National Parks Geology: A Journey Through Time

Posted on Nov 30, 2024 in Geology

Yellowstone National Park

Yellowstone’s formation is driven by its tectonic setting atop a hotspot, where magma rises from the Earth’s mantle. This hotspot fuels intense geothermal activity and volcanic eruptions. As the North American Plate moves westward, the stationary hotspot generates periodic eruptions, the most recent forming the Yellowstone Caldera. This activity is linked to extensional stress and normal faulting, pulling the crust apart and allowing magma to rise.

The park features unique landforms shaped by volcanic and glacial processes. The Yellowstone Caldera, a massive volcanic crater, is surrounded by rhyolite lava flows and volcanic plateaus. Geothermal features like geysers, hot springs, and fumaroles, including Old Faithful, are fueled by deep heat sources. The nearby Absaroka and Gallatin mountain ranges showcase rugged peaks formed from volcanic and sedimentary processes, uplifted by tectonic forces.

Yellowstone’s rocks reveal a complex history. Rhyolite and basalt, both volcanic rocks, are widespread due to eruptions. The surrounding mountains expose Precambrian metamorphic rocks (gneiss and schist), and Paleozoic and Mesozoic sedimentary rocks (limestone and sandstone), predating the hotspot’s influence. This rock variety, combined with geothermal and volcanic landforms, makes Yellowstone geologically unique.

Zion National Park

Zion’s badlands topography reflects ancient sedimentary environments. Eroded cliffs, steep slopes, and multicolored layers characterize this rugged landscape, primarily composed of sedimentary rocks—sandstones, shales, and mudstones. These rocks were deposited in ancient deserts, rivers, and shallow seas. The Navajo Sandstone, a prominent feature, reflects a vast ancient desert, evidenced by cross-bedding patterns indicating shifting sand dunes. These lithified sands form Zion’s iconic cliffs and canyons.

The rocks’ diverse colors, from white to deep red, reveal regional climate and iron oxidation levels, indicating fluctuations between wet and dry periods. Red hues signify iron oxidation during wetter times, while paler colors indicate arid periods. Marine and river deposits in layers like the Kayenta Formation reflect intervals of shallow seas or rivers, supporting diverse ecosystems. Fossils within these rocks reveal ancient plant roots, animal tracks, and dinosaur footprints, showcasing Zion’s geological history shaped by shifting climates and varied depositional environments.

Yosemite National Park

Late Cretaceous Paleogeography of North America

Eastern edge: Passive continental margin – Opening of the Atlantic Ocean.
Western edge: Active margin (subduction zone) – Development of a major mountain belt called the Western Cordillera – Volcanic arc – Farallon Plate subducting under the North American Plate.

Sierra Nevada Batholith

Represents ancient igneous rocks and shows eruption of volcanic activity. Development of granitic batholiths in the western US. An extensive chain of volcanoes developed as the Farallon Plate subducted beneath the western edge of North America. Subducting Farallon starts melting and causes volcanoes to form. Yosemite contains many different rocks, including those in the Sierra Nevada Batholith, Sierra Nevada, Central Valley (forearc valley), and Coast Ranges (accretionary wedge).

Batholiths

Rising magma chambers can cause eruptions. When they solidify, they create plutons. Balloon-shaped intrusions. Plutons are blob-shaped intrusions (ranging from tens of meters to tens of kilometers). Batholiths are groups of plutons (several hundred kilometers long and over 100 km wide). The Sierra Nevada range is an exposed batholith.

Magma Composition in the Sierra Nevada

Intermediate range (some higher silica, some higher mafic). Hydration melting ranges has intermediate. Granodiorite mostly in Sierra Nevada (between granite and diorite).

Texture of Igneous Rocks

• Fine-grained = fast cooling of magma = extrusive = volcanic
• Coarse-grained = slow cooling of magma = cooled underground = intrusive = plutonic
• Fine-grained with some coarse grains = porphyritic = slow cooling underground

Texture represents conditions of magma cooling: Glassy/Porphyritic/Phaneritic/Vesicular/Pyroclastic/Aphanitic (slow).

Igneous Rocks are Classified Based on:

1. Chemical Composition: Different magmas have different concentrations of major elements. The chemical composition of magma determines the minerals that form when it solidifies. Different types of magmas tend to occur in particular tectonic settings.
2. Texture

Where are These Different Silicate Rock Types Most Commonly Found Within the Earth?

