Unit 9 - Page 264 - 272

What is a subduction zone?

• A divergent boundary where plates move apart

• A location where oceanic crust is forced beneath continental crust

• A transform fault where plates slide past each other

• A region where mantle plumes rise to the surface

Answer explanation

Answer: B. A location where oceanic crust is forced beneath continental crust.

• Explanation: A subduction zone happens when one tectonic plate (usually oceanic) sinks under another plate (usually continental).

• Example: The Pacific Plate being forced under the North American Plate along the west coast of the U.S., forming the Cascadia subduction zone.

What type of magma is typically found at subduction zones?

• Basaltic magma

• Andesitic magma

• Rhyolitic magma

• Komatiitic magma

Answer explanation

Answer: B. Andesitic magma.

• Explanation: Subduction zones generate andesitic magma due to the melting of oceanic crust and sediments. This magma is thick and leads to explosive volcanic eruptions.

• Example: Mount St. Helens in the U.S. or Mount Fuji in Japan.

The Earth's oceanic crust is primarily composed of:

• Granite

• Basalt

• Sandstone

• Limestone

Answer explanation

Answer: B. Basalt.

• Explanation: Oceanic crust is made of basalt, a dense, dark volcanic rock, which forms when magma cools quickly underwater.

• Example: The crust under the Atlantic Ocean.

What distinguishes continental crust from oceanic crust?

• It is thinner and denser

• It is thicker and less dense

• It is primarily composed of basalt

• It lies only at subduction zones

Answer explanation

Answer: B. It is thicker and less dense.

• Explanation: Continental crust is thicker (up to 70 km) and lighter than oceanic crust, allowing it to float higher on the mantle.

• Example: The Himalayas sit on thick continental crust.

Mantle plumes are responsible for creating:

• Subduction zones

• Mid-ocean ridges

• Hotspots

• Transform faults

Answer explanation

Answer: C. Hotspots.

• Explanation: Mantle plumes are columns of hot material from the Earth's mantle that rise and create hotspots, forming volcanic islands.

• Example: Hawaii’s volcanic islands are formed over a hotspot.

A hotspot is characterized by:

• Volcanic activity unrelated to plate boundaries

• Earthquakes at transform boundaries

• Subduction zones with deep-focus earthquakes

• Divergent boundaries producing magma

Answer explanation

Answer: A. Volcanic activity unrelated to plate boundaries.

• Explanation: Hotspots form due to mantle plumes, which are independent of tectonic plate edges.

• Example: Yellowstone National Park is a hotspot volcano.

Lava is formed when:

• Magma is released onto the Earth's surface

• The mantle melts completely

• Subduction zones collide with transform faults

• Mantle plumes solidify underground

Answer explanation

Answer: A. Magma is released onto the Earth's surface.

• Explanation: Magma is molten rock inside the Earth. When it reaches the surface through volcanoes, it becomes lava.

• Example: Lava flows from the Kilauea volcano in Hawaii.

Mid-ocean ridges are found at:

• Subduction zones

• Divergent plate boundaries

• Transform boundaries

• Continental-continental collision zones

Answer explanation

Answer: B. Divergent plate boundaries.

• Explanation: At mid-ocean ridges, tectonic plates move apart, allowing magma to rise and create new oceanic crust.

• Example: The Mid-Atlantic Ridge in the Atlantic Ocean.

Partial melting occurs when:

• A subduction zone is uplifted

• Only part of a rock melts due to heat and pressure

• The entire crust melts at divergent boundaries

• The oceanic crust becomes completely solid

Answer explanation

Answer: B. Only part of a rock melts due to heat and pressure.

• Explanation: Rocks contain minerals that melt at different temperatures, causing only part of the rock to melt.

• Example: This happens at subduction zones, creating magma that leads to volcanoes.

The epicentre of an earthquake is:

• The location of the deepest earthquake focus

• Directly above the focus on the Earth's surface

• The boundary where magma rises to the surface

• The region of most intense lava flow

Answer explanation

Answer: B. Directly above the focus on the Earth's surface.

• Explanation: The focus is the point underground where the earthquake starts; the epicentre is the point on the surface directly above it.

• Example: In the 2010 Haiti earthquake, the epicentre was near Port-au-Prince.