If Rocks Could Talkâ¦
If Rocks Could Talk… Presented by the
McDowell Sonoran Field Institute a program of the
McDowell Sonoran Conservancy People Preserving Nature
Remembrance of Things Past: Types of Rocks •
Igneous
•
Sedimentary
•
Metamorphic
•
Rock cycle
Solidified from molten state (magma) Fast cooling lava Slow cooling granite Erosion, gravity and pressure Sand sandstone, mud shale Skeletons limestone “Changed” by reheating and pressure (but not re melted) Sandstone quartzite Shale mica/slate/schist Limestone marble “Recycling” of rock over geologic time due to mantle convection 2
Creation and Destruction of Rocks: Interior Structure of the Earth • Core Nickel iron Very hot (~5500° C or ~10,000° F)
• Mantle Many chemicals “Plastic” upper layer (magma) Convection currents in mantle drive tectonics
• Crust Thin under oceans, thicker on continents Crust floats on mantle and is dragged along as mantle material circulates 3
Creation and Destruction of Rocks: Plate Tectonics
•
Plate tectonics Earth’s crust divided into plates Ocean ridges are plate boundaries Magma comes up through ridges and expands Expansion of plates at ridges
•
causes “collisions” elsewhere
Subduction—one plate going beneath another Heat
magma
volcanoes
The Oldest Valley Rocks
•
Created during the assemblage of an ancient supercontinent by tectonic forces ~1.8 billion years ago
•
Coastal volcanoes activated by subduction produced thick lava – igneous rhyolite
•
Nearby shallow sea accumulated sediment that became sandstone
•
Continued subduction carried rhyolite and sandstone to metamorphic depths and produced meta-rhyolite (southern McDowells) and quartzite (western McDowells) ~1.7 billion years ago
Conception of Ancient Supercontinent (io9.com, Alasdair Wilkins)
Intrusion from Below: the Granite Invasion •
About 1.4 billion years ago, the foundation rocks were intruded from below by magma from the interior
•
This magma solidified into granite, coarse-grained where it cooled slowly and finer-grained where cooling was faster
•
Today this granite is visible around the Tom’s Thumb trailhead in the northeastern McDowells, in places near the Sunrise access area, and throughout north Scottsdale
Source: www2.brevard.edu
Source: paleogeology.blogspot.com/2008/11/igneous-structures.html
Geologic History Revealed in Mountains – Mazatzals
• Highest points ~7500 feet (e.g. Four Peaks) • Exposed ancient granite topped with older metamorphic quartzite (granite forming magma intruded from below)
7
Geologic History Revealed in Mountains – McDowells
• About 4000 feet high (East End, Thompson Peak) • Granite to northeast and southeast, older metamorphic rock •
elsewhere (the layers have been tilted over to the side) Smaller mountains in northern Scottsdale (Pinnacle Peak, Troon, Granite, etc) and Preserve land north of Dynamite are mostly granite
8
Northern Scottsdale Granite
Granite has three main components: milky white feldspar crystals, black biotite, and clear quartz crystals. 9
Northern Scottsdale Granite (continued)
10
Spheroidal weathering – weathering with rounded edges – is characteristic of granite. Granitic rock isn’t internally organized but rather is uniform throughout, so when it erodes there’s no preferred or consistent direction of fracture. This random erosion produces rounded contours. Large granitic masses often erode into what look like “piles” of rounded boulders.
Preserve Rocks (continued)
Much of the rock in the McDowells is metamorphic. In what ways does it look different from the granitic rock?
11
Intermission
If most rocks in the McDowell Mountains and northern Scottsdale had been formed by 1.4 billion years ago, what happened between then and now? A lot—but not until “recently”.
12
The (Very Long) Undersea Era
•
The foundation rocks and later intrusive granite were mostly underwater until Rockies formed
•
While underwater, these rocks were covered with sedimentary rock which eroded away during periods of uplift or sea level decline
•
Final erosion began 75 – 65 million years ago when the Rockies formed; little ancient sedimentary rock exists in the Valley 13
Interlude: Is there any sedimentary rock here? • •
The sedimentary rock from 65 – 75 million years ago has eroded away.
•
Young sedimentary rock is found in the head of Camelback, Red Mountain (below), the Papago Buttes and a few other places.
•
Sedimentary rock sometimes is found in the Preserve, mostly washed there from elsewhere.
Erosion of the newly uplifted rock produced new sediment which over time became “young” sedimentary rock.
