There are just under 2 billion years of Geologic History in Pinal County. Some pieces are missing due to erosion, but much is left. Here's a brief overview of that history.
Precambrian (Proterzoic)
2 billion years ago
There was no land in what is now Arizona; only ocean was present. The smaller North American continent consisted only of Wyoming and much of Canada.
Older Precambrian (Proterozoic)
The Earth was formed by about 4.5 billion years ago at the beginning of the Hadean Era. In Arizona, those earliest rocks are either buried very deep or have been eroded away completely.
1.7 billion years ago
There was volcanic activity and sediment deposition in this area associated with island arcs and other plate tectonic activity. The crocks were buried and metamorphose to for the Pinal Schist. The Pinal Schist is the oldest rock in the county. It has generally weathered to low hills. It is found behind the Sheriff’s Station on Hunt Highway along Walker Butte Granite Quarry Road, Gonzales Palss on Route 60 and many other places in southern Arizona.
There was no land in what is now Arizona; only ocean was present. The smaller North American continent consisted only of Wyoming and much of Canada. The “land” that is now Arizona was located near the South Pole.
The oldest rocks now found in Pinal County, and most of southern Arizona, are the Pinal Schist. This rock was formed in the early Precambian, about 1.7 billion years ago. These rocks are about the same age as the Vishnu Schist which makes up the very bottom of the Grand Canyon in northern Arizona. There are some slightly older rocks exposed at Bear Canyon, near Tucson.
Pinal Schist
Precambrian
1.7 billion years old
Pinal Schist is a metamorphic rock, formed from sandstone and shale. These earlier sedimentary rocks were compressed marine deposits from the bottom of an early sea that covered Arizona about 2 billion years ago.The Pinal Schist can be found at least from Arizona east to Texas and north into Colorado.
Younger Precambrian
1.2 to 1 billion years ago
1.65 billion years ago
Magma intruded into the Pinal Schist forming Granite. It is found in the hills in Solera and on Gary Road southwest of Walmart in Coolidge.
Granite
Precambrian
1.65 billion years old
Granites are mostly made up of the minerals quartz and feldspar with minor amounts of mica, amphiboles, and other minerals.
Granite is formed when magma cools under the surface of the Earth slowly enough that visible crystals form. After cooling, these Pinal County Granites were pushed up through the Pinal Schist.
There have been several episodes of granite formation in county history.
1.45 billion years ago
More granite magma intruded into this area. It forms Cholla Mountain north of Coolidge, is the rock in the Walker Butte Granite Quarry, and is the base of Walker Butte, Poison Butte, and the hills by the hospital.
Conglomerate
Precambrian
1.2 billion years old
Conglomerate is a sedimentary rock made of other rocks that have been cemented together. The rock pieces making up conglomerate are about 2 millimeters or more across, and usually have rounded surfaces due to their having been tumbled around underwater.
In Pinal County these rocks were formed in shallow seas or near the mouths of rivers along a shoreline.
The smaller rocks are older than the Conglomerate itself, and have been rounded by being tumbled in water.
Hornfels
Precambrian
1.2 billion years old
Hornfels is a metamorphic rock formed from sedimentary rocks that have been heated and partially melted by contact with hot igneous masses such as still-cooling granite.
Marble
Precambrian
1.2 billion years old
Marble is a metamorphic rock formed from Limestone. The Limestone in Pinal County was deposited in shallow seas, and later partially melted and squeezed to turn it into marble. Fossils found in the earlier limestone can sometimes be seen in Marble.
Quartzite
Precambrian
1.2 billion years old
Quartzite is a metamorphic rock formed almost entirely from quartz. It is formed from sandstone that has been heated and pressurized by burial. The sand grains are cemented together by silica.
Diabase
Precambrian
1.1 billion years old
The gold flecks in the diabase is pyrite (“fools gold”).
1 billion years ago
The area was pulled apart, sediments were deposited, and magma was intruded along the cracks forming diabase intrusions. They best seen at the Salt River Canyon, and are scattered around Pinal County.
