Oceanography Chapter 8

What three factors help rocks melt more easily?

Increasing temperature, decreasing pressure, and addition of water.

Where does magma form at spreading centers and what type of magma?

From decompression melting of hot asthenosphere → basaltic magma.

What is the main process of magma formation at mantle plumes?

Decreasing pressure in hot, buoyant mantle; basaltic magma (sometimes granitic if under continents).

What is the main process of magma formation at subduction zones?

Addition of water from subducting plate, plus decompression and friction → granitic/andesitic magma.

What is the "Ring of Fire"?

Subduction-zone volcanoes surrounding the Pacific Plate.

Which magma type is typical of oceanic crust?

Basalt.

Which magma type is typical of continental crust?

Granite.

What type of magma is intermediate in composition?

Andesite.

How old are the oldest continental rocks?

~4.2 billion years (zircons in Australia).

What are two hypotheses for early continent formation?

Horizontal tectonics (subduction) and vertical plume tectonics (upwellings).

What % silica and water do granitic vs basaltic magmas have?

Granitic: ~70% silica, up to 10% water. Basaltic: ~50% silica, 1–2% water.

How does silica affect magma? 

Increases viscosity.

How does water affect magma?

Lowers solidification temperature, escapes as magma rises.

Which magma usually erupts at surface vs crystallizes in crust?

Basaltic rises to surface; granitic crystallizes in crust.

What is a batholith vs stock?

Batholith >100 km², stock smaller.

What are sills and dikes?

Sill = horizontal intrusion, dike = vertical intrusion.

What is lava?

Magma at Earth’s surface.

What is a fissure eruption?

Low-viscosity lava flowing from cracks; large eruptions form flood basalts.

What shape is a shield volcano? What type of lava?

Broad, gentle slopes; basaltic, fluid lava.

What is a cinder cone?

Small, steep, symmetrical volcano made from pyroclastics; short-lived.

What is a composite cone (stratovolcano)?

Steep-sided, alternating lava and pyroclastic layers; very explosive.

What type of magma causes explosive eruptions?

Granitic/andesitic (high silica, high gas).

What is a caldera?

Large depression formed after a massive eruption and collapse of magma chamber.

Example of stratovolcano eruption in history?

Mt. Vesuvius (79 AD, Pompeii).

Example of caldera-forming eruption in the U.S.?

Yellowstone (last ~0.6 million years ago).

How can volcanic eruptions affect climate?

Ash blocks sunlight → cooling; CO₂ release → warming.

Which of the following tectonic environments does not generate large quantities of magma?

transform boundaries

A shield volcano commonly forms when

fluid basaltic magma builds a gently sloping mountain