Geol 4110
Mid-term Exercise- Every Rock Tells A Story
Spring 2007
Due Date: Thursday, April 12, 2007
Objective: To go beyond rock identification by investigating the geologic STORY that
the rocks of Minnesota tell.
Material: 17 rock samples that are representative of the major geologic terranes of
Minnesota.
Rock
Collection Location
Geologic Province/Terrane
1. Gneiss
Morton (Minnesota R.)
MRV gneiss terrane
2. Greenstone
Ely
Superior province
3. Metagraywacke
Lake Vermilion
Superior province
4. Iron Formation
Tower/Soudan
Superior province
5. Granite
Embarrass
Superior province
6. Iron Fm
Eveleth
Penokean orogen
7. Garnet-mica schist
Little Falls
Penokean orogen
8. Granite
St. Cloud
Penokean orogen
9. Quartzite
New Ulm
Sioux Quartzite terrane
10. Basalt
Two Harbors
Midcontinent Rift
11. Gabbro
Duluth
Midcontinent Rift
12. Rhyolite
Silver Bay
Midcontinent Rift
13. Sandstone
Stillwater
Paleozoic Platform
14. Limestone
St. Paul
Paleozoic Platform
15. Shale
Cottonwood
Cretaceous Seaway
16. Grey Till
Redwood County
Quaternary
17. Red Till
Mille Lacs County
Quaternary
Gp
I
I
I
I
I
II
II
II
II
III
III
III
IV
IV
IV
V
V
Informational resources:
Powerpoint Lectures (downloadable from class website)
Physical geology textbooks (in-class “library”)
Geologic Map of MN (posted in class; downloadable from MGS website)
Geology of Minnesota – Teachers Guide
Popular Books: Minnesota’s Geology; Minnesota Underfoot; Geology on Display
(in-class “library”)
MGS website – www.geo.umn.edu
Minnesota at a Glance Series
Virtual Egg Carton
Bedrock and Surficial Geologic Maps of MN
DNR website (Digging into Minnesota Minerals) –www.dnr.state.mn.us/minerals
Assignment:
Choose one rock type from each of the five groups (I-V) and report in writing on the
following for each:
Physical Characteristics – describe of the obvious physical characteristic of the rock you
have chosen (e.g., color, crystal size, layered features, fossils, ripple marks, etc.)
Geologic Environment - describe the geologic environment in which this type of rock
typically forms on earth; if sedimentary, suggest possible depositional
environments; if igneous, suggest possible tectonic environments; if metamorphic,
suggest possible tectonic environments where such metamorphism may take place
and what original the sedimentary or igneous rock may have been.
Age – knowing the location and geologic terrain that the rock came from, determine the
age or possible age range of the rock; for Precambrian rocks (1-12), the age
should be given within 100 million years or better; for Phanerozoic rocks (13-15),
the age should be within 10 million years; for Quaternary rocks (16-17), the age
should be within 10,000 years of the probable age.
The Story – In words and pictures, describe the geologic event that formed the terrane
and the role this rock played in that event. It is OK to copy illustrations from
other sources (e.g., class powerpoint presentations; be sure to cite the source), or
to draw your own. Be sure the illustration tells something about the geologic
story or about the process by which the rock formed (don’t just insert a picture of
a rock). Also be sure that important features of the illustration are labeled or
described in a figure caption.
Grading Rubrik
Total Points – 50 (10 per sample)
Grammar
Physical
Characteristics
Geologic
Environment
Age
0
Poor grammar;
many spelling
errors; hard to
read
0.5
Most obvious
descriptive
elements missing
0.5
Incomplete or
largely incorrect
description
0
Age way off or
not reported
Geologic Story
0.5
Incomplete or
largely incorrect
description of the
geologic event
and rock’s role
Illustrations
0
Picture not used
0.25
Poor grammar;
some spelling
errors; somewhat
hard to read
1
Some obvious
descriptive
elements missing
1
Somewhat
complete and
correct
description
0.5
Age somewhat
correct
1
Somewhat
complete and
correct
description of the
geologic event
and rock’s role
0.5
Picture used, but
does not
illustrate story or
process
1.5
Some minor
descriptive
elements missing
1.5
Mostly complete
and correct
description
0.5
Good grammar;
rare spelling
errors; very easy
to read
2
Complete
description
Possible Pts
3
0.5/sample
12
2/sample
2
Complete and
correct
description
12
2/sample
0.75
General (terrane)
age correct
1
Specific rock age
correct
6
1/sample
2
Mostly complete
and correct
description of the
geologic event
and rock’s role
3
Complete and
correct
description of the
geologic event
and rock’s role
18
3/sample
1
Picture
somewhat
illustrates story
or process
1.5
Picture used that
appropriately
illustrates story
or process
9
1.5/sample
Example:
Rock
X) Conglomerate
Collection Location
Duluth
Geologic Province/Terrane
Midcontinent Rift
Physical Characteristics – reddish, poorly sorted (sand to pebble sizes), faint hint of
bedding defined by subtle variations in grain size; contains dark gray to dark purplish
basalt? Rounded pebbles (some speckled with white minerals = amygdules) in a
coarse dark sandy matrix
Geologic Environment - Conglomerate forms in a high energy sedimentary environment
like a stream channel in a topographically rugged area. With the pebbles being
composed of basalts, this indicates that the sediments were eroded from a mafic
volcanic terrain.
Age – according to the state geologic map, the rocks in the Duluth area are mostly mafic
igneous rocks which are part of the Midcontinent Rift formed about 1.1 billion ago.
The Story – The Midcontinent Rift formed when the North American continent (then
known as Laurentia) began to split apart about 1.1 billion years ago. The rift formed
along a 2000-km-long arcuate break that extended in two arms from the Lake
Superior region – one to the SW to Kansas; the other to the SE to Lower Michigan
(figure). As the crust broke and thinned, mafic magma generated in the earth’s
mantle rose to the surface. Most of this magma erupted layer upon layer of lava flow
into an ever widening and deepening rift valley. During times when the lava
eruptions stopped, rivers might have flowed over the surface eroding the lava flows
and depositing coarse sediment on top of them. Later lava flows would have buried
and heated the coarse poorly sorted sediments and converted them into a sedimentary
rock to form an interflow conglomerate.
Figure showing the shape of the Midcontinent Rift. Arrows show the direction of rifting
cause by impact of a mantle plume head. Arrows in Grenville Mountains show the
direction of compression that caused the rifting to stop. From Miller, 2002.