asthenosphere rises and fills the space between the plates, cools, and hardens onto the edges of the separating plates, creating new oceanic crust.

• Most are found on the ocean floor and follow the mid-ocean ridges.

• Rift valley– Steep, narrow valley formed as lithospheric plates

separate– Often formed where continents are separated by plate

movement


– Example: Red Sea occupying a huge rift valley by the African and Arabian plates separating


– Convergent boundaries• Boundaries formed by two lithospheric plates that are

moving toward each other• Result in one of three kinds of collisions based on crusts

involved– Continental crust colliding with oceanic crust

» Since oceanic crust is denser, it will be subducted, or forced, under the less dense continental crust

» Subduction zone: region where one lithospheric plate moves under another

» Ocean trench: deep valley in the ocean that forms along a subduction zone


» Continental volcanic arc: mountains formed in part by volcanic activity caused by the subduction of oceanic lithosphere beneath a continent

» As the oceanic plate moves down into a subduction zone, it melts and becomes part of the mantle material.

» Some of the magma formed rises to the surface through the continental crust and produces volcanoes.


– Continental crust colliding with continental crust» Neither plate is subducted since they have similar

densities.» Edges are crumpled, uplifted, and create large

mountain ranges.» Example: Himalayan Mountain Range

– Oceanic crust colliding with oceanic crust» Deep ocean trench is formed when one is

subducted under another.» Part of the subducted plate melts, and resulting

magma rises to the surface along the trench to form an volcanic island arc (chain of volcanic islands formed along an ocean trench)


Convergent Plate Boundaries


– Transform fault boundaries• Boundaries formed by two lithospheric plates that are

sliding past each other• Do not slide smoothly, scrape each other, get caught up to

build up force, and eventually release, causing minor tremors to major earthquakes

• Example: San Andreas Fault


Transform Plate Boundaries



Section 9.4Mechanisms of Plate Motion

• Causes of plate motion– According to the theory of plate tectonics, the lithosphere is

broken into separate plates that ride on the denser asthenosphere with the asthenosphere being warped and moved based on convection currents (movement in a fluid caused by uneven heating in which hotter magma rises and cooler magma sinks)

– Convection: transfer of heat through the movement of a fluid material, such as magma

– Heat from Earth’s core and mantle cause some magma in the lower asthenosphere to become hotter and less dense, causing it to rise above the more dense material. When the hot magma reaches the base of the lithosphere, it cools, becomes more dense, and sinks again.

Chapter 9: Plate Tectonics-- Section 9.4: Mechanisms of Plate Motion --

– The cooling magma is pushed to the side by new hot magma that rises, causing the lithospheric plate to be carried along with the moving magma.

– Evidence for convection currents• Recent studies of the ocean floor in which heat flow is

higher along plate boundaries where two plates are separating that elsewhere on the ocean floor, explaining temperature differences.

• Questions of whether convection currents alone are strong enough to move the plates at rates suggested by geological evidence.

Chapter 9: Plate Tectonics-- Section 9.4: Mechanisms of Plate Motion --

Chapter 3: Rocks

• The Rock Cycle• Igneous Rocks• Sedimentary Rocks• Metamorphic Rocks

Chapter 3: Rocks

Section 3.1The Rock Cycle

• Magma– Hot, molten rock that originates inside the earth– Parent material for all rocks

• Types of rocks– Igneous rock

• Rock formed from cooled and hardened magma• Comes from the Latin meaning “from fire”• Is called “lava” if it cools at the earth’s surface• Examples: basalt, obsidian, pumice, granite

– Sedimentary rock• Rock formed from hardened deposits of sediments

– Fragments of rock that result from the breaking of rocks, minerals, and organic matter

– Carried away and deposited by water, ice, and wind

Chapter 3: Rocks-- Section 3.1: The Rock Cycle --

• The hardened deposits are cemented together over time.• Examples: conglomerate, sandstone, gypsum, halite,

coal, limestone– Metamorphic rock

• Rock formed from other rocks as a result of intense heat, pressure, or chemical processes

• Metamorphic = “changed form”• Examples: marble, quartzite, schist, gneiss, slate

• The rock cycle– Series of processes in which rock changes from one type to

another and back again– Steps of the rock cycle

• Once igneous rock has formed, a number of processes break down the rock into sediments.


• The sediments are compacted and hardened to form sedimentary rocks.

• The sedimentary rocks are subjected to high temperature and/or pressure within the earth to become metamorphic rocks.

• Heat and pressure increase to melt the metamorphic rock into magma to later form igneous rocks.

– Not all rocks go through a complete rock cycle.• Igneous rock may never be exposed to Earth’s surface

and therefore never be broken down into sediments to form sedimentary rock.

• Some igneous rock may receive enough pressure and temperature early on to become metamorphic rock.

