APES Unit 4
Continental vs. Oceanic Crust
Q: How does continental crust differ from oceanic crust?
A: Continental crust is thicker, less dense, and older. Oceanic crust is thinner, denser, and younger (basalt).
Why Oceanic Crust Subducts
Q: Why does oceanic crust subduct under continental crust?
A: It’s denser and thinner, so it sinks beneath continental crust.
Divergent Boundary Definition
Q: What happens at a divergent boundary?
A: Plates move apart as rising magma forms new crust, mid-ocean ridges, and rift valleys.
Convergent Boundary Definition
Q: What happens at a convergent boundary?
A: Plates collide; the denser plate subducts, forming trenches, volcanoes, mountains, and island arcs.
Transform Boundary Definition
Q: What happens at a transform boundary?
A: Plates slide past each other, causing earthquakes.
Why Plates Move (Convection Currents)
Q: What causes tectonic plates to move?
A: Convection currents in the mantle: hot magma rises, cool magma sinks, dragging plates with it.
Oceanic–Continental Convergence
Q: What forms when oceanic crust subducts under continental crust?
A: Trenches, coastal volcanic mountain ranges (like the Andes), earthquakes, tsunamis.
Oceanic–Oceanic Convergence
Q: What forms when two oceanic plates collide?
A: Deep trenches and volcanic island arcs (Japan, Philippines).
Continental–Continental Convergence
Q: What forms when two continental plates collide?
A: Large non-volcanic mountain ranges (Himalayas).
Mid-Ocean Ridge
Q: What is a mid-ocean ridge and how does it form?
A: An underwater mountain chain formed at divergent boundaries where magma rises and creates new crust.
Rift Valley
Q: Where do rift valleys form?
A: At divergent boundaries on land when continental crust splits apart.
Subduction Zone
Q: What is a subduction zone?
A: A region where one plate sinks beneath another, creating trenches, volcanoes, and earthquakes.
Hotspots
Q: What is a hotspot?
A: A stationary plume of hot magma that forms volcanoes in the middle of plates (Hawaii, Iceland).
How Island Arcs Form
Q: How do volcanic island arcs form?
A: From oceanic–oceanic subduction; melting produces magma that rises to create a chain of volcanic islands.
How Trenches Form
Q: How do ocean trenches form?
A: When an oceanic plate subducts beneath another plate at a convergent boundary.
San Andreas Fault Boundary Type
Q: What type of boundary is the San Andreas Fault?
A: Transform boundary.
Convection Cycle Steps
Q: What are the steps of the mantle convection cycle?
A:
Core heats magma → it rises.
Magma spreads plates apart at divergent boundaries.
Magma cools → becomes denser → sinks.
Sinking plate may subduct and melt → cycle repeats.
Magnetic Striping / Seafloor Spreading Evidence
Q: How does magnetic striping support plate tectonics?
A: Alternating magnetic bands on either side of mid-ocean ridges show new crust forming and spreading.