Download Objective 1 Reading Guide – pages 141-147 Mid

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Objective 1 Reading Guide – pages 141-147
Mid-Ocean Ridges
1. The East Pacific rise is just one of many ____________________ ridges that wind beneath Earth’s
_________________.
2. In the mid-1900s, scientists mapped the mid-ocean ridges by using __________________. Sonar is a device
that _____________ sound waves off underwater objects and then records the _______________ of these
sound waves.
3. Mid-ocean ridges curve like the seam of a baseball along the _____________________.
4. Most of the ___________________ in the mid-ocean ridge system lie hidden under hundreds of meter of
water. But in few places the ridge pokes above the ____________________.
5. The island of __________________ is part of the mid-ocean ridge that rises above the surface in the North
_______________Ocean.
6. A ______________-sided _________________ splits the top of some mid-ocean ridges.
What is Sea-Floor Spreading?
1. Harry Hess, an American __________________, was one of the scientists who studied mid-ocean ridges.
2. In 1960, _____________ proposed a radical idea. He suggested that a process he called sea-floor
________________ continually adds new material to the _____________ _____________.
3. In sea-floor spreading, the sea floor _______________ apart along both sides of the mid-ocean
_______________ as new crust is _____________. As a result, the _____________ floors move like conveyor
belts, carrying the ___________________ with them.
4. Sea-floor spreading __________________ at the mid-ocean ridge, which forms along a crack in the
_____________ crust.
5. Along the ridge, ___________________ material that forms several kilometers beneath the surface rises and
_____________. At the same time, ________________ rocks move outward on both side of the ridge. As the
molten material ___________, it forms a strip of solid ______________ in the ____________ of the ridge.
Evidence for Sea-Floor Spreading
1.
Several types of evidence supported Hess’s _____________ of sea-floor spreading: __________________ of
molten material, magnetic stripes in the rock of the ocean floor, and the ________________ of the rocks
themselves.
Evidence from Molten Material
1. In the 1960s, scientists found evidence that ____________ material is indeed erupting along mid-ocean
ridges.
2. The scientists dived to the ocean floor in ______________, a small submarine built to withstand the crushing
_________________ four kilometers down in the ocean.
3. In the ridges central valley, Alvin’s crew found strange rocks shaped like _________________ or like
toothpaste squeezed from a tube. Such rocks form only when ________________ material hardens quickly
under ___________________.
Evidence from Magnetic Stripes
1. You read earlier that Earth behaves like a giant ______________, with a north pole and south pole.
2. Surprisingly, Earth’s magnetic poles have ________________ themselves many times during Earth’s history.
The last reversal happened 780,000 years ago.
3. Scientists discovered that the rock that makes up the ocean floor lies in a _______________ of magnetized
“stripes.” These _______________ hold a ________________ of reversals in Earth’s magnetic field.
4. As the rock _________________, the iron bits inside lined up in the direction of the Earth’s magnetic poles.
5. Using sensitive instruments, scientists record the ____________________ ___________________ of rocks on
both sides of the mid-ocean ridge.
6. They found that stripes of rock that formed when Earth’s magnetic field pointed north ________________
with stripes of rock that formed when the magnetic field pointed south.
Evidence from Drilling Samples
1. The final proof of sea-floor spreading came from rock samples obtained by ________________ into the ocean
floor.
2. __________________ from the sea floor were brought up through the pipes. Then scientists determined the
_________________ of the rock samples.
3. They found that the ___________________ away from the ridge the samples were taken, the
___________________ the rocks were. The _____________________ rocks were always in the center of the
ridges.
Subduction at Trenches
1. How can the ocean floor keep getting __________________ and _______________? The answer is that the
ocean floor generally does not just keep _____________________. Instead, the ocean floor
_____________________ into deep underwater canyons called deep-ocean _____________________.
2. At a deep-ocean trench, the ocean crust bends ___________________.
3. In a process taking tens of millions of years, part of the ocean floor _________________ back into the mantle
at deep-ocean trenches.
The Process of Subduction
1. The process by which the ocean floor sinks ___________________ a deep-ocean trench and back into the
mantle is called _________________.
2. Sea-floor ________________ and subduction work together. They ________________ the ocean floor as if it
were on a giant conveyor belt.
3. _____________ oceanic crust is hot. But as it moves away from the mid-ocean ridge, it _______________
and becomes more _______________. Eventually, gravity pulls this older, denser crust down
_________________ the trench.
Subduction and Earth’s Oceans
1. The processes of subduction and sea-floor spreading can change the _____________ and
__________________ of the oceans.
2. Because of these processes, the ocean floor is ________________ about every 200 __________________
years.
3. The vast _______________ Ocean covers almost one third of the planet. And yet it is _________________.
4. Sometimes a deep ocean _________________ swallows more oceanic crust than a mid-ocean ridge can
__________________.