• Other exceptions



The Rock Cycle

Chapter 3: Rocks

Section 3.2Igneous Rocks

• Two groups of igneous rock– Intrusive igneous rock

• Rock formed from the cooling of magma beneath Earth’s surface

• Intrudes, or enters, into other rock masses beneath Earth’s surface

• Examples: granite– Extrusive igneous rock

• Rock formed from molten lava that rapidly cools and hardens on Earth’s surface

• Extrudes, or exits, onto Earth’s surface• Examples: basalt

• Texture of igneous rocks– The size of the crystalline grains in igneous rock

Chapter 3: Rocks-- Section 3.2: Igneous Rocks --

– Largely determined by the cooling rate of the magma or lava that formed the rock

– Texture differs in intrusive igneous rock and extrusive igneous rock

• Intrusive igneous rock– Formation: magma cools and hardens slowly deep

underground– Crystalline grains

• Coarse-grained• Large because of long cooling time, allowing large

crystals to form– Examples: granite



Granite (Intrusive Igneous Rock)• Extrusive igneous rock

– Formation: lava cools and hardens rapidly on Earth’s surface

– Crystalline grains• Fine-grained• Small because of quick cooling, not allowing large crystal

grains to form– Examples: basalt



Basalt (Extrusive Igneous Rock)– Porphyry

• Igneous rock composed of large and small crystals• Formed from when magma cools slowly at first and then

more rapidly as it gets closer to the surface, causing large crystals caught in a mass of smaller crystals


– Other textures• Obsidian, or volcanic glass

– Extremely rapid cooling occurs where crystals cannot form

– Glassy appearance


• Pumice– During extremely rapid cooling, gases escaping from

the magma get trapped in the rock and form bubbles.– Some pumice can float on water because of the air in it

causing it to be less dense than water.


• Composition of igneous rocks– Mineral composition of an igneous rock is determined by the

chemical composition of the magma from which the rock develops.

– Three families of igneous rocks• Granite• Basalt• Diorite


GraniteFormation: formed from felsic magma, which is high in

silica

Mineral contents:• Orthoclase feldspar and quartz (light colored minerals)• Quartz• May also have plagioclase feldspar, hornblende, and muscovite

mica

Color: generally light-colored

Examples: granite (cg), rhyolite (fg), obsidian (“volcanicglass”)



Granite Rhyolite


Obsidian

BasaltFormation: formed from mafic magma, which is high in

iron but low in silica

Mineral contents:• Plagioclase feldspar and augite• May also have contain olivine, biotite mica, and hornblende (dark

colored ferromagnesian minerals)

Color: generally dark-colored

Examples: basalt (fg), gabbro (cg)



Basalt Gabbro

DioriteMineral contents:• Plagioclase feldspar, hornblende, augite, and biotite mica• Contains little to no quartz

Color: generally medium-colored

Examples: diorite (cg), andesite (fg)



Diorite Andesite

• Igneous Rock Structures– Intrusions (underground rock masses made up of intrusive

igneous rocks)• Batholith

– Largest type of igneous intrusion, covering over 100 km2 and reaching a depth of thousands of meters

– Means “deep rock”– Form the cores of many major mountain ranges, such

as the Sierra Nevada in California and the Coast Range in British Columbia



Batholith

• Stock– Igneous intrusion with an area less than 100 km2

– Similar to a batholith but much smaller



Composite of formations

• Laccolith– Flat-bottomed intrusion that pushes overlying rock

layers into an arc– Means “lake of rock”– Sometimes can be identified by the small dome-

shaped mountains they push up on Earth’s surface– Many are located beneath the Black Hills of South

Dakota.


• Sill– Sheet of hardened magma that forms between and

parallel to layers of rock– Vary in thickness from a few centimeters to hundreds

of meters and can extend horizontally for several kilometers

– Many are located in Big Bend National Park in Texas.



Sill • Dike– Igneous intrusion that cuts across rock layers– Common in areas of volcanic activity– Differ from sills because they cut across rock layers

instead of parallel to them.



Dikes


Summary of Intrusions

– Extrusions (surface rock masses made up of extrusive igneous rocks)

• Volcano– Formed when lava erupts onto Earth’s surface and

forms a cone of extrusive igneous rock surrounding a central vent through which lava can flow

– “The Ring of Fire”



Volcano

• Volcanic neck– Solidified central vent of a volcano– Created when a volcano stops erupting for a long

period of time, the lava in the central vent cools and solidifies, and the cone surrounding it deteriorates to leave the hardened lava

– Shiprock in New Mexico is an example.



