The Condensed,
Superficial, and Very
Generalized Version of
Arizona’s Geologic
History:
how it was
formed
what remains
identifying what
we find
By Jan Sherwood
ARIZONA Raised Relief Map
Rocks of Arizona Today
When trying to
decide what rock
you are holding,
walking over, or
viewing from a
distance, focus
on where you are
and what the
structure of the
rock is.
In order to understand
Arizona rocks, we need to
know a bit of basic
geology.
Basic principles of geology:
Earth is made up of plates that move by convection taking place from
within.
Plates can move 3 ways: apart, diving under another, and slipping
sideways.
Plates carry the land and seafloor bed masses with them as they move.
Basic rock types:
Igneous rocks form directly from magma (lava). Arizona’s igneous rocks are pegmatite, granite , and diorite formed in batholiths
(picture the shape of Half Dome at Yosemite), sills, dykes, and laccoliths under ground (unerupted magma = granite) . Basalt
(Hawaii’s lava), andesite, dacite, and rhyolite are extruded on the surface, and welded tuff results from explosive pyroclastic flows.
Sedimentary rocks form from layers of broken debris that is left over from other rock, transported by water, wind, or gravity, then
compacted and cemented together. Arizona’s sedimentary rocks are sandstone, mudstone, conglomerate (cemented small rocks),
and limestone (cemented tiny sea skeletons)
Metamorphic rocks get their name from meta (change) and morph (form). Any rock can become a metamorphic rock, usually this
involves burial leading to a rise in temperature and pressure. The minerals of the original rock changes. Arizona’s metamorphic rocks
are quartzite, gneiss and schist.
Proterozoic
2.5 billion to 570 million years ago (mya)
•Arizona was near the present day South
Pole
•Plates were moving carrying island arcs
which then joined to the continent
(evidence of 3 large arcs joining to form
Arizona)
•Volcanoes were erupting; eroding into
shallow seas inland from each arc
attachment
Granite formed underground when magma
didn’t reach the surface and can be found
at: Big Horn, Black Canyon, Deem Hills,
Eagletail, Estrella, Goldfield, Lake Pleasant,
McDowell, Picacho Peak, Pinnacle Peak,
Prescott, San Tan, South Mountain, Spur
Cross, Superstition, White Tank, and
Wickenburg areas.
Rocks metamorphosed into gneiss, these
can be found at: Big Horn, Estrella, Phoenix
Mt Preserve, Picacho Peak, South
Mountain, White Tank, and Wickenburg
areas. Schist can be found in: Big Horn,
Estrella, Lake Pleasant, Phoenix Mt
Preserve, Picacho Peak, San Tan, and
Wickenburg areas.
Quartz Peak, Sierra Estrella Mountains: granite, gneiss,
quartzite, and pegmatite
pegmatite – similar to
granite but comprised
entirely of crystals at least
1cm in diameter
granite - even crystals of quartz, mica,
hornblende (black) & feldspar (pink or white)
Have you squeezed through this
granite of South Mountain?
gneiss – same
composition as
granite but
metamorphic –
stripes and bands
Schist of Piestewa Peak
Schist (easily splitting into
layers, metamorphic rock)
Granite of McDowell Mountains
weathers into boulders, spires, and
weird shapes
Camelback Mountain - body is granite
Granite and gneiss (notice the wavy
stripes) of the White Tank Mountains
Lake Pleasant area, granite and schist
Paleozoic
570 – 240 million years ago (mya)
•Arizona was near the present day Equator (but
equator was turned 90 degrees – it ran from
Mexico to Canada)
•Plates were moving apart and quieting down
•Shallow seas came in, seas went out, over and
over
•Deposits of sediments and volcanic rocks
accumulated along the shore and in shallow
seas
•Sand dunes formed during dry periods
•Some deposits were capped with limestone –
•Fossils found of: trilobites, coral, snails, clams,
and 1st fish
www.somewhereelseland.com
•Ice age came and species died out
•Land flooded
•Another Ice age came
•There is a warming trend, coal deposits result
in NE corner
•Another extinction
•Fossils found of: amphibians, reptiles
•Another extinction
sandstone
•Sedimentary rocks formed beneath sea,
multiple flooding and retreating resulting in
alternating rock bands of Monument Valley and
Sedona: sandstone, mudstone, conglomerate,
and limestone
Sedona alternating bands of sandstone, mudstone,
limestone, sand dune sandstone, and conglomerates
Limestone forming a greyish-white
ring around each formation.
Limestone is harder than
sandstone and often forms a ridge.
