Structural Geology
Primary Igneous and Impact Structures
Lecture 4 – Spring 2016
1
Sheet Intrusions
Around a
Volcano
• Figure 2-21 in text
2
Ring Dikes
• The dikes dip steeply
away from the
volcano, or are nearly
vertical, and have a
circular trace in map
view
3
Cone Sheets
• Similar to ring dikes
except the dip is
toward the center of
the volcano
4
Radial Dikes
Photo: Wiley Wales
• Vertically dipping
beds which radiate out
from the center of a
volcano like the
spokes on a wheel
• Shiprock, New
Mexico as seen from
an ultralight
5
Laccolith
Formation
• If the loading above a sill is weak in one area, the
intrusion may bulge upward to form laccolith
• Laccoliths parallel existing strata along their base,
but bulge up in the center
6
The Great Dike of Zimbabwe
• Bisects the entire length of
Zimbabwae in southern
Africa
• One of the prominent
visual features easily
recognized from low orbit
• The volcanic rocks which
make up the dike are
about 1.2 billion years old
The straight line of the dike is offset in places by faults which
are often occupied by streams flowing through the fractures
7
Shallow
Intrusions
Image of a sheet like (tabular)
intrusion cutting the Madison
limestone in Spearfish canyon in
the Black Hills
• Shallow intrusions
have sharp contacts,
because the large
temperature difference
between the magma
and the country rock
leads to very different
rock types
8
Migmatite Photo
• Migmatite with
transitional border to
the amphibolite
• Scale 13 cm
• Location: Skattøra
gneiss, Troms, north
Norwegian
Caledonides
9
Pahoehoe Image
• Surface of pahoehoe is billowy
and undulating and its skin is
smooth and continuous
• The plastic skin is wrinkled by
flow movement causing the ropy
structure seen on the surface of
some flows such as this pahoehoe
flow in Hawaii
• Thin, congealed skin remains
plastic over the red hot fluid
interior
10
Aa Image
• Flows over steeper terrain
move more rapidly and tend to
be of the Aa type while slower
flows tend to form pahoehoe
• Galapagos Islands
• Aa lava in general is more
viscous than pahoehoe
• The outer skin tends to
form a rubbly, sharpedged surface
• As the flow moves along,
the surface rubble at the
edge of the flow falls off
of the front edge and is
over-ridden
11
Pillow
Basalt
• These "pillows" are formed mostly in basalt lavas
when eruptions occur beneath the sea, under glaciers,
or in areas where lava spills into a lake or marshy
ground
• These basalt pillows near Oamaru, New Zealand,
formed more than 25 million years ago
12
Nuée ardentees
Photograph of a
pyroclastic flow by
Heilprin, 1902 – Mt.
Pelée
• Photos taken several
weeks after initial
nuée ardentees
13
Devil’s
Postpile
• Polygonal columns of the Devil's Postpile in California average
about 46 cm across
• Jointing of this sort may attain a high degree of regularity
• The horizontal joints divide the columns into a series of segments
• Such columnar jointing is found in all types of lava flows,
including ash flows
• It is also conspicuous in dikes and other intrusions
14
Devil’s Tower
• Devils Tower, an
isolated pillar of
basalt located in
Wyoming, shows
characteristic
columnar cooling
features
• According to
Native American
stories, the
vertical lines, or
cracks, were the
claw marks of a
giant bear
15
Barringer Crater, Arizona
• Meteor (Barringer) Crater was
the first terrestrial crater
recognized as an impact
structure back in the 1920's
• There are about two
dozen obvious craters
on earth
• Shallow, bowl-shaped
excavation, 1 km in
diameter, with an
upraised sub-circular
rim, and is
extraordinarily wellpreserved
16
Sudbury Impact Breccia
• Impact breccia on
northern rim of the
Sudbury impact crater
• This major asteroid
collision occurred 1.85
billion years ago
during the early
Proterozoic
17
Impact
Dynamics
• When an impact occurs,
the rock initially is pushed
away from the center, but
then elastic-rebound
occurs and the rock surges
back and up in the center
• The movement causes
faults to develop and
brecciates the rock
18
Shock Lamellae in Quartz
• Quartz subject to very
high pressure fluctuations
develops shock lamellae
• Shocked quartz crystal
from the K/T boundary
layer of the Raton Basin,
Colorado/New Mexico
• Photo by Glen Isett, US
Geological Survey
19
Gravity Map of Sudbury
• This map portrays the horizontal
gradient of the gravity anomaly
associated with the Sudbury
Structure, after correcting for
deformation that took place since
its formation about 1850 million
year age
• This illustrates that the structure
could very well have been circular,
supporting the hypothesis that it
was in fact formed by impact
20