Chapter 2
Journey to the Center of the
Earth
•
The Earth’s
magnetic field is a
dipole magnet. This
means it has a north
and a south pole.
The force generated
by a magnetic field
repels charged
particles.
• Magnetic field
lines show
trajectories along
which magnetic or
charged particles
would flow.
•
The solar wind contains electrically charged particles that are deflected by the
Earth’s magnetic field. The magnetosphere is the region inside this shield.
•
The Van Allen radiation belts (located 3,000 km and 10,500 km out from the
Earth) are important because they trap those solar wind particles and cosmic
rays that get through the magnetosphere that would otherwise bombard the
Earth with dangerous radiation.
The atmosphere is a very thin
layer that surrounds the earth
The atmosphere is mostly nitrogen and oxygen with a small proportion of trace
gases (labeled as other gases on the diagram). Trace gases consist of compounds
like carbon dioxide (CO2) and methane (CH4). Although trace gases are present in
very small amounts, they play important roles in climate warming because they
tend to absorb and reflect back to the surface long-wave radiation, thus promoting
heating..
Venus and Mars
both have
atmospheres
made up of CO2
gas.
Pressure and temperature increase with depth in the
Earth. At the center of the Earth, pressure is 3.6
million times greater than at the surface and
temperatures can reach 4,300oC, nearly as hot as the
Sun’s surface. The rate of temperature change with
depth is called the geothermal gradient.
• The troposphere is the lowest layer of the atmosphere and
this is where air undergoes convection. Convection refers
to a circulation pattern induced by temperature differences
in a fluid. Warm air (fluid) rises, cooler air (fluid) sinks.
Convective cells also occur deep within the earth where
warm rock rises, then sinks as it cools. Most geologists
believe this process is what controls plate movement.
• Land covers about 30% of the Earth’s surface, oceans 70%.
• Groundwater is all surface water plus the water that fills
the openings underground. The hydrosphere is that plus the
water vapor in the atmosphere.
•The topography of the Earth shows different kinds of landscapes such as
mountains, valleys, and plains. Similar topography also exists beneath the
oceans: submarine plains, oceanic ridges (mountains), and deep trenches
(valleys).
•The solid earth is
mostly made of iron
(35%), oxygen
(30%), silicon
(15%), and
magnesium (10%).
The remaining 10%
consists of the other
88 naturallyoccurring elements.
The lighter elements
were blown away
by the solar wind
during the formation
of the Earth and
other planets.
Categories of earth materials
•
Carbon-containing compounds are generally called organic chemicals (oil,
protein, fat, plastic, and rubber). However, carbon-containing compounds such as
pure carbon (C), carbon dioxide (CO2), carbon monoxide (CO), lime (CaO)and
calcium carbonate (CaCO3) are considered inorganic.
Some geologic vocabulary to know:
•
Minerals are a solid inorganic substance where the atoms are arranged in an
orderly pattern or crystalline lattice. Some common minerals include quartz,
feldspar, diamond, calcite.
•
Precipitation occurs when dissolved atoms come together to form a solid.
•
A crystal is a single coherent example of a mineral.
• A grain is a fragment of a mineral.
• Rocks are aggregates of crystals or grains.
• Igneous rock crystallize or solidify from molten rock (magma if below
the ground, lava if above the ground).
• Sedimentary rock arise from the cementation of loose grains (sand,
mud, etc.) and through chemical precipitation (from the ocean or bodies
of water).
• Metamorphic rocks arise from heat and pressure-induced alteration of
existing rock (without melting).
• Glasses are physically solid structures in which the atoms are disordered
(for example, obsidian, commercial glass). These materials were cooled
so quickly from the molten to the solid state that the atoms didn’t have
time to form a crystalline lattice structure.
Categories of earth materials (cont’d)
• Metals are solids comprised of metallic elements only such as gold, iron, and
copper.
• Melts are hot liquids that crystallize at surface temperatures to form igneous
rocks. When they occur within the Earth, they are called magma. Extruded
melts are called lava.
• Volatiles are substances that are stable in a gaseous state at the Earth’s
surface.
The Earth’s Interior – a layered sphere
• The Earth’s interior consists of a series of layers. Geologists identify three
main layers: (1) the crust, (2) the mantle, and (3) the core. The crust is a
relatively thin skin (7-10 km beneath oceans, 25-70 km beneath the land
surface) that lies over the mantle.
• There are two kinds of crust:
• Oceanic crust consists of an iron-rich and silica poor type of rock
called basalt (fine-grained, above the surface) or gabbro (coarsegrained, below the surface). Iron-rich rocks are also known as mafic or
ultramafic rocks.
• Continental crust tends to be silica-rich and therefore is lighter than
ocean crust. Continental crust “floats” on the asthenosphere, the
warmer, more plastic part of the mantle.
• The mantle, which is made up of ultramafic rock, can be divided into 3
layers: upper mantle, transition zone, and lower mantle. The crust
and the upper mantle are known as the lithosphere. The underlying,
warmer and more plastic portion of the mantle is known as the
asthenosphere.
• The inner core can be divided into an outer core of liquid iron alloy and
an inner core of solid iron alloy. Within the Earth’s outer core, there is
convective flow. Scientists believe the flow of the outer core creates the
Earth’s magnetic field.
What is an earthquake?
•
When rock within the crust suddenly breaks, an earthquake results. An
earthquake is a vibration. This vibrational energy moves as seismic waves
through the solid earth, the oceans, and on the surface. Earthquakes can
produce a new fracture in the earth’s crust or can cause sliding or movement
on an existing fracture or can occur on a “blind fault” (an old fracture that
has left no visible signs on the surface). Sliding on a existing fracture is
called a fault.
•
The speed at which seismic waves travel can change abruptly beneath the
surface and is dependent upon the layers through which the seismic waves
are traveling. The boundary at which the velocity changes is called a
seismic-velocity discontinuity. A discontinuity represents a definitive
change in the nature of the material inside the earth such as a change in
density or compressibility.
•
Major discontinuities define the boundaries between the mantle and the
core. The Moho seismic discontinuity defines the boundary between the
crust and the mantle.
The Crust
• Oceanic crust underlies the sea floor and is less than
10 km thick. It is composed primarily of mafic (ironrich) igneous rocks.
• Continental crust ranges from 25 km thick to more
than 70 km thick and can contain a variety of rock
types but, on average, is less mafic and more silicic
than oceanic crust. Rocks that are more mafic tend to
be heavier than silicic rocks. The continental crust
“float” on the asthenosphere because it is less dense
and lighter in relation to the more mafic and heavier
oceanic crust. Continental crust usually consists of
rocks different from the mantle.
The Mantle
• The Earth’s mantle is about 2,885 km thick and is the largest
part of the Earth. The mantle is made up of ultramafic rock
called peridotite, a rock rich in iron and magnesium but
poor in silica.
• The mantle consists of the upper mantle (to 400 km), the
transition zone (400 km to 670 km), and the lower mantle
(670 km to the core boundary). Mantle rock is relatively
solid but can also flow very slowly (like softened wax).
• Temperature in the mantle generally increases with depth but
not uniformly. This creates a convective flow pattern.
• The mantle composition consists mostly of iron and
magnesium-rich minerals.
The Core
• The core of the earth consists of two phases.
• There is an outer core (between 2,900 and
5,155 km deep) and an inner core (from 5,155
to 6,371 km).
• The outer core is a liquid iron alloy because of
the extremely high temperatures in that region.
• The flow of the outer core generates the Earth’s
magnetic field.
• The inner core is a solid iron alloy that remains
solid despite the high temperatures because of
the tremendous pressure.
End of Chapter 2