Sedona crossbedding in
sandstone from sand dunes
Conglomerate
(notice the imbedded
pebbles) formed
from eroded earlier
rocks carried by
rivers from ancestral
Rocky Mountains
•MESOZOIC
65 – 251 mya
•Arizona is emerging from the sea (which still
covered much of CA and NV). Most of the state is a
vast low-lying landscape.
•Fossils found of dinosaurs, lizards, birds, and
mammals
Granite of White Tank
•Northern AZ (Colorado Plateau ) deposits of
sandstone, shale (mudstone), conglomerate
(Canyon de Chelly)
•Forests form in NE (fossilized logs of Petrified
Forest National Park)
•Southern AZ volcanic field – gold, silver, and
copper (Bisbee) - hot water circulating near
magma forms the minerals
•Massive extinction
•Northern AZ = desert (like Sahara) with blowing
sand
Basalt of Hieroglyphic Mountains, Lake Pleasant
Area
•Very little new rock was added:
Granite of White Tank Mountains
Some basalt of Hieroglypic
Mountains,
gold in Vulture Mine
area
Sandstone of Papago Peak
Diorite intruded dikes of Big Horn
Mountains
Papago Peak sandstone
Diorite – similar to granite but with no quartz
Big Horn Mountain basalt, andesite, dacite, and rhyolite layers. As you
hike, look for diorite dikes cutting through the layers.
CENOZOIC
66
mya
to present
Goldfield Mountains– layers of rhyolite, basalt lava flow, pyroclastic ash and debris flow
The state is divided into 3 sections:
tuffs. The Goldfields lie just north of the Superstitions which were formed about 29 mya
1. Colorado Plateau uplifted
when a caldera (large magma chamber that collapses) occurred, rhyolite poured out, ash
2. transition zone - NW to SE (Mogollon
and rocks exploded covering a vast region.
rim) a crumbling plateau
3. Basin & Range - S & W – wide-spread
faulting, stretched and rifted, large
sections dropped down, valley filled
with debris
Widespread volcanic activity occurred
throughout the state’s south and
west. Basalt , rhyolite, dacite, and,
andesite flowed from cracks, cinder
cones built up, fast moving pyroclastic
flows carrying rock and gas exploded
with ash and cemented into tuff.
Granite formed underground.
Basalt, dacite, andesite, rhyolite,
By 20 mya Arizona was covered with
and welded tuff ash samples
1000’s of feet of volcanic rock.
Basalt can be found at: Big Horn, Black
Canyon, Deem Hill, Eagletail, Estrella,
Goldfield, Lake Pleasant, Phoenix Mt
Preserve, Prescott, San Tan, Sedona,
Spur Cross, and Superstitions.
Andesite
can be found at: Big Horn, Lake
Sunset crater north of Flagstaff,
Pleasant and Picacho Peak.
basalt and cinder cones
Dacite can be found at: Big Horn,
Goldfield and Wickenburg
Rhyolite can be found at:
Big Horn, Goldfield, Lake Pleasant,
Superstition, and Wickenburg.
Tuff can be found at: Black
Canyon, Goldfield, Phoenix Mt
Preserve, Picacho Peak, Prescott, San
Tan, Spur Cross, Superstition, and
Eagletail Mountains basalt, notice the
Wickenburg
Granite can be found at: McDowell,
layers of various flows
Pinnacle Peak, South Mt, and White
Tank
Camelback Mountain’s
sandstone head formed
about 30 mya (remember
it’s body is over 1.5 billlion
years old???) A geologic
mystery
Rhyolite layers and welded tuff of Chiricahua National
Monument in SE Arizona. At about 26.9 mya another caldera
collapsed, exploding rhyolite, ash and rocks to the depth of
over 1000 feet, then dacite welled up. Over time freezing and
thawing of water and erosion have carved the cliffs and
columns we see today.
All the features eroded, rock debris was blown, washed, or fell down
from the mountains, and filled the valleys.
Under Pebble Creek the deposits total about 2 miles in depth. You’d
have to dig down a mile or two through sand and gravel before you
hit bedrock.
GEOLOGY DESSERTS
chalcedony - forms when
ground water with dissolved
quartz fills a cavity near the
surface, as the water
dissolves away, layers line the
cavity
mica – super large crystals that form as
granite cools. Silvery, thin, flat, flexible
flakes that you can see light through
(isinglass – used to make windows long
ago)
geode- a hollow, spherical
crystal-filled volcanic bubble
quartzite - large crystals form in
pegmatite and quartzite chunks
litter the ground in many areas
And now for the test
OK, I’ll make it easier by adding roads
Generally
Pink shades are granites
formed underground
Orange shades are
volcanic rocks extruded
above ground
Green, Blue, and purple
shades are sediments
formed under the inland
seas
Grey are deposits from
most recent erosion
Can you find Pebble Creek? See I-10, I-8, I-40 and I-17 in yellow? US 60/Grand Ave is the diagonal red.