• Ultramafic: Mantle and parts of the lower crust (peridotite)
• Mafic: Oceanic crust and continental crust (basalt and gabbro)
• Intermediate: Continental crust (andesite and diorite)
• Felsic: Continental crust (rhyolite and granite)

These trends set the stage for shaping topography, plate tectonics, magmatism, and surface/biotic processes.

Geomorphic Development of Yosemite Valley

• 10–3 Ma: Rivers carved the canyon.
• Pleistocene glaciations: Glaciers carved out the canyon walls. Glaciers grew extensively and created glacial valleys. As cooled, re-established rivers and rockfalls.
• 50 Ma: Landscape consisted of rolling hills, broad valleys, and meandering streams.
• 10 Ma: A more dissected landscape ensued as the whole range was uplifted and tilted westward (river cut deeper into valleys and cool/drier climate).
• 3 Ma: Canyon landscape developed with continued uplift. Raging Merced River cut its canyon.
• 1 Ma to 250,000 years ago: At least one, and perhaps more, glacial advances filled Yosemite Valley to its brim. Valley gouged and quarried into a U-shaped trough with steep walls.
• 30,000–10,000 years ago: Glacier retreated and melted (caused deep excavation). Glacier created that U-shaped valley profile.

Hawaii Volcanoes National Park

The Hawaiian Islands formed above a volcanic hotspot—an area of high heat energy originating deep within the Earth and rising in a plume to the upper mantle. High heat and lower pressures at the base of the lithosphere facilitate melting. The Island of Hawaii is the tip of a conglomeration of shield volcanoes. Construction of the islands came from lava flows and activity by volcanoes. Shield volcanoes are long and flat.

Types of Volcanoes Reflect Magma Viscosity and Eruption Style

Eruption style is directly related to gas content and viscosity. High-viscosity magmas are higher in silica and produce light-colored volcanic rocks that trap gas and erupt explosively. Low-viscosity magmas are low in silica and produce dark volcanic rocks. Gas escapes, resulting in effusive eruptions.

Mauna Loa: An Enormous Shield Volcano

Kohala, Hualalai, and Mauna Kea are older volcanoes north of the hotspot. Mauna Loa and Kilauea are very active volcanoes directly above the hotspot. Loihi is a new volcano forming to the south.

Hawaiian Islands: Shield Volcanoes, Basaltic Eruptions

Mafic, basalt, gabbro tier.

Types of Basalt Lava in Hawaii

• Pahoehoe lava flows: Characterized by a smooth and ropy surface and tend to be relatively thin. In map view, flows tend to be narrow and elongate.
• A’a lava flows: Typically blocky, usually 3–20 meters thick, and the lava rolls over itself on the ground like a tank track.

Why the Change in Composition of Rocks?

Temperature, flow velocity, and gas content affect lava viscosity and whether it forms as pahoehoe or a’a. If lava cools slowly and does not move too fast, it forms smooth, ropy pahoehoe. If it cools quickly and moves fast, it can tear into clinkery a’a. There is no systematic chemical difference between the two lava types; lava with identical compositions can form both. Viscosity is the critical factor influencing the transition from pahoehoe to a’a.

Volcanic Activity in Hawaii Volcanoes National Park

Calderas form due to the evacuation of magma chambers. The top of the volcano then falls through.

Volcanic Features in Hawaiian Parks Due to Low-Silica Basaltic Volcanism

1. Broad shield volcanoes: Thin, fluid lava flows that extend for tens of miles. Built up from thousands of eruptions.
2. Fissure zones: Cracks along the flanks of volcanoes where eruptions commonly occur.
3. Lava surfaces: Smooth, ropy pahoehoe lava; blocky, rough a’a lava.
4. Cinder cones: Gas-laden lava shoots into the air, raining down as cinders.
5. Lava tubes: Crust forms and insulates flowing lava below. “Arteries” that allow lava to flow long distances.

Located in the center of the Pacific Plate.

What Causes Hotspots?

Associated with thermal anomalies that start deep in the Earth (causes the rise of mantle material that rises to the lithosphere). Decompression melting. Pressure drops when hot rock is carried to shallower depths (same with rifts, mantle plumes, and mid-ocean ridges).

