Chapter 3: Plate Tectonics Lesson 2: Sea-Floor Spreading

New ______ material from inside Earth rises along the _____ and eventually cools and hardens to form solid ____.

molten, ridge, rock

The ridges form the longest ________ ______ on earth.

mountain ranges

In sea-floor spreading, new crust is added to the _____ _____. Older strips of rock then move _______ from either side of the ridge.

ocean floor, outward

Mid-ocean ridges run along the ______ of some ocean floors.

middle

a deep valley along the ocean floor beneath which oceanic crust slowly sinks toward the mantle.

deep-ocean trench

Scientists have found three different types of evidence to support the theory of sea-floor spreading: A. Pillow shaped ____ _____ found in the valleys of the ridges proves that molten material exists there. B. The pattern of ________ _____ in rocks on either side of a mid-ocean ridge is the same. C. Drilling samples show that the youngest rocks are always found at the ______ of the ridges.

lava rocks, magnetic stripes, center

Mid-ocean ridges form ____ ______ __ _________ ____ _____ __ ____ ___ _____ _____.

long chains of mountains that rises up from the ocean floor

an undersea mountain chain where new ocean floor is produced.

mid-ocean ridge

The oceanic crust created along a ______________ is destroyed at a ________________.

mid-ocean ridge, deep-ocean trench

The process of sea-floor spreading begins at a _________ ______ where a _____ is formed.

mid-ocean ridges, crack

Subduction occur because: A. New oceanic crust moves away from the _______________ and cools. B. As it cools it becomes more _____. C. As it moves, the cool, dense crust may collide with the edge of a _________. D. _______ then pulls the older, denser oceanic crust down beneath the ______ and back into the mantle again.

mid-ocean ridges, dense, continent, gravity, trench

_______ crust bends downward at deep-ocean trenches so that part of the ocean floor sinks back into the ______.

oceanic, mantle

the process by which molten material adds new oceanic crust to the ocean floor.

sea-floor spreading

the process by which oceanic crust sinks beneath a deep-ocean trench and back into the mantle at a convergent plate boundary.

subduction

Some mid-ocean ridges have a ______ that runs along their center.

valley