14
Things Get Interesting: The Volcanic Era 25 MYA – 15 MYA • Ancient basement rocks mostly
unaffected by 1+ billion years of submergence and uplift
• Erosion removed almost all the rock that accumulated underwater
• New rock appeared with local
volcanic activity ~25 million years ago Superstition and Goldfield Mountains are volcanic caldera remnants (top) Lava flows: northern Phoenix Mountains (Moon Hill), top of Brown’s and Cone Mountains in northern Scottsdale, hills around Shea/136th (bottom) 15
The Volcanic Era: 25 MYA – 15 MYA (continued)
• Occasional explosive eruptions produced tuff (welded ash) layers visible on Brown’s Mountain • Flowing vs. exploding lava Chemical composition of magma (high vs. low silicon) Gas pressure Note “vesicular” basalt at Brown’s Mountain Light colored horizontal tuff layer visible in Pass Mountain, eastern Mesa
16
Brown’s and Cone Mountains
Notice how different Brown’s Mt. (flat top at far left) looks from Cholla Mt. (far right). Cholla is granite formed from magma that solidified deep underground ~1.4 billion years ago. It has characteristic spheroidal weathering and looks like a huge pile of rounded boulders. Most of the upper part of Brown’s Mt. is volcanic in origin and was formed at/near the surface in the last 10 – 20 million years. 17
Brown’s and Cone Mountains (continued)
18
The Latest Chapter: The Basin and Range 15 MYA - Present
•
Geological history of central Arizona is unique
•
Pacific oceanic ridge swerved east toward edge of North America
•
This ridge—a source of fresh magma from below— was carried beneath the southwest
The Basin and Range 15 MYA – Present (continued)
•
East – west spreading along the subducted ridge continued under the North American plate
•
This underground spreading stretched the brittle crust above it in an east – west direction
•
Parts of the southwest stretched up to 500%, which caused crustal faulting in the direction perpendicular to the stretching and the creation of blockfault ranges including the McDowells
•
This process built most of the parallel north – south basins and ranges in southern and western Arizona
Basin and Range Alignment
21
Main Scottsdale Rocks – Summary Volcanic Basalt and Tuff (mostly east central and occasional northern Scottsdale)
~10 – 20 MYA Metamorphic Quartzite and Meta Rhyolite (mostly western and southern McDowells, respectively)
~1.7 BYA Intrusive Igneous Granite (mostly northeast/southeast McDowells and northern Scottsdale)
~1.4 BYA
22
References and Suggested Reading • A Natural History of the Sonoran Desert by the Arizona Sonora Desert Museum (Arizona Sonora Desert Press, 2000) • Geology of Arizona by Nations and Stump (Kendall/Hunt, 1981) • Guide to Rocks and Minerals by Mottana, Crespi and Liborio (Simon & Schuster, 1978) • Roadside Geology of Arizona by Chronic (Mountain Press, 1983) • Ancient Landscapes of the Colorado Plateau by Ron Blakey and Wayne Ranney (Grand Canyon Association, 2008) • The Broken Land by Frank L. DeCourten (The University of Utah Press, 2003) Please note that although the information presented is believed to be correct, is derived from reputable sources, and has been reviewed by a geologist, MSC does not guarantee its accuracy. 23
McDowell Sonoran Field Institute a program of the
McDowell Sonoran Conservancy People Preserving Nature
Remembrance of Things Past: Types of Rocks •
Igneous
•
Sedimentary
•
Metamorphic
•
Rock cycle
Solidified from molten state (magma) Fast cooling lava Slow cooling granite Erosion, gravity and pressure Sand sandstone, mud shale Skeletons limestone “Changed” by reheating and pressure (but not re melted) Sandstone quartzite Shale mica/slate/schist Limestone marble “Recycling” of rock over geologic time due to mantle convection 2
Creation and Destruction of Rocks: Interior Structure of the Earth • Core Nickel iron Very hot (~5500° C or ~10,000° F)
• Mantle Many chemicals “Plastic” upper layer (magma) Convection currents in mantle drive tectonics
• Crust Thin under oceans, thicker on continents Crust floats on mantle and is dragged along as mantle material circulates 3
Creation and Destruction of Rocks: Plate Tectonics
•
Plate tectonics Earth’s crust divided into plates Ocean ridges are plate boundaries Magma comes up through ridges and expands Expansion of plates at ridges
•
causes “collisions” elsewhere
Subduction—one plate going beneath another Heat
magma
volcanoes
The Oldest Valley Rocks
•
Created during the assemblage of an ancient supercontinent by tectonic forces ~1.8 billion years ago
•
Coastal volcanoes activated by subduction produced thick lava – igneous rhyolite
•
Nearby shallow sea accumulated sediment that became sandstone
•
Continued subduction carried rhyolite and sandstone to metamorphic depths and produced meta-rhyolite (southern McDowells) and quartzite (western McDowells) ~1.7 billion years ago
Conception of Ancient Supercontinent (io9.com, Alasdair Wilkins)
Intrusion from Below: the Granite Invasion •
About 1.4 billion years ago, the foundation rocks were intruded from below by magma from the interior
•
This magma solidified into granite, coarse-grained where it cooled slowly and finer-grained where cooling was faster
•
Today this granite is visible around the Tom’s Thumb trailhead in the northeastern McDowells, in places near the Sunrise access area, and throughout north Scottsdale
Source: www2.brevard.edu
Source: paleogeology.blogspot.com/2008/11/igneous-structures.html
Geologic History Revealed in Mountains – Mazatzals
• Highest points ~7500 feet (e.g. Four Peaks) • Exposed ancient granite topped with older metamorphic quartzite (granite forming magma intruded from below)
7
Geologic History Revealed in Mountains – McDowells
• About 4000 feet high (East End, Thompson Peak) • Granite to northeast and southeast, older metamorphic rock •
elsewhere (the layers have been tilted over to the side) Smaller mountains in northern Scottsdale (Pinnacle Peak, Troon, Granite, etc) and Preserve land north of Dynamite are mostly granite
8
Northern Scottsdale Granite
Granite has three main components: milky white feldspar crystals, black biotite, and clear quartz crystals. 9
Northern Scottsdale Granite (continued)
10
Spheroidal weathering – weathering with rounded edges – is characteristic of granite. Granitic rock isn’t internally organized but rather is uniform throughout, so when it erodes there’s no preferred or consistent direction of fracture. This random erosion produces rounded contours. Large granitic masses often erode into what look like “piles” of rounded boulders.
