Geology Quiz 3

What are the Columbia River Basalts (CRB)?

Massive layers of flood basalt in Eastern Washington formed ~16 million years ago.

Why are they called “flood basalts”?

Because lava flooded across the land surface, covering huge areas.

Where did CRB lava come from?

Fissures—long cracks in the Earth’s crust, not the ocean.

When did CRB eruptions begin?

Around 16.7 million years ago.

When was peak lava volume in the CRB?

Between 16.5–16.1 million years ago.

How thick is one typical CRB lava flow?

About 100 feet.

How many basalt layers make up the CRB?

Around 300 stacked flows.

How thick is the middle CRB basalt section?

About 3 miles thick.

Why is Eastern Washington so flat?

Flood basalts buried the previous rugged landscape.

Why did the land sink below sea level during CRB eruptions?

The weight of dense, mafic (45% silica) lava depressed the crust.

What biological effect did CRB eruptions have?

Created a temporary biological dead zone.

How often might individual flood basalt eruptions occur?

Roughly every 1,000 years.

What phase of CRB will be covered on the quiz?

Grande Ronde Phase. (16.6-161 m.y) 73% of the total flood basalts. 

Where are vesicles located in basalt flows?

At the top—gas bubbles rise and get trapped.

What does pillow basalt indicate?

Eruption underwater (river, lake, or ocean).

What does the absence of pillow basalt indicate?

No water present during eruption.

How do columnar basalts form?

Cooling contraction causes cracks that propagate downward.

What are feeder dikes?

Vertical intrusions that transported lava upward from depth.

Why do basalt flows insulate deeper lava?

Hardened flows form a crust barrier that traps heat.

What does petrified wood at the base of basalt flows indicate?

Calm periods allowed forests to grow between eruptions.

How long has the Cascade Arc existed?

About 46 million years.

What is the typical lifespan of a Cascade stratovolcano?

About 2 million years.

What happens when a cone volcano dies?

It erodes and becomes a “ghost volcano.”

What volcanic event formed Crater Lake?

The eruption and collapse of Mount Mazama.

How explosive was the Mazama eruption compared to Mt. St. Helens?

~43 times more explosive.

What volcanic product can travel over 100 miles?

Lahars (volcanic mudflows).

Which volcanic product travels the farthest from the vent?

Ash fall.

Why can lahars outlive volcanoes?

Rain, melting snow, and landslides remobilize volcanic debris.

What does repeated cone formation in one area indicate?

Long-lived subduction and magma supply.

What plate once subducted under western North America?

Farallon Plate.

What is the Juan de Fuca Plate?

A remnant of the Farallon Plate.

Did the San Andreas Fault exist 40 million years ago?

No—subduction dominated then.

What formed the San Andreas Fault?

Pacific Plate meeting North America and creating a transform boundary.

When did the San Andreas Fault begin forming?

Around 20 million years ago.

Why were North American volcano arcs similar to the Andes 40 MYA?

Continuous ocean–continent subduction.

Why isn’t Mt. Olympus a volcano?

It’s made of sedimentary/metamorphic rocks (shale, slate), not volcanic rock.

What geologic process built the Olympic Mountains?

Accretion of scraped-off ocean sediments at a subduction zone.

What rock types are common in the Olympic Peninsula?

Shale and slate.

What is a mélange?

Chaotic mixture of ocean sediments and crustal fragments.

What is a subduction complex?

Scraped-off oceanic material forming a secondary mountain range.

What is a forearc basin?

Low area between the volcanic arc and subduction complex that fills with sediment.

What does diorite in the Sierra Nevada indicate?

Former volcanic arc—now extinct (ghost volcanoes).

What explains the huge size of flood basalts?

A massive, long-lived hotspot.

What does clockwise rotation help explain?

Shape and prominence of the Olympic Peninsula.

What major mudflow occurred 5,600 years ago near Mt. Rainier?

Osceola Mudflow.