Observations of the Hawaiian Island Chain

Island of Hawaii

• No vegetation over brown (because of fairly recent eruptions)
• Large and rounded topography

Island of Kauai

• Smaller island
• Much more green
• Many ridges and valleys created by erosion

Islands get older as you go more northwest, also more eroded, shorter. These observations are consistent with volcanoes forming as the Pacific Plate moves northwestward over a hotspot.

Why is Hawaii a Line of Volcanic Islands?

Stages of Hotspot Island and Seamount Development

1. The surface of the plate rises as it rides over the hot, expanded region of the hotspot.
2. Magma forms as pressure drops on the rising plume of hot mantle.
3. Basaltic lava pours out on the seafloor, piling up as a volcano.
4. Magma from thousands of eruptions builds up shield volcanoes that rise above sea level.
5. Over 0.5 million years, a very high island can form.
6. Lesser amounts of material can continue to erupt after an island moves off the hotspot.
7. The region cools, lowering the island.
8. Volcanism ceases as the region sinks and erodes, leaving only corals above water.
9. Continued cooling and subsidence drowns the volcano, leaving a seamount.

National Parks in Hawaii Reveal Two Stages of Island Development

• Hawaii Volcanoes National Park shows a high island.
• Haleakala National Park shows a lower island.

Hawaiian Islands: Part of a Much Longer Chain of Volcanic Islands and Seamounts

Hawaiian Islands (forming today) – (21 Ma) Hawaiian Ridge (45 Ma) – Emperor Seamounts (80 Ma). Northwestern plate motion.

Yellowstone Geology

Volcanic Eruptions of High-Silica Magmas: Rhyolite Lava and Ash Flow Tuff (Highly Viscous)

• Rhyolitic lava: Forms highly viscous lava flows; typically light-colored with subtle thin banding (flow banding).
• Tuff: Forms from an explosive rhyolitic eruption of rock fragments, pumice, crystals, and volcanic ash called pyroclastic flows; typically light-colored but includes fragments.

In Yellowstone, 1.3-million-year-old deposits of thick layers of stream deposits (gravels) and rhyolitic tuff.

How Does a Supervolcano’s Size Compare to Other Volcano Types?

About 2,000 times the volume of material that came out of Mount St. Helens in 1980 (St. Helens is about 1 mile long and the Yellowstone caldera is 400). The amount of material that discharges into the atmosphere affected climate in the past.

Features in Parks along Continental Hotspot Track

1. Changing Volcanism:
   • Initial massive basalt outpourings, head of rising mushroom plume.
   • Chain of rhyolite volcanic centers, stem of mushroom plume – analogous to the chain of Hawaiian islands developed on a continent.
   • Late-stage basalt occurs after a portion of the plate moves off the hotspot.
2. Explosive Volcanic Eruptions: Due to silica enrichment of original basaltic magma. Massive eruptions of light-colored rhyolite, pumice, and obsidian form gigantic collapse calderas.
3. Shallow Earthquakes (Magnitude 4 and less): Earthquakes can be caused by rising magma or hot groundwater movement. But most earthquakes are due to the regional faults that have been formed due to crustal stretching and mountain building. Significant earthquakes can be created, such as the large earthquake of Hebgen Lake in 1959, but most are small.
4. Geothermal Features: Developed where groundwater encounters rock heated by shallow magma over hotspot (geysers, hot springs, mud pots).

How is Yellowstone Magmatism Related to Regional Volcanism in the Northwest?

There are a series of caldera eruptions that move west to east (hotspot pattern like Hawaii). Hotspot volcanism in oceanic lithosphere settings produces basaltic lava like in Hawaii because oceanic lithosphere is only one type of rock. A hotspot surfacing beneath an ocean forms a chain of volcanic islands.

Continental Hotspot Volcanism

1. Stage 1: Initial rise of mantle plume.
2. Stage 2: Melting of asthenosphere to generate basaltic magma.
3. Stage 3: Eruption of the basaltic lava as voluminous “flood basalts” (come out of fissures).
4. Stage 4: Melting of the continental lithosphere to generate rhyolitic magma (and caldera-forming super eruptions).

