Unit 3 - Page 63 - 67

What is a subduction zone?

• A place where two continental plates move apart

• A region where one tectonic plate is forced beneath another

• A type of transform boundary

• A location where magma rises to form mid-ocean ridges

Answer explanation

Answer: B) A region where one tectonic plate is forced beneath another
Explanation:
A subduction zone is where one tectonic plate (usually an oceanic plate) sinks beneath another plate into the Earth's mantle. This process happens at deep ocean trenches and causes earthquakes and volcanoes.

Example:

• The Pacific Plate is subducting under the North American Plate at the Aleutian Trench, leading to volcanoes in Alaska.

The asthenosphere is best described as:

• A rigid outer layer of the Earth

• The portion of the Earth's mantle that is solid and brittle

• A partially molten, ductile layer beneath the lithosphere

• A part of the inner core

Answer explanation

Answer: C) A partially molten, ductile layer beneath the lithosphere
Explanation:
The asthenosphere is a soft, hot, and semi-liquid layer below the Earth's rigid outer shell. It allows tectonic plates to move on top of it.

Example:

• Imagine a conveyor belt in a factory. The moving belt represents the asthenosphere, and boxes on top are the tectonic plates.

What is the Benioff Zone?

• The part of a mid-ocean ridge where new crust forms

• A region of intense earthquake activity along a subducting plate

• The highest point of a volcanic island arc

• A section of the lithosphere that remains rigid

Answer explanation

Answer: B) A region of intense earthquake activity along a subducting plate
Explanation:
The Benioff Zone is the deep earthquake zone where a subducting plate sinks into the mantle, causing earthquakes at different depths.

Example:

• The Japan Trench experiences deep earthquakes as the Pacific Plate is forced under the Eurasian Plate.

Ocean trenches are formed as a result of:

• Sea-floor spreading

• Subduction of an oceanic plate beneath another plate

• Transform faults grinding against each other

• Hotspot volcanic activity

Answer explanation

Answer: B) Subduction of an oceanic plate beneath another plate
Explanation:
An ocean trench is a deep underwater valley formed when one oceanic plate bends downward into the mantle.

Example:

• The Mariana Trench in the Pacific Ocean is the deepest place on Earth, created by the subduction of the Pacific Plate under the Mariana Plate.

The trench outer rise is located:

• At the edge of a continental shelf

• Along the mid-ocean ridge where plates separate

• On the oceanic plate just before it bends and subducts

• On the back-arc side of an island arc

Answer explanation

Answer: C) On the oceanic plate just before it bends and subducts
Explanation:
The outer rise is a small bulge or uplift that happens before a plate bends downward into a trench.

Example:

• Picture a diving board: the end bends downward, but before that, it slightly rises—that’s the trench outer rise.

What is the primary characteristic of an outer slope in an ocean trench?

• It is a gently sloping area before the trench

• It is a steep area where the subduction occurs

• It marks the boundary between oceanic and continental crust

• It is the region where mid-ocean ridges form

Answer explanation

Answer: A) It is a gently sloping area before the trench
Explanation:
The outer slope is the part of the ocean floor that gradually slopes down toward the trench before dropping steeply.

Example:

• The outer slope of the Peru-Chile Trench gently slopes before reaching the deep part of the trench.

The inner slope of an ocean trench is usually:

• Steeper than the outer slope

• Less steep than the outer slope

• Completely flat due to sediment accumulation

• Covered by mid-ocean ridge formations

Answer explanation

Answer: A) Steeper than the outer slope
Explanation:
The inner slope is the side of the trench closer to the overriding plate and is steeper because of crustal movement and sediment buildup.

Example:

• The inner slope of the Mariana Trench is steeper than the outer slope due to tectonic compression.

What is another term for a subduction complex?

• Accretionary prism

• Mid-ocean ridge

• Ocean trench

• Rift valley

Answer explanation

Answer: A) Accretionary prism
Explanation:
An accretionary prism is a pile of sediments and crust that gets scraped off a subducting plate and builds up near the trench.

Example:

• Think of a bulldozer pushing dirt—the dirt piles up in front, just like an accretionary prism in a subduction zone.

An island arc is formed when:

• A continental plate collides with another continental plate

• An oceanic plate subducts beneath another oceanic plate

• A hotspot volcano forms in the middle of an ocean

• A transform fault moves two plates past each other

Answer explanation

Answer: B) An oceanic plate subducts beneath another oceanic plate
Explanation:
Island arcs form when one oceanic plate sinks under another, causing magma to rise and create volcanic islands.

Example:

• The Philippines and Japan are island arcs formed by oceanic plate subduction.

Why do earthquakes occur frequently in subduction zones?

• Due to the melting of the subducted plate

• Because of magma pushing through the crust

• Due to the intense pressure and friction between the subducting and overriding plates

• Because of the collision of two continental plates

Answer explanation

Answer: C) Due to intense pressure and friction between the subducting and overriding plates
Explanation:
When one plate pushes under another, stress builds up and is suddenly released as an earthquake.

Example:

• The 2011 Japan earthquake was caused by the Pacific Plate subducting under the Eurasian Plate.