Glossary of Jacob

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a large group of stars held together by gravity
Milky Way
Our outer galaxy
a band of colors made when white light is broken up
expansion redshift
the shift of a spectrum of a galaxy toward longer(redder)wavelengths due to the expansion of space
big bang
the beginning of the universe, when the density of the universe was very high
background radiation
electromagnetic radiation left over from the big bang
an extremely bright, extremely distant, high-energy source
Earth's solid, rocky surface containing the continents and ocean floor
original horizontality
the idea that many kinds of rocks form in flat, horizontal layers
continental drift
the idea that a supercontinent split apart into pieces, the continents, which drifted in time to their present locations
sea-floor spreading
the idea that new crust is forming at ridges in the sea floor, spreading apart the crust on either side of the ridges
hot,molten rock below Earth's surface
plate tectonics
the idea that Earth's surface is broken into plates that move
Earth's layer beneath the crust
where plates collide, the sliding of a denser ocean plate under another plate
a huge in the crust, at or below the surface, the sides of which may show evidence of motion
the point where an earthquake starts, where rocks begin to slice past each other
seismic wave
a vibration that spreads out away from a focus when an earthquake happens
the point on Earth's surface directly above the focus
the shaking of the crust after the the initial shaking of an earthquake
a sensitive device that detects the shaking of the crust
the amount of energy released by an earthquake
hot spot
a very hot part of the mantle, where magma can melt through a plate moving above it
a central opening in a volcanic area through which magma may escape
magma that reaches Earth's surface and flows out of a vent
a cuplike hollow that forms at the top of a volcano around the vent
cinder-cone volcano
a steep-sided cone that forms from e eruptions of hot rocks, ranging from particles to boulders
shield volcano
a wide, gently sloped cone that forms from flows of lava
composite volcano
a cone formed from e eruptions of hot rocks followed by a flow of lava, over and over
geothermal energy
heat from below Earth's surface
fold mountain
a mountain made mostly of rock layers folded by being squeezed together
fault-block mountain
a mountain made by huge tilted blocks of rocks separated from surrounding rocks by faults
the breaking down of rocks into smaller pieces by natural processes
the picking up and removal of rock particles
a mixture of weathered rock, decayed plant and animal matter, living things, air, and water
soil horizon
any of the layers of soil from the surface to the bedrock below
water that soaks into the soil and rock by collecting in spaces between rock particles
material in soil formed by the breakdown of plant and animal material
mass wasting
the downhill movement of Earth material caused by gravity
the dropping off of sediment
a huge sheet of ice and snow that moves slowly over the land
a jumble of many sizes of sediment deposited by a glacier
a deposit of many sizes of sediment in front of or along the sides of a glacier

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