Regents Earth Science – Unit 11: The Dynamic CrustCrustal Changes

Principle of Original Horizontality - sedimentary rocks form in a horizontal position

• any change in the horizontal position is evidence of crustal change

horizontal

tilted folded faulted

• Mountains with sea fossils indicate that the crust has been uplifted

• Fossils of shallow water animals found at great depths in the oceans indicate

crust “submergence”

Patterns of Crustal Movements

When plotted on a map, earthquakes, mountains, and volcanoes all occur in the same areas

• called zones of crustal activity

EarthquakesEarthquakes are the sudden movement of the Earth's crust

• most earthquakes occur along plate boundaries

Causes of Earthquakes:

1. Pressure due to crustal plate movement

2. Pressure “Unloading” due to the retreat of glaciers

3. Other sources of pressure

Reference Tables p. 5

FW HW

Earthquakes

Earthquakes occur at Plate Boundaries:

1. Divergent - shallow, minor earthquakes

2. Convergent - deep, strong earthquakes

3. Transform - shallow, moderate earthquakes

Three Types of Faults Associated with Plate Boundaries:

1. Normal Fault

2. Reverse Fault

3. Strike-Slip

Fault

Faults1. Normal Fault - associated with divergent plate boundaries

• footwall moves up

• hanging wall moves down

2. Reverse Fault - associated with convergent plate boundaries

• footwall moves down

• hanging wall moves up

• a low angle reverse fault is called a thrust fault

Faults

3. Strike-Slip Fault - associated with Transform plate boundaries

Seismic WavesFault - break in the rock of the Earth's crust where movement has occurred

Focus - point beneath the Earth's surface where fault movement releases seismic energy (waves)

Epicenter - point on Earth's surface directly above the focus

Seismic Waves – the energy released by the earthquake

The instrument used to record and measure these waves is called a seismograph

• The recording made is called a seismogram

Seismic Waves

When an earthquake is recorded, two waves of energy appear

• there are two kinds of waves the energy travels in:

compression waves and shear (transverse) waves

Types of Seismic Waves:

1. P-Waves – compression wave

• primary wave – 1st to arrive at a seismic station

• travel through solids and liquids

• travel fast

• particle motion is in the same direction as wave movement

• do little damage

2. S-Waves – shear wave

• secondary wave – 2nd wave to arrive at a seismic station

• travel only through solids

• travel slow

• particle motion is perpendicular to wave motion

Note: when the speeds the waves travel are plotted, the

difference in the times they arrive can be found, and thus the

distance from their starting point - the epicenter of an

earthquake

Locating EpicentersBecause seismic waves travel at different speeds, we can determine the distance from an earthquake

• as 2 objects move at different speeds, the farther they move, the farther they get from each other

Finding Epicenters:

1. Find the difference in arrival times

of P and S waves on seismograms

from 3 different recording stations

• P-wave arrival time = 9 hours 24 minutes

• S-wave arrival time = 9 hours 27 minutes

• Difference in arrival time = 3 minutes

Locating Epicenters

2. Find the distance to the epicenter by using the arrival time difference and the P and S wave Travel Time chart in Reference Tables:

Distance to epicenter = 1800 km

Reference Tables p. 11

3. Use the map scale to find the radius of a circle that equals thedistance to the epicenter

• 3 plots are needed to pinpoint where the epicenter is

epicenter

difference = 3 minutes

Locating Epicenters

4. To determine the origin time, find how long the P-wave

took to travel from the epicenter's distance and subtract this

time from the arrival time of the P-wave

• P-wave travel time = 3 min 40 sec

• P-wave arrival time = 9 hrs. 24 min. (from step 1)

9 hrs. 24 min - 3 min. 40 sec. = 9 hrs. 20 min. 20 sec

• Origin Time = 9hrs. 20 min. 20 sec.

1800 km = 3 min., 40 sec P-wave travel time

3 min 40 sec. P-wave

travel time

Earthquakes

Richter Scale - scale used to express the strength (energy released) by and earthquake

• scale ranges from 1 to 10

• each increase in magnitude is a ten

times increase in energy

3 min 40 sec. P-

wave travel

time

Earthquakes

Mercalli Scale - scale used to show the damage caused by an earthquake

• scale ranges from

I to XII.

