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UNIT 7 Notes: The Forces the Effect Earth
Earth’s Interior
The Science of Geology
Geology is the study of planet Earth. Scientists that study the
forces that make and shape Earth are called geologists.
Geologists study the chemical and physical characteristics
of rock.
Studying Surface Changes
There are two forces that change Earth’s surface features:
1.Constructive forces: shape the surface by building surface
features. (To help remember constructive forces, think about
a construction site. At a construction site they are building
buildings, so constructive forces build surface features.)
Destructive forces: slowly wear away surface features. (To
help remember destructive forces, think about the word
destroy. Destructive means to destroy features.
Indirect Evidence-Seismic Waves
Geologists are not able to travel to the center of Earth
because the temperature is too hot and the pressure is too
great. Geologists must relay on other evidence to study the
Earth’s interior. Geologists study how seismic waves
travel through the Earth to identify the layers of the
Earth. Seismic waves travel faster through solids and
slowest through gases. By measuring seismic waves
speed, geologists can determine the interior of the Earth.
A Journey to the Center of Earth
Temperature and pressure both increases from the surface to
the center.
The Earth has three main parts:
1. The crust is a layer of rock that forms Earth’s outer skin.
There are two types of crust basalt (the crust under the
ocean) and granite (the crust under the continents),
-Basalt is a dark, dense rock making the oceanic crust.
-Granite is a light-colored rock with large crystals making
continental crust.
2. The mantle is a layer of hot rock between the crust and
core. The lithosphere is a rigid layer that includes the
upper mantle and the crust. The astenosphere is a soft
layer just below the lithosphere in the mantle
3. The core is made of iron and nickel. The core is two parts,
the outer core and inner core.
- Outer core: a layer of molten metal surrounding the inner
- Inner core: a dense ball of solid metal
Earth’s Magnetic Field
The rising and sinking of the hot liquid metal in the outer core
creates convection currents. Convection currents in the
liquid outer core force the inner core to spin faster than the
rest of Earth creating Earth’s magnetic field.
Convection Currents and the Mantle
Heat transfer is the movement of energy from a warmer object to a cooler object.
Three types of heat transfer:
– Conduction
– Convection
– Radiation
• Conduction: heat transfer by direct contact
Convection: heat transfer by the movement of a heated fluid.
 Convection is caused by temperature and density differences.
– Density: how much mass there is in a volume of a substance.
– Increasing temperature decreases density
 Convection Currents: Circular motion of fluid caused by the
rising and sinking of heated and cooler fluid
• Radiation: energy transfer by electromagnetic waves.
Convection in Earth’s Mantle
Heat from Earth’s core and from the mantle itself causes the convection
currents in the mantle.
Drifting Continents
Continental Drift
In 1910, Alfred Wegener hypothesized that all the continents were once
adjoined and have since drifted apart.
Pangaea: super continent.
Continental drift: continents slowly moved over Earth’s surface.
Evidence of Continental Drift
 Example: South Africa and Argentina mountains align
 Example: Europe and North America coal fields align
Fossils: traces of organisms preserved in rock
 Similar fossils located on different continents
 The climate of specific locations has changed because the
position of these places on Earth’s surface changed.
 Geologists in the early 1900s did not accept Wegener’s hypothesis.
 Wegener could not provide a satisfactory explanation for the force that pushes
or pulls the continents.
Sea-Floor Spreading
Mid-Ocean Ridge
 Mid-Ocean Ridge: longest mountain chain in the world
 The top of the mid-ocean ridge is split by a steep-sided valley.
 Sea-Floor Spreading: the process of adding new material to the ocean floor.
Evidence for Sea-Floor Spreading
 Magnetic stripes
 Ocean floor rock lies in patterns of magnetized strips.
 Drilling samples
 The Glomar Challenger took samples of rock.
 The farther from the ridge, the older the rocks.
Subduction at Deep-Ocean Trenches
 Deep-ocean trenches: downward bend in the oceanic crust
 Subduction: the process of the ocean floor sinks beneath a deep-ocean trench
and back into the mantle.
 Gravity pulls the older, less dense oceanic crust beneath the trench.
Subduction and Earth’s Oceans
 Pacific Ocean
 Shrinking because the many trenches swallows more oceanic crust than
 Atlantic Ocean
 Expanding because it only has a few short trenches.
The Theory of Plate Tectonics
A Theory of Plate Motion
 Plates: separate broken sections of the lithosphere.
 Plate tectonics: theory that pieces of Earth’s lithosphere are in constant, slow
motion, driven by convection currents in the mantle.
– Explains formation, movement, and subduction of Earth’s plates.
Plate Boundaries
Plate boundaries: lines where pieces of lithosphere meet
 Faults: breaks in Earth’s crust where rocks have slipped past each other.
 Transform boundaries: two plates move past other in opposite directions
– Earthquakes
 Divergent Boundaries: Two plates move apart
– Ocean
 Mid-Ocean Ridge
– Land
 Rift valley: a deep valley formed at divergent boundaries on land
The Continents’ Slow Dance
 Convergent boundaries: Two plates move together
– Collision results
Density determines the top plate
 Oceanic/oceanic plates –subduction occurs
 Oceanic/continental–oceanic plate sinks.
 Continental/continental– mountain ranges form
 Plates move about 1 to 10 cm per year.
Changing Earth’s Surface
Forces in the Lithosphere
Plate movements produces stress in rock.
– Stress adds potential energy to the rock until it changes shape or breaks and
– Stress leads to deformation ( a change in the rock’s shape or volume).
Forces in the Lithosphere
 3 types of deformation
– Shearing: rock slips in opposite directions
– Tension: pulls rock
– Compression: pushes rock together
Fault Movements
Faults usually occur along plate boundaries (where lithosphere breaks).
– Strike-Slip Faults: rock on either side of the fault slip past each
other sideways.
– Normal Faults: fault is at an angle
 Hanging wall: half of the fault above
 Footwall: half of the fault below
– Reverse Faults: formed by compression forces and blocks toward each other
Mountain building caused by:
 Plates collide
 Fault-block mountains: blocks of crust slid along normal faults to form
Land Subsidence
Land subsidence occurs when land surface sinks as a result of geologic
processes or human activities.
– Plate movements can cause the formation of rift valleys and ocean basins.
– As uplift raises one part of the crust, land subsidence occurs in an adjoining
Volcanic Mountains
 Volcano: a weak spot in the crust where molten, rock-forming magma comes to
the surface.
 Volcanic activity builds mountains made of lava rock and other volcanic
Locations of Volcanoes
 Diverging Plate Boundaries
– Rift Valley
– Mid-Ocean Ridge
 Converging Boundaries
– Subduction occurs
 Hot Spot Volcanoes
– An area where magma from mantle melts through the crust