Hotspot right under Yellowstone today. Continental hotspot volcanoes produce bimodal volcanic rocks … of rhyolite and basalt with nothing compositionally in between. Columbia River basalts (18 million years old). Rocks represent the earliest stage of hotspot magmatism when the plume is large and first impinging on the base of the lithosphere. Plate motion shows a consistent clockwise rotation of the Pacific Northwest (plate moving west).

Geothermal Activity in Yellowstone National Park

The Hydrologic Cycle

Groundwater is a component of the hydrologic cycle. Hydrologic cycle processes: evaporation, transpiration, precipitation, infiltration, runoff.

The Underground Reservoir

Some precipitation enters the subsurface via infiltration. Soil properties and vegetation govern infiltration rate. Infiltrated water adds to soil moisture and groundwater. Soil moisture wets the soil. Some is wicked up by roots, some is evaporated.

What Controls Movement of Groundwater?

Groundwater resides in subsurface pore spaces. Pores are open spaces within sediment or rock. Total volume of open space is termed porosity. Geologic materials exhibit a wide range of porosities. Permeability is the ease of water flow due to pore interconnectedness. Highly permeable material allows water to flow readily. Water flows slowly through less permeable material. Many large and straight flow paths enhance permeability.

Aquifers and Aquitards

Both are commonly on top of one another (interlayered). Aquifer: Sediment or rock that transmits water easily. Aquitard: Sediment or rock that hinders water flow.

Groundwater Flow

Unsaturated and saturated zones are separated by the water table (unsaturated does not have water, saturated has water). Groundwater flows slowly under the influence of gravity. Rivers and streams are expressions of the water table. Flow in the unsaturated zone is straight downward. In the saturated zone, flow is more complicated (governed by gravity and pressure gradients).

Types of Hydrothermal Features

• Geysers: Hot springs with constrictions in their plumbing, which causes them to periodically erupt to release the pressure that builds up.
• Hot Springs: Pools of geothermally heated water.
• Fumaroles: AKA steam vents. These hot features lack water in their system and instead constantly release steam.
• Travertine Terraces: Hot springs that rise through limestone, dissolve the calcium carbonate, and deposit the calcite that makes the travertine terraces.
• Mudspots: Hot springs that are acidic enough to dissolve the surrounding rock. Typically also lack water in their systems.

Geysers: A Special Type of Spring with Three Features

1. Heat source: Hot magmatism below Yellowstone geyser basins provides the heat needed to produce geyser eruptions.
2. Water source: Snowmelt from the surrounding mountain ranges and rainfall soaks into the ground. This water flows into the geyser basins as groundwater; however, it becomes heated as it travels further down.
3. Plumbing in the groundwater system: All geysers have a surface vent connected to a pipe-like crack system that has narrow constrictions connected to deeper, wider chambers.

Hot Springs: The Most Common Hydrothermal Features

Form where there is not a severe restriction in groundwater flow to the surface. Hot water can flow more continuously to the surface without erupting. Convection currents constantly circulate the water, preventing it from getting hot enough.

Thermophiles and the Intense Coloration of Thermal Pools

Blue color of the water: The intense blue color of some springs results when sunlight passes into their deep, clear waters. Blue, a color visible in light, is scattered the most and the color we see.
Oranges/yellows: Intense orange and yellow colors around the edges are caused by heat-loving microbes that thrive in the extreme environment.

Fumaroles (AKA Steam Vents)

Occur when a hydrothermal feature has so little water in its system that the water boils away before reaching the surface. Steam and other gases emerge from the feature’s vents.

Travertine Terraces: Formed from Limestone

Limestone is a common type of carbonate rock, made mostly of calcite. Thermal water rises through the limestone in the subsurface, carrying high amounts of the dissolved calcium carbonate. At the surface, CO₂ is released and calcium carbonate is deposited, forming travertine, the chalky white mineral forming the rock of travertine.

Mudspots

A geothermal feature that forms in low-lying areas where thermal waters are impeded by an aquitard, typically clay, and are unable to percolate deeper into the subsurface. Thermal water beneath the depression causes steam to rise through the ground, heating the collected surface water and causing a gooey mix of bubbling clay, water, and hydrogen sulfide gas.

Petrified Forest National Park

Regional Geology of the Southwest

National parks of Arizona and Utah. Formations can be traced long distances. Overlap is seen in the sequences of rock types. Overlapping rock columns are used to build a composite. Chinle rock formation is Triassic-aged, while Kaibab is Permian.