TsunamiTsunami - gigantic sea wave caused by an earthquake on the ocean floor

• travel fast: 400-500 mph

• 50-100 ft. height

Earth's Interior

Seismic waves are used to infer Earth's interior:

• P-waves refract (bend) when passing into a

material of different density

• S-waves cannot pass through liquids

P-wave S-wave

Earth's interior has 4 major zones (based on seismic waves)

• Outer Core is liquid (S-waves do not pass through)

• temperature, pressure, and density increases towards the center

1. Crust - outermost, least dense

2. Mantle - heat is transferred by convection

3. Outer Core - metallic, liquid

4. Inner Core - metallic, most dense

Reference Tables p.10

Earth's Interior

• Earth's Structure:

• Lithosphere - rigid crust and upper mantle

• Asthenosphere - plastic-like layer of the mantle

Volcanoes

Volcano - cone-shaped mountain built of lava and/or volcanic ash

Types of Volcanoes:

1. Shield - form from lava flows, found at hot spots

2. Cinder Cone - explosive, found at convergent boundaries

3. Composite - explosive and non-explosive, found at convergent

boundaries

Volcanoes

• Location of volcanoes:

1. Plate Boundaries:

2. Hot Spots: Hawaiian Islands

• ocean crust moves over a mantle plume

• islands form over the plume

• islands furthest from the plume are oldest

Volcanic Features

• Igneous Intrusion - molten rock that moves

up/through preexisting rock and cools and

solidifies

Continental DriftContinental Drift - idea that the continents move over the surface of the Earth like rafts in water

• proposed by Alfred Wegner in early 1900's

• continents were once a single large landmass called Pangea

Evidence for Continental Drift

1. Coastlines of Continents - fit together like a "jig-saw" puzzle

Evidence for Continental Drift2. Fossil Clues - fossils of ancient life found on widely

separated continents

• Mesosaurs - fresh water reptile

• Glossopteris - ancient seed fern with heavy seeds

(too heavy for wind transport)

3. Rock Clues - similarities in rock types and mountain ranges

Evidence for Continental Drift

4. Climate Clues - rocks found today near equator have evidence of glaciation

• coral limestone - lived in tropical seas is found

today in New York

• coal forms from plants that lived in warm,

swampy environments is found in cold climates

of N. America and Antarctica

5. Paleomagnetic Clues - Earth's North Pole appears to wander as we look back in time with the continents in their present positions (A)

• If the land masses are shifted back in time to where the rocks would have been when they formed, the poles don’t appear to wander at all (B)

Wegener’s Evidence:

1. “Jig-Saw Continents”

2. Matching Mountains/Rock Layers

3. Fossil Clues

4. Climate anomalies

5. Polar Wandering

• Scientists did not take his theory seriously, because he couldn’t explain how continents could “drift across oceans”

• his theory was “shelved” for over 50 years before new evidence was found that ultimately supported his idea and even expanded on it

Sea Floor Spreading

After WWII, scientists set out to map the ocean floors, and

in the process, found some interesting data that

would explain Wegener’s theory

• Scientists mapped the ocean floor and noticed it contained huge mountain ranges, underwater volcanoes, and trenches

Sea Floor Spreading - principle that oceanic crust spreads out at mid-ocean ridges

Evidence for Sea Floor Spreading1. Age Evidence - as distance from mid-ocean ridge increases, the age of the rock increases

Evidence for Sea Floor Spreading

2. Magnetic Evidence - magnetic anomalies in

the iron bearing basalt rock of the ocean floor

• rocks that formed during

different time periods were

magnetized to opposite

polar directions, and

matched on opposite sides

of the mid-ocean ridges

• 3. Iceland –Iceland is located on the Mid-Atlantic Ridge

• new crust is formed by volcanic eruptions

• the islands can be measured and increase in size about the same rate as your fingernails grow!

Sea Floor Spreading could explain the movement of Wegener’s continental drift and opened the

door to understanding how the continents could have moved and why climates and fossils of

today vary from the past

• this theory could not explain why the sea floor was split and being pulled and stretched apart

Plate Tectonics

Plate Tectonics - theory that the Earth's surface is composed of about a

dozen rigid plates that carry the continents as they move relative to

one another

• unifies the theories of continental drift and sea-floor

spreading

• lithospheric plates (crust and rigid mantle) float on

the asthenosphere (plastic-like layer of the mantle

where heat is transferred by convection)

• Lithospheric Plates:

Reference Tables p.5

• Earth's interior is hot - heat must escape

• heat is transferred to the surface by

convection

• Convection in the Earth's mantle is the

driving force behind plate tectonics

• Three Types of Plate Boundaries:

1. Divergent

2. Convergent

3. Transform

Plate Boundaries

1. Divergent Plate Boundary - where two plate move apart

• occurs at mid-ocean ridges and rift valleys

ex.: Mid-Atlantic Ridge

2. Convergent Plate Boundary - where two plate come together/collide

• subduction - ocean plate (more dense, basalt 3.0g/cm3) sinks underneath a continental plate (less dense, granite 2.7g/cm3)

ex.: Nazca Plate and S. America ex.: India and Asia

3. Transform Plate Boundaries - two plates slide side horizontally relative to one another

ex.: San Andreas Fault