Geoscience of National Parks Exam 2

What is the full range of geologic time represented by the age of rocks exposed at the bottom of Grand Canyon, to the top of Bryce Canyon?

About 2 billion years to ~50 million years, representing nearly half of Earth’s geologic history from Precambrian rocks in the Grand Canyon to Eocene rocks at Bryce Canyon.

What is unique about Bryce Canyon physiographically?

Bryce Canyon contains the youngest and highest elevation sedimentary rocks of the Colorado Plateau, making it the top of the Grand Staircase sequence.

We have discussed that the name “Bryce Canyon” is a misnomer. Why? What is the main erosional feature that forms Bryce Canyon NP more accurately called?

It is not actually a canyon because it was not carved by a river. It is a series of amphitheaters eroded into the Paunsagunt Plateau.

How was the Claron Formation deposited and during what geologic time period?

The Claron Formation was deposited during the Paleogene (mainly Eocene) in freshwater lakes, streams, and floodplains.

What are the compositions, age, and types of rocks in the Claron Formation? What process causes the colors?

The Claron Formation is limestone, mudstone, and siltstone deposited about 50–60 million years ago. The red/orange/pink colors come from iron oxidation (hematite). The white unit lacks iron and is composed mostly of pure limestone.

How did uplift of the Paunsagunt Plateau lead to the formation of Bryce Canyon?

When the Colorado Plateau uplifted (~5–6 million years ago), streams like the Paria River began eroding the edge of the Paunsagunt Plateau, carving back the plateau edge and creating Bryce Canyon’s amphitheaters.

Physical weathering

Breaking rock into smaller pieces without changing its composition (example

Chemical weathering

Alters the chemical composition of rock (example

How do hoodoos form and what role does cross-hatched jointing play?

Hoodoos form when erosion removes rock along vertical joints, leaving tall spires. Freeze-thaw weathering widens joints while chemical weathering weakens the rock. Cross-hatched jointing creates intersecting fractures that isolate rock columns which erode into hoodoos.

Where is Arches National Park located and what are its physiographic characteristics?

It is located in southeastern Utah near Moab on the Colorado Plateau. It is a high desert environment with sandstone cliffs, fins, and over 2,000 natural arches.

What is the general rock type of Arches NP and its depositional environments?

The rocks are mainly sandstone formed in desert (eolian dunes), shallow marine, and continental environments.

What is the Paradox Formation and how did it form?

The Paradox Formation is a thick layer of evaporite salt deposits formed when an ancient sea repeatedly evaporated during the Pennsylvanian period (~300 million years ago). It lies beneath the arch-forming sandstones.

What is a salt anticline and how does it form?

A salt anticline forms when buried salt layers flow upward because salt is less dense and plastic. This upward movement uplifts overlying rock layers.

How do salt anticlines become salt grabens and lead to arches?

As salt moves away or dissolves, the overlying rocks collapse, creating grabens (down-dropped blocks) and fractures. These fractures become joints where erosion can carve arches.

What are the steps in arch formation?

1. Joints form in sandstone

2. Erosion widens joints into fins

3. Water and weathering create small openings

4. Openings enlarge into arches

What is the life-cycle of an arch?

Crack formation → fin formation → opening develops → arch enlarges → arch weakens → collapse leaving isolated rock spires.

What is the difference between an arch and a natural bridge?

Arch: Formed mainly by weathering and erosion in rock fins. Natural bridge: Formed by running water (stream erosion) cutting through rock.

How do Natural Bridges and Rainbow Bridge form?

They form when a stream cuts through a meander bend, eventually eroding through the rock and leaving a bridge over the stream.

Stream piracy

When one stream captures the flow of another.

Stream abandonment

The original channel dries up or becomes inactive. These processes can leave rock bridges behind.

Describe the ages and types of rocks at Canyonlands NP.

The rocks range from about 300 million years to 150 million years old and include sandstones, limestones, and shales.

What are the two major rivers the park is built around?

The Colorado River and the Green River.

How did the needles and towers form?

Weathering along vertical joints caused rock to break into columns. Cross-hatched joints isolate rock blocks which erosion shapes.

What role did the Colorado River and the Paradox Formation play in creating grabens?

Movement and dissolution of salt in the Paradox Formation caused the overlying rock to collapse.

What is an entrenched meander and how does it form?

An entrenched meander forms when a meandering river becomes deeply incised into bedrock due to uplift, preserving the curved shape of the river.

What geologic feature is Upheaval Dome and what evidence supports it?

It is believed to be an impact crater. Evidence includes shocked quartz, breccia, and circular deformation of rock layers.

What is biological soil crust and why is it important?

Biological soil crust is made of cyanobacteria, mosses, and lichens. It stabilizes soil, prevents erosion, and helps retain moisture for desert plants.

General physiography of the Cascade Range.

The Cascade Range is a north–south chain of volcanoes in the Pacific Northwest formed by subduction volcanism.

Why do volcanoes erupt?

Eruptions occur when gas-rich magma rises toward the surface, pressure builds, and gases expand and escape explosively or effusively.

Mafic lava

Iron-rich, low silica, low viscosity, quiet eruptions.

Felsic lava

High silica, high viscosity, gas-rich, explosive eruptions.

The Pacific Ring of Fire is defined by what type of tectonic boundary?