Preserve Rocks (continued)
Much of the rock in the McDowells is metamorphic. In what ways does it look different from the granitic rock?
11
Intermission
If most rocks in the McDowell Mountains and northern Scottsdale had been formed by 1.4 billion years ago, what happened between then and now? A lot—but not until “recently”.
12
The (Very Long) Undersea Era
•
The foundation rocks and later intrusive granite were mostly underwater until Rockies formed
•
While underwater, these rocks were covered with sedimentary rock which eroded away during periods of uplift or sea level decline
•
Final erosion began 75 – 65 million years ago when the Rockies formed; little ancient sedimentary rock exists in the Valley 13
Interlude: Is there any sedimentary rock here? • •
The sedimentary rock from 65 – 75 million years ago has eroded away.
•
Young sedimentary rock is found in the head of Camelback, Red Mountain (below), the Papago Buttes and a few other places.
•
Sedimentary rock sometimes is found in the Preserve, mostly washed there from elsewhere.
Erosion of the newly uplifted rock produced new sediment which over time became “young” sedimentary rock.
14
Things Get Interesting: The Volcanic Era 25 MYA – 15 MYA • Ancient basement rocks mostly
unaffected by 1+ billion years of submergence and uplift
• Erosion removed almost all the rock that accumulated underwater
• New rock appeared with local
volcanic activity ~25 million years ago Superstition and Goldfield Mountains are volcanic caldera remnants (top) Lava flows: northern Phoenix Mountains (Moon Hill), top of Brown’s and Cone Mountains in northern Scottsdale, hills around Shea/136th (bottom) 15
The Volcanic Era: 25 MYA – 15 MYA (continued)
• Occasional explosive eruptions produced tuff (welded ash) layers visible on Brown’s Mountain • Flowing vs. exploding lava Chemical composition of magma (high vs. low silicon) Gas pressure Note “vesicular” basalt at Brown’s Mountain Light colored horizontal tuff layer visible in Pass Mountain, eastern Mesa
16
Brown’s and Cone Mountains
Notice how different Brown’s Mt. (flat top at far left) looks from Cholla Mt. (far right). Cholla is granite formed from magma that solidified deep underground ~1.4 billion years ago. It has characteristic spheroidal weathering and looks like a huge pile of rounded boulders. Most of the upper part of Brown’s Mt. is volcanic in origin and was formed at/near the surface in the last 10 – 20 million years. 17
Brown’s and Cone Mountains (continued)
18
The Latest Chapter: The Basin and Range 15 MYA - Present
•
Geological history of central Arizona is unique
•
Pacific oceanic ridge swerved east toward edge of North America
•
This ridge—a source of fresh magma from below— was carried beneath the southwest
The Basin and Range 15 MYA – Present (continued)
•
East – west spreading along the subducted ridge continued under the North American plate
•
This underground spreading stretched the brittle crust above it in an east – west direction
•
Parts of the southwest stretched up to 500%, which caused crustal faulting in the direction perpendicular to the stretching and the creation of blockfault ranges including the McDowells
•
This process built most of the parallel north – south basins and ranges in southern and western Arizona
Basin and Range Alignment
21
Main Scottsdale Rocks – Summary Volcanic Basalt and Tuff (mostly east central and occasional northern Scottsdale)
~10 – 20 MYA Metamorphic Quartzite and Meta Rhyolite (mostly western and southern McDowells, respectively)
~1.7 BYA Intrusive Igneous Granite (mostly northeast/southeast McDowells and northern Scottsdale)
~1.4 BYA
22
References and Suggested Reading • A Natural History of the Sonoran Desert by the Arizona Sonora Desert Museum (Arizona Sonora Desert Press, 2000) • Geology of Arizona by Nations and Stump (Kendall/Hunt, 1981) • Guide to Rocks and Minerals by Mottana, Crespi and Liborio (Simon & Schuster, 1978) • Roadside Geology of Arizona by Chronic (Mountain Press, 1983) • Ancient Landscapes of the Colorado Plateau by Ron Blakey and Wayne Ranney (Grand Canyon Association, 2008) • The Broken Land by Frank L. DeCourten (The University of Utah Press, 2003) Please note that although the information presented is believed to be correct, is derived from reputable sources, and has been reviewed by a geologist, MSC does not guarantee its accuracy. 23
