Volcanoes and Earthquakes

Grade 6 · Science · Study Guide

Understand the powerful geologic events that reshape Earth's surface.

Beneath Earth's crust, tremendous forces are constantly at work. Two of the most dramatic results are volcanoes and earthquakes — both driven by the movement and heat of tectonic plates.

An earthquake is a sudden shaking of the ground caused by the release of energy along a fault — a crack in Earth's crust where rocks have moved. When tectonic plates grind against each other (especially at transform and convergent boundaries), stress builds up over time. When the stress becomes too great, rocks suddenly slip, releasing energy as seismic waves that shake the ground outward in all directions. The focus (or hypocenter) is the point inside Earth where the earthquake starts. The epicenter is the point on the surface directly above the focus. Scientists measure earthquake strength using the Richter scale or the moment magnitude scale.

Volcanoes form where magma (molten rock underground) reaches the surface. When magma erupts from a volcano, it is called lava. Volcanoes often occur at convergent plate boundaries (where subducting plates melt into magma) and at divergent boundaries (like mid-ocean ridges). They also form at hot spots — areas where a plume of unusually hot mantle rises through the crust. Hawaii's islands were formed by a hot spot as the Pacific Plate slowly moved over it.

Volcanic eruptions release lava, ash, and gases. They can be catastrophic — the 1980 eruption of Mt. St. Helens in Washington released energy equivalent to 27,000 atomic bombs. But volcanoes also build new land and release minerals that enrich soil.

Earthquake Magnitude Scale (Richter)
Magnitude	Description	Typical Effects
< 2.0	Micro	Not felt by people
2.0 – 3.9	Minor	Felt slightly by some
4.0 – 5.9	Light to Moderate	Felt widely, minor damage
6.0 – 6.9	Strong	Significant damage possible
> 7.0	Major / Great	Severe damage, can be catastrophic

Key terms

Earthquake: A sudden shaking of Earth's crust caused by the rapid release of energy along a fault.
Fault: A crack or break in Earth's crust along which rocks can move.
Seismic waves: Energy waves that travel through Earth and cause the shaking felt during an earthquake.
Focus (hypocenter): The underground point where an earthquake originates.
Epicenter: The point on Earth's surface directly above the earthquake's focus.
Volcano: An opening in Earth's crust through which magma, gases, and ash erupt onto the surface.
Magma: Molten rock beneath Earth's surface.
Lava: Magma that has reached Earth's surface through a volcanic eruption.

Flashcards

Earthquake: Sudden ground shaking caused by energy released along a fault.
Fault: A fracture in Earth's crust along which movement occurs.
Epicenter: The surface point directly above where an earthquake originates (the focus).
Volcano: An opening in Earth's crust where magma erupts as lava, ash, and gases.
Magma vs. Lava: Magma = molten rock underground; Lava = magma that has reached the surface.
Hot spot: A fixed plume of hot mantle material that burns through the moving tectonic plate above it, creating volcanoes.

Practice questions

What is the point on Earth's surface directly above the earthquake's origin called?
1. Focus
2. Fault
3. Epicenter
4. Hot spot
Show answer
Answer: Epicenter
The epicenter is on the surface directly above the focus (the actual underground origin). The epicenter is where shaking is usually strongest.
Magma is called lava after it erupts onto Earth's surface.
Show answer
Answer: True
The same molten rock has two names: magma when underground, lava when it erupts and flows on Earth's surface.
A crack in Earth's crust where rocks have moved is called a ___.
Show answer
Answer: fault
Faults are fractures where tectonic stress has caused rocks to move. The San Andreas Fault, for example, runs 1,300 km through California.
The Hawaiian Islands were formed by which volcanic feature?
1. Divergent boundary
2. Transform boundary
3. Subduction zone
4. Hot spot
Show answer
Answer: Hot spot
A hot spot is a stationary plume of hot mantle material. As the Pacific Plate moves over it, a chain of volcanic islands forms — this created Hawaii.
Seismic waves travel outward from the epicenter of an earthquake.
Show answer
Answer: False
Seismic waves travel outward from the FOCUS (underground origin), not the epicenter. The epicenter is where waves first reach the surface.

Fun facts

About 90% of all earthquakes occur in the 'Ring of Fire' — a horseshoe-shaped zone of tectonic plate boundaries encircling the Pacific Ocean.
The deepest volcanic eruption ever recorded occurred 4 km underwater in the Pacific Ocean — the lava cooled so fast it formed glass-like pillows.
Hawaii is moving toward Japan at about 10 cm per year because the Pacific Plate is slowly drifting northwest over a hot spot.

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