The Chinle Formation (Late Triassic, 225–208 Ma)

As seas retreated, a major river system developed along the southwestern states, originating perhaps in Texas and flowing northwest. Petrified right on a river. Lowland terrestrial environments, including river channels, floodplains, swamps, and small lakes. The rocks of the Chinle Formation suggest a strongly seasonal subtropical climate with an upsection transition to an increasingly arid climate. Colorful layers in the Chinle Formation represent ancient soil horizons along river floodplains (coloration due to the presence of various minerals; shows oxidation or reduction). While the red and green layers contain the same amount of iron and manganese, differences in color depend on the position of the groundwater table when the ancient soils were formed. In soils where the water table was high, a reducing environment existed due to a lack of oxygen in the sediments, giving the iron minerals in the soil a greenish or bluish hue. Reddish soils were formed where the water table fluctuated, allowing the iron minerals to oxidize.

Floodplain and Swamp Depositional Environments

Floodplains and swamps receive mostly fine-grained sediments and are often the sites of rich organic deposits. Paleosols (ancient soil horizons), coal beds (resulting from compression of organic matter), plant fossils, symmetrical ripples, and concretions are common features typical of floodplains.

Paleogeographic Reconstruction of the Chinle River System

Triassic Conifers (Tropical)

Araucarioxylon arizonicum is an extinct species of conifer that is the state fossil of Arizona (species known from massive tree trunks that weather out of the Chinle Formation in desert badlands). High quality of preservation and little decay suggests uprooting, transport, and swamp burial occurred in relatively short timespans, possibly during monsoonal floods.

Fossils of the Chinle Formation

• Petrified conifer logs
• Fossilized plants (needles, pollen, twigs)
• Reptile bones and footprints

Fossils are clues to the past, allowing reconstruction of ancient ecosystems and environments.

Silicification and Formation of Petrified Wood

Needs moisture, time, and aqueous silica in the groundwater. Groundwater is enriched in silica derived from the felsic volcanic ash. Permineralization: Mineral matter fills voids and surrounds cells or pores without destroying delicate cellular structure.

Silica and Fossilization

Silica is one of the most abundant minerals in Earth’s crust and is present in both crystalline and amorphous forms. Quartz: Crystal growth in space. Some silica forms amorphous or cryptocrystalline. Opal is hydrated amorphous silica.

Source for Silica-Enriched Fluids

Analog: Eruption of volcanic ash (Mount St. Helens).

Tectonic Setting of the Chinle Formation

Subduction plate down southwest away. Volcanoes from subduction erupted and gave silica to Petrified Forest. Inland from a major subduction zone along western North America. Sediment river system with a drainage basin in Texas.

Glacier National Park

Mountain Belts Formed by Compressive Tectonic Stress

• Cordilleran (Andean style): Glacier, Rocky Mountain, Arctic Gates, and Denali National Parks. Mountains form due to deformation of the continental plate during convergence.
• Collisional (Himalayan): Shenandoah, Blue Ridge, Great Smoky Mountain, and Acadia National Parks. Mountains form due to the collision between two continents.

Glacier National Park is found in northwestern Montana (part of the Northern Rocky Mountains).

North American Cordillera

Includes the mountainous region in the west. Extends from Alaska and northern California to southern Mexico. Represents multiple phases of tectonic deformation that affected the western part of North America.

Glacier National Park

Long, linear, rugged mountain ranges.

Andean-Type (Cordilleran) Mountain Belts

Products of oceanic plate subduction under continental crust.
Trench: Where oceanic plate is being subducted.
Subduction zone: Major zone of ruptures and earthquakes.
Accretionary wedge: Sediment scraped off from subduction (covering trench, maybe).
Volcanic arc and batholith (plumbing system) below.
Fold and thrust belt: Where continental plate is being shortened or thickened.
Foreland sedimentary basin: Folding and thrusting causes accumulation of sediment.

Deformation (Changes in Shape, Size, Location, Orientation in Rocks) Caused by Three Types of Stress

Compression (compression causes shortening). At shallow depths, shortening occurs by brittle deformation along faults where one rock mass is thrust over another. At deeper levels where temperatures are high, compressional forces squeeze and fold rock masses.

