If Rocks Could Talkâ¦
If Rocks Could Talk…
©2013 McDowell Sonoran Conservancy
1
Dan Gruber Master Steward
McDowell Sonoran Conservancy
©2013 McDowell Sonoran Conservancy
2
Remembrance of Things Past: Types of Rocks Igneous • Solidified from molten state (magma) • Fast-cooling lava • Slow-cooling granite
©2013 McDowell Sonoran Conservancy
3
Remembrance of Things Past: Types of Rocks Sedimentary • Erosion, gravity and pressure • Sand sandstone; Mud shale • Skeletons limestone
©2013 McDowell Sonoran Conservancy
4
Remembrance of Things Past: Types of Rocks Metamorphic • “Changed” by reheating and pressure (but not re-melted) • Sandstone quartzite; • Shale mica/slate/ schist • Limestone marble
©2013 McDowell Sonoran Conservancy
5
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
©2013 McDowell Sonoran Conservancy
6
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
©2013 McDowell Sonoran Conservancy
7
Creation and Destruction of Rocks: Plate Tectonics Plate tectonics • Earth’s crust divided into plates • Ocean ridges are plate boundaries • Magma comes up through the ridges and expands • Expansion of plates at ridges causes “collisions” elsewhere
©2013 McDowell Sonoran Conservancy
8
Creation and Destruction of Rocks: Plate Tectonics
Subduction – one plate going beneath another • Heat magma volcanoes
©2013 McDowell Sonoran Conservancy
9
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
©2013 McDowell Sonoran Conservancy
10
The Oldest Valley Rocks • 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
©2013 McDowell Sonoran Conservancy
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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 forming the granite, coarse-grained where it cooled slowly and finer-grained where cooling was faster ©2013 McDowell Sonoran Conservancy
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Intrusion from Below: The Granite Invasion • 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
©2013 McDowell Sonoran Conservancy
13
Geologic History Revealed in Mountains – Mazatzals
©2013 McDowell Sonoran Conservancy
14
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)
©2013 McDowell Sonoran Conservancy
15
Geologic History Revealed in Mountains – McDowells
©2013 McDowell Sonoran Conservancy
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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 ©2013 McDowell Sonoran Conservancy
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Northern Scottsdale Granite
Granite has three main components: milky white feldspar crystals, black biotite, and clear quartz crystals. ©2013 McDowell Sonoran Conservancy
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Northern Scottsdale Granite
©2013 McDowell Sonoran Conservancy
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Northern Scottsdale Granite 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 or fracture. This random erosion produces rounded contours. Large granitic masses often erode into what looks like “piles” of rounded boulders. ©2013 McDowell Sonoran Conservancy
20
Preserve Rocks
©2013 McDowell Sonoran Conservancy
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Preserve Rocks
Much of the rock in the McDowells is metamorphic. In what ways does it look different from granitic rock?
©2013 McDowell Sonoran Conservancy
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Intermission
©2013 McDowell Sonoran Conservancy
23
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”.
©2013 McDowell Sonoran Conservancy
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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 ©2013 McDowell Sonoran Conservancy
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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 ©2013 McDowell Sonoran Conservancy
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Interlude: Is there any sedimentary rock here?
©2013 McDowell Sonoran Conservancy
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Interlude: Is there any sedimentary rock here? • The sedimentary rock from 65 – 75 million years ago has eroded away. • Erosion of the newly uplifted rock produced new sediment which over time became “young” sedimentary rock. • Young sedimentary rock is found in the head of Camelback, Red Mountain (photo), the Papago Buttes and a few other places. • Sedimentary rock sometimes is found in the Preserve, mostly washed there from elsewhere.
©2013 McDowell Sonoran Conservancy
28
Things Get Interesting: The Volcanic Era 25 MYA – 15 MYA
©2013 McDowell Sonoran Conservancy
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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 ©2013 McDowell Sonoran Conservancy
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Things Get Interesting: The Volcanic Era 25 MYA – 15 MYA
Superstition and Goldfield Mountains are volcanic caldera remnants ©2013 McDowell Sonoran Conservancy
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Things Get Interesting: The Volcanic Era 25 MYA – 15 MYA
Lava flows: northern Phoenix Mountains (Moon Hill), top of Brown’s and Cone Mountains in northern Scottsdale, hills around Shea/136th ©2013 McDowell Sonoran Conservancy
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Things Get Interesting: The Volcanic Era 25 MYA – 15 MYA • 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
©2013 McDowell Sonoran Conservancy
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Volcanic Era 25 MYA – 15 MYA
©2013 McDowell Sonoran Conservancy
34
Volcanic Era 25 MYA – 15 MYA Notice how different Brown’s Mt. (flat top at far left) looks from Cholla Mt. (far right). Cholla is granite formed 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.