Convergent plate boundaries (subduction zones).

What is the tectonic setting of Mount Rainier and Crater Lake?

They occur at an ocean-continent convergent boundary where the Juan de Fuca Plate subducts beneath the North American Plate.

What type of melting fuels the Cascade magmatic arc?

Flux melting, caused by water released from the subducting plate.

Why are Cascade volcanoes inland instead of at the trench?

Magma forms above the subducting slab at depth, which occurs inland

How old is Mount Rainier?

Approximately 500,000 years old.

How do Cascade volcanoes form?

The Juan de Fuca Plate subducts, releasing water that causes flux melting in the mantle, producing magma that rises to form volcanic arcs.

What type of igneous rocks form the Cascade volcanoes?

Mainly andesite and basalt.

Why are Cascade volcanoes silica-rich?

Magma interacts with and melts continental crust, increasing its silica content.

What is welded tuff?

A volcanic ash deposit that becomes compacted and fused together by heat.

What is a stratovolcano?

A steep volcano built from layers of lava, ash, and volcanic debris.

What kind of volcanoes are Mount Rainier and Crater Lake?

Stratovolcanoes

Why is Mount Rainier a hazard?

It poses hazards including lahars, pyroclastic flows, ash fall, and landslides.

What is a lahar?

A fast-moving volcanic mudflow made of water, ash, and rock debris.

What informs a lahar forecast model?

Data such as topography, river channels, past deposits, glacier melt potential, and eruption history.

What is a glacier and how does glacial ice move?

A large mass of flowing ice that moves due to gravity and internal deformation.

Are there glaciers on Mount Rainier? About how many?

Yes, about 25 named glaciers, the most of any mountain in the contiguous United States.

How is the setting similar to Mount Rainier?

Both formed from subduction volcanism in the Cascade Range.

How does a stratovolcano become a caldera lake?

A massive eruption empties the magma chamber, the volcano collapses inward forming a caldera, which later fills with water.

What is the geologic history of Crater Lake?

It formed after Mount Mazama erupted catastrophically about 7,700 years ago and collapsed into a caldera.

What rocks are present at Crater Lake?

Tuff: Consolidated volcanic ash.

Tephra: Air-fall volcanic fragments.

Pumice: Gas-rich frothy volcanic rock.

Welded Tuff: Compacted hot ash deposits.

Rhyodacite: Silica-rich volcanic rock.

Ignimbrite: Deposits from pyroclastic flows.

Scoria: Vesicular basaltic rock.

What was the eruptive sequence that formed the caldera?

A Plinian eruption followed by ring-vent eruptions and collapse of the magma chamber.

What is a pyroclastic flow?

A fast-moving avalanche of hot gas, ash, and rock fragments.

Why is the Pinnacles deposit compositionally zoned?

The bottom contains lighter silica-rich material, while the top contains darker, more mafic material, showing different magma layers erupted.

How deep and wide is Crater Lake? What is the water quality?

Depth: ~1,943 ft (deepest lake in the U.S.)

Width: ~5–6 miles

Water: Extremely clear and pure, with low nutrients and limited aquatic life.

Where is the Hawaiian Archipelago and what is its physiography?

It is located in the central Pacific Ocean and consists of volcanic islands with tropical climate, steep volcanoes, and lush vegetation.

How do hotspot island chains form?

A stationary mantle hotspot melts the lithosphere, forming volcanoes while the Pacific Plate moves over it, creating a chain.

Does the hotspot move?

No, the tectonic plate moves over the stationary hotspot.

What trends exist along the Hawaiian Islands?

Toward the northwest the islands become older, lower, more eroded, and volcanically inactive.

How old is Hawaii

How old is Maui

~1–2 million years

How old is Kauai

~5 million years

Will a new Hawaiin island form?

Yes, the Lō‘ihi seamount is growing and may become a new island in the future.

What is the source and composition of Hawaiian magma?

Mantle plume magma, primarily basaltic.

What process causes magma to rise?

Mantle convection and decompression melting.

What are the magma characteristics?

High temperature, low viscosity, low silica, and relatively gentle eruptions.

What rock type forms Hawaiian volcanoes?

Basalt.

What types of volcanoes formed Hawaii and Haleakala?

Shield volcanoes.

What are characteristics of shield volcanoes?

Very large, broad, gently sloping volcanoes formed by fluid basaltic lava flows.

How does a Hawaiian-style caldera form?

When magma drains from beneath the summit, the surface collapses inward.

What are rift zones?

Linear fractures where magma travels outward from the volcano summit.

Lava lake

A pool of molten lava in a crater.

Lava tube

A tunnel formed when the outer lava cools but molten lava continues flowing inside.

What caused the 2018 collapse event?

Magma drained from the summit to rift eruptions, causing the caldera floor to collapse.

Why is Mauna Loa considered the tallest mountain?

Measured from the ocean floor to summit, it is over 33,500 ft tall.

Which park is actively erupting?

Hawaii Volcanoes National Park, because it sits directly over the active hotspot.

What are AA and Pahoehoe lava flows?

AA flows flow characteristics?

Cooler, thicker, rough and clinkery.

Pahoehoe flow characteristics?

Hotter, smoother, ropy lava.

Why do Hawaiian islands have black sand beaches?

Basaltic lava cools quickly in water and breaks into black sand particles.