Fold and Thrust Belt: A Large-Scale Mountain System Formed by Thickening and Shortening of the Continental Lithosphere

Folded Rocks: Anticlines and Synclines

During ductile deformation (requires time, heat, and pressure), rocks can be bent into a series of wave-like undulations called folds. Anticlines are upfolded or arched sedimentary layers (oldest strata are in the center; rocks tilt away from hinge). Synclines are downfolded or troughs of rock layers (youngest strata are in the center; rocks tilt toward the hinge).

Fold Geometry

Depending on their orientation, anticlines and synclines can be described as:
Symmetrical: Limbs of the fold are mirror images of each other.
Asymmetrical: Limbs of the fold are not identical.
Overturned: One or both limbs are moved beyond vertical.

Evolution of the North American Cordillera

Orogeny: Process of mountain building through tectonic deformation of the Earth’s lithosphere (orogen – mountain belt). Mesozoic subduction along western North America represents a convergent plate boundary. The Sierra Nevada batholith is evidence of the ancient (Mesozoic) volcanic arc. But compressional stresses affected the continental interior as well as the Sevier mountain belts and Laramide Rocky Mountains.

Rocky Mountains of the West Formed During Multiple Orogenic Events (Formed During Two Major Phases of Contractional Deformation)

• Late Jurassic–early Cenozoic “The Sevier Orogeny”: Characterized by low-angle thrust faulting and folding of sedimentary strata. Lots of crustal shortening and formation of long, linear mountain belts.
• Early–mid Cenozoic: Characterized by high-angle reverse faulting and uplifted crystalline basement. Relatively less crustal shortening and discontinuous mountains.

The Belt Supergroup: Uplifted Proterozoic Rocks

Belt Supergroup rocks in Glacier National Park make up the Lewis Range (hanging wall block), and Chief Mountain lies to the east of the main thrust fault. These rocks were deposited in a 1.6–1.2 Ga sea basin and show little to no metamorphism despite their age. Footwall block made up of Cretaceous shale.

The Lewis Thrust Fault

Became active about 170 Ma as a result of compressional stress in the North American Cordillera. Deformation along the Lewis Thrust and other structures caused uplift of the Rocky Mountains. Chief Mountain is an example of the hanging wall of the Lewis Thrust. The mountain is an isolated remnant of the hanging wall block “thrust sheet.”

Mountain Belts Formed by Compressive Tectonic Stress

Movement along the Lewis Thrust Fault displaced mainly Precambrian rocks over Cretaceous strata in what is now Glacier National Park. Erosion sculpted the thrust sheet and created an isolated remnant of the sheet called Chief Mountain.

Continental Rifts (Where Parts of Continental Lithosphere are Being Stretched)

Many national parks and landscapes in the west are located in continental rifts (e.g., Teton National Park, Death Valley National Park, White Sands National Park, New Mexico). Broadly, these are mostly in the Basin and Range Province and the narrow plate. Normal faulting occurs in the Basin and Range Province.

What Causes Rocks to Deform?

Generally, Rocks Deform in One of Two Ways: They Break or They Bend

Behavior Depends on:

• Rock properties (mineralogy, preexisting weakness)
• External conditions (pressure, temperature)
• Magnitude and rate of the applied stress

When a rock breaks, it is called brittle deformation. Any material that breaks into pieces exhibits brittle behavior. When a rock bends or flows, like clay, it is called ductile deformation.

Brittle Structures: Faults and Fractures

A fracture is a break in the rocks that make up the Earth’s crust. Joints are fractures in a rock where there has been no rock movement. Most joints appear in parallel groups or “sets.” A fault is a break in the rocks that make up the Earth’s crust, along which rocks on either side have moved past each other. Fault motion is the cause of earthquakes, as rocks break in response to the buildup of tectonic stress. Many earthquakes over millions of years result in large fault displacement and produce spectacular mountains and topography.

How Do Rocks Deform?

Stress is the pressure or tension exerted on a material object, i.e., the force that deforms rocks. Three types of stress:
Compressional stress: Squeezes a rock and shortens it (reverse and thrust faults).
Tensional stress: Pulls apart a rock and stretches it (normal faults).
Shear stress: Produces distortion or shearing of a rock (strike-slip faults).

At shallow depths, rocks exhibit brittle fracture. At deeper crustal depths, rocks deform by ductile flow. When stresses acting on a rock exceed its strength, the rock will deform by fracturing, faulting, or flowing. Deformation is a general term that refers to all changes in the shape or position of a rock body in response to stress.