= Phoenix
Let’s Review
Multi-colored large grained rock is probably pegmatite.
If it has smaller grains it’s probably granite.
If it’s only black and white, it’s diorite.
All of these were buried deep within the crust, we have
a lot of granite around Phoenix.
Heavy, fine-grained rocks are probably basalt (black, often
rusty-colored), andesite (reddish), dacite (grey), veryfine grained rhyolite (lots of colors, yellowish in the
Goldfield Mts.). These flowed or exploded from
volcanoes or cracks in the earth. Mt St Helens is dacite,
remember seeing the explosion on the news? There are
a lot of these fine-grained rocks in the area also.
If it looks like cemented BBQ ash, it’s tuff. Yes, it’s just
cemented ash blown out of an exploding volcano.
Multi-colored rocks (with crystals) that have stripes or wavy
lines, are gneiss.
If it is flaky and shiny, it’s schist.
It it’s mostly pure white, shiny, porcelain looking with no
crystals showing, it’s quartzite.
These were other older rocks which were buried and heated
so they’ve changed their appearance: larger crystals or
lined up crystals.
If it looks like cemented sand, it’s sandstone. You can rub it
and sand comes off.
If it looks like cemented gravel, it’s conglomerate. It’s hard to
break apart the small cemented rocks, but you can see
each older rock in the new rock.
If it looks like mud that’s hardened, it’s mudstone.
Limestone is trickier, since it’s partially made of sea skeletons,
but we often can’t see them. If you are in Sedona, look
for the horizontal whitish-grey stripe going around the
formations.
Also when in Sedona, look for the sandstone made from sand
dunes, you will see these parallel swooping lines of
cross-bedded sandstone.
So remember, when trying to determine what a rock is, think
of where you are and how it looks. Maybe you’ll get
lucky and find dessert!!
Thanks to www.respository.azgs.az.gov for help in finding out about the rocks of
Arizona.
Maps from www.uwgb.edu/dutchs/StateGeolMaps/ArizGMap
Check out the interactive map at www.gemland.com
Pretty accurate, but a few disagreements with the AZGS documents.
Many of the rock sample images are from www.geology.com
basalt
andesite
dacite
Basic surface igneous rocks
As the amount of silica (quartz)
increases from basalt to andesite
to dacite to rhyolite the color of
the rock gets lighter. Welded tuff
explodes out as ash and rock
fragments, then is cemented
together.
rhyolite
Welded tuff – notice the angular
rocks and ash pieces welded
together
pegmatite
granite
Basic igneous rocks formed underground
As you go from pegmatite to granite to diorite, the
size and amount of the crystals change. Pegmatite
cooled slowest resulting in the largest crystals.
(Your granite counter probably is more pegmatite
than granite.) Granite cools a bit faster growing
smaller crystals. Diorite is quartz poor.
diorite
gneiss – notice how the
crystals are lined up
quartzite – usually white,
can be orange, pink, or
purple depending upon the
type and amount of iron
schist – it looks like you could
easily split off the crystals
Basic metamorphic rocks
Gneiss, schist, and quartzite were
changed deep underground from
other, older rocks. The heat and
pressure from being buried lined up
the minerals, recrystallized them and
erased the original structure.
sandstone
conglomerate
cross-bedded sandstone from sand dunes
limestone
Basic sedimentary rocks
These are the layered, cemented, broken debris
from other rock: sandstone from sand,
conglomerate from pebbles, limestone from sea
creatures and sea floor debris. Buried sand dunes
still show the wind direction that created them in
cross-bedded sandstone.
Predominant Rock Types Easily Found
Trail Area
metamorphi
c
igneous
pegmatite
granite
diorite
basalt
andesite
dacite
rhyolite
Big Horn
*
*
*
*
*
*
Black Canyon
*
*
Deem Hill
*
*
Eagletail
*
*
*
*
Goldfield
*
*
Lk Pleasant
*
*
McDowell
*
Estrella
*
gneiss
schist
*
*
*
*
sandstone
*
*
*
*
*
*
*
*
*
*
*
*
*
Pinnacle Pk
*
Prescott
*
*
*
San Tan
*
*
*
*
*
Spur Cross
*
*
Superstition
*
*
*
White Tank
*
Wickenburg
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
S Moutain
limestone
*
Picacho Pk
Sedona
conglomerate
*
*
Phx Mt Pre
tuff
sedimentary
*
*
These are the predominant types, outcroppings of other rocks are also present
.