Paleozoic
541 to 251 million years ago
During this time, North America was moving east to form the super continent of Pangea and the west coast was passive. The sea rose and fell over Arizona several times depositing mainly limestone. These are the rocks seen at the Grand Canyon. In Pinal County most rocks of this ages have been eroded away. The best local place to see them is in the road cuts just east of Superior.
Limestone
Paleozoic, Mississippian
330 million years old
This Limestone was formed in shallow seas that covered Pinal County 330 million years ago. Limestone is formed from shells, corals, algae and other debris that drops to the bottom in clear shallow seas.
Mesozoic
252 to 66 million years ago
Pangea was breaking apart, the Atlantic Ocean was forming and North America started to move west. Southern Arizona was pushed up into mountains and many of the older rocks are eroded away.
80 to 50 million years ago (a time in western North America referred to as the Laramide Orogeny by geologists)
The subducting plate of the Pacific Ocean moved east under Arizona. Granite magma intruded into the area and the copper deposits were formed. This granite is present at Twin Buttes west of the Hunt Highway, at Grayback Mountain east of Florence and at the Boulders on the Florence Kelvin Highway. Florence Copper, Ray Mine, Sacatan Mine (in northwest Casa Grande), and many other ore deposits formed at this time.
East of Arizona the Western Interior Sea formed and retreated, covering what is now the Great Plains.
Granite
Cretaceous
70 million years old
The late Cretaceous in Arizona was a time of great volcanic activity. This was part of the Laramide orogeny (80-35 million years ago). This was a time of great mountain building throughout western North America.
Cenozoic
66 million years ago to the present
66 million years ago
North America continued to move west and the Laramide activity continued in the area until about 50 million years ago.
30 to 15 million years ago
Igneous (volcanic) activity started again in Pinal County, this time in the form of explosive volcanoes mainly centered in the Superstition Mountains. The rocks are light colored tuffs and rhyolites. In this area, they form the Superstition Mountains, Picketpost Mountain, North Butte, South Butte, and Box Canyon. These rocks are also present on the hills by the hospital.
15 million years ago to Present
The San Andreas Fault formed in California and southern Arizona was pulled apart forming the Basin and Range topography. The ranges are pushed up and the basins (valleys) dropped down. Some basins are over 2 miles deep and filled with sediment eroded off the ranges.
Perlilte/Obsidian
Miocene Epoch
15 to 25 million years old
Perlite and Obsidian are volcanic glass. Obsidian is formed when some lavas cool extremely quickly without the ability to form crystals, which take some time to grow. Perlite is formed when the same lavas cool rapidly in the presence of water.
Tuff
Miocene Epoch
15 to 25 million years old
Tuff is an igneous rock formed from volcanic. This Tuff was formed during a series of volcanic eruptions in southern Arizona.
Late Tertiary (Pliocene)
8 to 5 million years ago
Basalt magma moved up from the mantle along the cracks caused by the pulling apart forming the Basin and Range. In Pinal County the volcanic activity associated with the basalt was from 8 to 5 million years ago. In other parts of Arizona, it started earlier and the last activity of this type in the state was at Sunset Crater north of Flagstaff, less than 1,000 years ago. The basalt here is found on top of Walker Butte, Hospital Hills, Poston Butte, and several other hills.
Basalt
Miocene Epoch
6 million years old
Basalt is a igneous rock that cools fairly quickly from lava, fast enough that only very small crystals can form – sometimes not visible without a magnifying glass. In Arizona it is mostly found forming the tops of mesas.
Cinder Volcanic Rock
From Vulcan Materials Quarry, Florence, AZ
About 6 million years old
Cinder, also called scoria, is a volcanic rock formed when lava cools fairly quickly. Gases dissolved in the molten rock form bubbles as it cools, but not all of them are able to escape into the air before the rock hardens.
Today
Erosion continues lowering all hills and filling all basins. Miles of rock have been eroded away from over this area in the last 1.7 billion years.
Modified from an article by Dr Ray Grant, Director, Pinal Geology and Mineral Museum.