Strike-Slip Faults: Vertical or Steeply Inclined Fault Across Which Crustal Blocks Move Laterally (Side by Side) (e.g., San Andreas Fault)

Forms under shear stress. Causes lateral displacement and local extension and convergence. Associated with transform plate tectonic settings. Are further defined by the relative direction of horizontal displacement. The relative motion across a strike-slip fault is determined by standing on one side of the fault and watching which way the other side moves (right-lateral and left-lateral).

Hanging wall block: A rock surface above the fault.
Footwall block: The rock surface below the fault.

Normal Faults

Hanging wall moves down relative to the footwall block. Forms under tensional stress. Causes rock thinning and stretching. Associated with divergent plate tectonic settings.

Reverse and Thrust Faults

Hanging wall moves up relative to the footwall block.
Reverse fault: High-angle fault.
Thrust fault: Low-angle fault.

Forms under compressive stress. Causes rock thickening and contraction. Associated with convergent plate tectonic settings.

Oblique-Slip Faults

Fault motion has both horizontal and vertical components of offset. Form due to either transpressional (transform + compression) and transtensional (transform + extension).

Continental Rifts

Many national parks and landscapes in the west are located in continental rifts (mostly in the Basin and Range Province and the narrow Rio Grande Rift).

Normal Faulting Occurs in the Basin and Range Province (Continental Extension)

Horst: Uplifted fault block (mountain range).
Graben: Down-dropped fault block (valley).

Continental Rifts

Zones of lithospheric deformation and normal faulting. Form due to tensional stresses in the lithosphere. Can be wide provinces (Basin and Range Province) or narrow zones (Rio Grande Rift).

Grand Teton National Park

Uplifted footwall fault block above Teton Fault escarpment as Teton Range. Rift valley, down-dropped hanging wall block below Teton Fault as Jackson Hole.

The Teton Fault

Active and capable of generating earthquakes. Fault scarp within glacial deposits (20,000 years old) is along the mountain front. Geoscientists have dated the two most recent earthquakes at 4,800 and 8,000 years ago.

Structural Cross-Section of the Teton Range and Jackson Hole Valley

Teton Range being uplifted while Jackson Hole Valley is dropping.

The Teton Fault

Rocks in the Teton Range are Precambrian crystalline basement. Paleozoic sedimentary rocks (pre-faulting). Cenozoic rift fill deposits (strata). Faulting began 24 Ma, but the modern Teton Range and Jackson Hole are largely the results of fault displacement within the last 5 Ma.

Paleozoic Sedimentary Rocks Atop the Teton Range Record West Tilting of the Teton Range

The Basement Geology: Very Old Rocks

The oldest rocks in Grand Teton National Park are the Webb Canyon Gneiss, which has a U-Pb date of 2.68 Ga. The gneiss (highly metamorphosed rock) represents metamorphism and deformation of original sandstone, limestone, and shale that were deposited in a marine environment.

Age of Cratonic Basement (>2.5 Ga)

Metamorphic Rocks in Grand Teton National Park

Webb Canyon Gneiss (2.68 Ga). Formed during continental collision between tectonic plates, analogous to the Himalayan mountain-building setting. High pressure and temperatures deep within the collision zone caused the rocks to deform, metamorphose, and partially melt. Different minerals change at different conditions, shown in metamorphic rocks. Metamorphic rocks form by the alteration of other rocks at high temperatures and pressures. Metamorphism causes chemical (mineralogical) and textural changes in igneous, sedimentary, and other metamorphic rocks.

Metamorphic Environments

Temperature and pressure both increase with depth from the Earth’s surface conditions. Sedimentary environments → Metamorphic environments → Igneous environments.

Metamorphic “Grades”

1. Metamorphic grade is the degree to which the parent rock changes during metamorphism, and these grades reflect the temperature and pressure conditions.
   • Low grade: Low temperature and low pressure
   • High grade: High temperature and high pressure
2. During metamorphism, the rock remains essentially solid. In high-grade metamorphic conditions, some melting of rocks will occur.

Metamorphic Textures (Foliated)

Low-grade metamorphism → High-grade metamorphism (slate, phyllite, schist, gneiss; slaty, phyllitic sheen, schistose, gneissic).