Dike and Volcanic Neck

• Lava plateau– Raised, flat-topped area made of layers of hardened

lava– Formed from lava that flows out of long cracks in

Earth’s surface, spreads out over a vast area, and fills valleys and covers hills



Arizona Lava Plateau

Chapter 3: Rocks

Section 3.3Sedimentary Rocks

• Formation of sedimentary rock– Sedimentary rock is formed from the accumulations of

different types of sediment.– Processes that form sedimentary rock

• Compaction– Process in which air and water are squeezed out of

sediments– Caused by the weight of overlying sediments to exert

pressure to squeeze out the air and water• Cementation

– Process in which dissolved minerals left by water passing through sediments hold the sediments together

– “Glue between rocks”

Chapter 3: Rocks-- Section 3.3: Sedimentary Rocks --

• Classes of sedimentary rock Clastic sedimentary rock

• Rock made up of fragments of pre-existing rocks• Fragments carried away from their source by water, wind,

or ice that are then deposited• Clastic sedimentary rocks are classified by the size of the

sediments they contain.– Gravel-sized fragments– Sand-sized fragments– Clay-sized fragments

• Gravel-sized fragments– Conglomerate

» Sedimentary rock composed of rounded gravel or pebbles cemented together by minerals


» Individual pieces of sediment can easily be seen.


– Breccia» Elastic sedimentary rock composed of angular

fragments cemented together by minerals


• Sand-sized fragments– Called sandstones– Quartz is the major component of sandstone.– Has pores through which groundwater and crude oil

can move


• Clay-sized fragments– Called shale– Particles cemented together under pressure into flat

layers that are flaky and will easily split apart.


Chemical sedimentary rock• Rock formed from minerals that have been dissolved in

water• Types

– Rocks formed from dissolved minerals that precipitate (settle) out of the water as a result of a change in temperature

» Limestone


– Evaporites» Rocks formed when water evaporates and leaves

behind the minerals dissolved in the water» Examples - gypsum (on the left) and halite (on the

right)


Biochemical sedimentary rock• Rock formed from the remains of organisms• Examples

– Coal (shown on the left)– Limestone (shown on the right)– Chalk


• Sedimentary rock features– Stratification

• Layering of sedimentary rock• Occurs when there is a change in the kind of sediment

being deposited• Beds (stratified layers) vary in thickness depending on

how long each type of sediment was being laid down.• Cross-bedding

– Layers not stratified horizontally– Often occur as a result of sediments being deposited

by wind



Cross-bedding

• Graded bedding– Occurs as different sizes and shapes of sediment

settle to different levels– Largest and heaviest grains generally settle at the

bottom.


– Ripple mark• Formed by the action of wind or water on sand• May be preserved when sand becomes sandstone


– Mud crack• Occur when muddy deposits dry and shrink, causing the

mud to crack• Occur in river flood plains and dry lake beds• When area is flooded again, new deposits fill in the cracks

and preserve them as the mud hardens to become solid rock.


– Fossil• Trace or remains of an organism in sedimentary rock• Occurs as a result of sediments burying remains or

leaving impressions in the rock


– Concretion• Nodule of rock with a

different composition from that of the main rock body

• Geodes– Crystal cavities

found in rocks– Formed by

groundwater depositing dissolved quartz or calcite inside cavities of rocks that then crystallize


Chapter 3: Rocks

Section 3.4Metamorphic Rocks

• Metamorphism– Changing one type of rock to another by heat, pressure, and

chemical processes– Most occurs deep beneath the surface of Earth where heat

and pressure are the greatest.• All metamorphic rock is formed from existing igneous,

sedimentary, or metamorphic rock.• Types of formation of metamorphic rock

– Contact metamorphism• Change in the structure and mineral composition of a rock

surrounding an igneous intrusion• Heat from magma of the intrusion changes the structure

and mineral composition of the surrounding rock.

Chapter 3: Rocks-- Section 3.4: Metamorphic Rocks --

– Regional metamorphism• Metamorphism that affects rocks over large areas during

periods of tectonic activity• Occurs over an area of thousands of square kilometers• Tectonic plates generate great heat and stress to change

rock.• Relates to contact metamorphism since volcanism and

magma movement often go with tectonic activity• Classification of metamorphic rocks

– Foliated rock• Rock that has visible parallel bands• Ways to form


– Extreme pressure may flatten mineral crystals in the original rock and push them into parallel bands.

– Minerals of different densities separate into bands, producing alternating light and dark bands.

• Examples– Slate– Schist– Gneiss




Foliated slateFoliated gneiss • Slate can turn into schist with greater heat and pressure, and even more heat and pressure changes schist into gneiss.

– Unfoliated rock• Rock that does not have visible bands• No bands of crystals• Examples

– Quartzite (from the sedimentary rock sandstone)– Marble (from the sedimentary rock limestone)



MarbleQuartzite

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Chapter 9: Plate Tectonics - LPSisite.lps.org/mgeist/N-GE-U1-2.pdf · in places, producing mountain ... –A surface feature in the seafloor produced by the descending plates