©2013 McDowell Sonoran Conservancy
35
Volcanic Era 25 MYA – 15 MYA
©2013 McDowell Sonoran Conservancy
36
Volcanic Era 25 MYA – 15 MYA Volcanic rocks form the upper layers of Brown’s Mt (on right). The top is basalt, probably from the lava flows that originated in the Superstitions. Below that is a thick layer of tuff, a lightcolored rock formed as a result of explosive volcanic eruptions. This tuff layer also is exposed in a long, low ridge running east from the mountain. Below the tuff is another basalt layer from earlier lava flows. At the bottom is a 1.4 billion year old granite. Everything between the ancient granite at the bottom and the 10-20 million year old volcanic rocks at the bottom is missing, probably eroded away.
©2013 McDowell Sonoran Conservancy
37
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 ©2013 McDowell Sonoran Conservancy
38
The Latest Chapter:
The Basin and Range 15 MYA - Present
• 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
©2013 McDowell Sonoran Conservancy
39
The Latest Chapter:
The Basin and Range 15 MYA - Present
• Parts of the southwest stretched up to 500%, which caused crustal faulting in the direction perpendicular to the stretching and the creation of block- fault ranges including the McDowells • This process built most of the parallel north – south basins and ranges in southern and western Arizona ©2013 McDowell Sonoran Conservancy
40
The Latest Chapter: Basin and Range Alignment Notice how the mountain ranges (gold) in southwestern Arizona mostly are parallel to each other and aligned NW-SE.
©2013 McDowell Sonoran Conservancy
41
Main Scottsdale Rocks – Summary
©2013 McDowell Sonoran Conservancy
42
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)
©2013 McDowell Sonoran Conservancy
43
This presentation was created by a citizen scientist with the McDowell Sonoran Field Institute, the research center of the McDowell Sonoran Conservancy. The information presented is derived from reputable sources and has been reviewed by a geologist.
©2013 McDowell Sonoran Conservancy
44
©2013 McDowell Sonoran Conservancy
1
Dan Gruber Master Steward
McDowell Sonoran Conservancy
©2013 McDowell Sonoran Conservancy
2
Remembrance of Things Past: Types of Rocks Igneous • Solidified from molten state (magma) • Fast-cooling lava • Slow-cooling granite
©2013 McDowell Sonoran Conservancy
3
Remembrance of Things Past: Types of Rocks Sedimentary • Erosion, gravity and pressure • Sand sandstone; Mud shale • Skeletons limestone
©2013 McDowell Sonoran Conservancy
4
Remembrance of Things Past: Types of Rocks Metamorphic • “Changed” by reheating and pressure (but not re-melted) • Sandstone quartzite; • Shale mica/slate/ schist • Limestone marble
©2013 McDowell Sonoran Conservancy
5
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
©2013 McDowell Sonoran Conservancy
6
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
©2013 McDowell Sonoran Conservancy
7
Creation and Destruction of Rocks: Plate Tectonics Plate tectonics • Earth’s crust divided into plates • Ocean ridges are plate boundaries • Magma comes up through the ridges and expands • Expansion of plates at ridges causes “collisions” elsewhere
©2013 McDowell Sonoran Conservancy
8
Creation and Destruction of Rocks: Plate Tectonics
Subduction – one plate going beneath another • Heat magma volcanoes
©2013 McDowell Sonoran Conservancy
9
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
©2013 McDowell Sonoran Conservancy
10
The Oldest Valley Rocks • 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
©2013 McDowell Sonoran Conservancy
11
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 forming the granite, coarse-grained where it cooled slowly and finer-grained where cooling was faster ©2013 McDowell Sonoran Conservancy
12
Intrusion from Below: The Granite Invasion • 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
©2013 McDowell Sonoran Conservancy
13
Geologic History Revealed in Mountains – Mazatzals
©2013 McDowell Sonoran Conservancy
14
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)
©2013 McDowell Sonoran Conservancy
15
Geologic History Revealed in Mountains – McDowells
©2013 McDowell Sonoran Conservancy
16
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 ©2013 McDowell Sonoran Conservancy
17
Northern Scottsdale Granite
Granite has three main components: milky white feldspar crystals, black biotite, and clear quartz crystals. ©2013 McDowell Sonoran Conservancy
18
Northern Scottsdale Granite
©2013 McDowell Sonoran Conservancy
19
Northern Scottsdale Granite 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 or fracture. This random erosion produces rounded contours. Large granitic masses often erode into what looks like “piles” of rounded boulders. ©2013 McDowell Sonoran Conservancy
20
Preserve Rocks
©2013 McDowell Sonoran Conservancy
21
Preserve Rocks
Much of the rock in the McDowells is metamorphic. In what ways does it look different from granitic rock?
©2013 McDowell Sonoran Conservancy
22
Intermission
©2013 McDowell Sonoran Conservancy
23
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”.
©2013 McDowell Sonoran Conservancy
24
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 ©2013 McDowell Sonoran Conservancy
25
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 ©2013 McDowell Sonoran Conservancy
26
Interlude: Is there any sedimentary rock here?
©2013 McDowell Sonoran Conservancy
27
Interlude: Is there any sedimentary rock here? • The sedimentary rock from 65 – 75 million years ago has eroded away. • Erosion of the newly uplifted rock produced new sediment which over time became “young” sedimentary rock. • Young sedimentary rock is found in the head of Camelback, Red Mountain (photo), the Papago Buttes and a few other places. • Sedimentary rock sometimes is found in the Preserve, mostly washed there from elsewhere.
©2013 McDowell Sonoran Conservancy
28
Things Get Interesting: The Volcanic Era 25 MYA – 15 MYA
©2013 McDowell Sonoran Conservancy
29
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 ©2013 McDowell Sonoran Conservancy
30
Things Get Interesting: The Volcanic Era 25 MYA – 15 MYA
Superstition and Goldfield Mountains are volcanic caldera remnants ©2013 McDowell Sonoran Conservancy
31
Things Get Interesting: The Volcanic Era 25 MYA – 15 MYA
Lava flows: northern Phoenix Mountains (Moon Hill), top of Brown’s and Cone Mountains in northern Scottsdale, hills around Shea/136th ©2013 McDowell Sonoran Conservancy
32
Things Get Interesting: The Volcanic Era 25 MYA – 15 MYA • 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
©2013 McDowell Sonoran Conservancy
33
Volcanic Era 25 MYA – 15 MYA
©2013 McDowell Sonoran Conservancy
34
Volcanic Era 25 MYA – 15 MYA Notice how different Brown’s Mt. (flat top at far left) looks from Cholla Mt. (far right). Cholla is granite formed 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.
©2013 McDowell Sonoran Conservancy
35
Volcanic Era 25 MYA – 15 MYA
©2013 McDowell Sonoran Conservancy
36
Volcanic Era 25 MYA – 15 MYA Volcanic rocks form the upper layers of Brown’s Mt (on right). The top is basalt, probably from the lava flows that originated in the Superstitions. Below that is a thick layer of tuff, a lightcolored rock formed as a result of explosive volcanic eruptions. This tuff layer also is exposed in a long, low ridge running east from the mountain. Below the tuff is another basalt layer from earlier lava flows. At the bottom is a 1.4 billion year old granite. Everything between the ancient granite at the bottom and the 10-20 million year old volcanic rocks at the bottom is missing, probably eroded away.
©2013 McDowell Sonoran Conservancy
37
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 ©2013 McDowell Sonoran Conservancy
38
The Latest Chapter:
The Basin and Range 15 MYA - Present
• 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
©2013 McDowell Sonoran Conservancy
39
The Latest Chapter:
The Basin and Range 15 MYA - Present
• Parts of the southwest stretched up to 500%, which caused crustal faulting in the direction perpendicular to the stretching and the creation of block- fault ranges including the McDowells • This process built most of the parallel north – south basins and ranges in southern and western Arizona ©2013 McDowell Sonoran Conservancy
40
The Latest Chapter: Basin and Range Alignment Notice how the mountain ranges (gold) in southwestern Arizona mostly are parallel to each other and aligned NW-SE.
©2013 McDowell Sonoran Conservancy
41
Main Scottsdale Rocks – Summary
©2013 McDowell Sonoran Conservancy
42
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)
©2013 McDowell Sonoran Conservancy
43
This presentation was created by a citizen scientist with the McDowell Sonoran Field Institute, the research center of the McDowell Sonoran Conservancy. The information presented is derived from reputable sources and has been reviewed by a geologist.
©2013 McDowell Sonoran Conservancy
44
