Transcript Geology of Acadia National Park

Geology
&
Geography
of
Acadia
National Park
Geology &
Geography of
Acadia National
Park
• Granite underlies most of
Acadia
• Very resistant bedrock
• Mostly pink course-grained
Magma Intrusion Into MDI Area
• On Mount Desert Island, igneous rock cooled
deep within the crust under deep layers of
sedimentary and metamorphic rocks
• The sedimentary and metamorphic rocks were
folded into a high mountain range, the
Appalachians
• As time passed, the rocks were eroded away
exposing the igneous rocks
Theory of Magma Intrusion Into MDI Area
• A pluton (magma mass) intruded and was
influenced in different ways at different times
which produced rock that shows blending of
composition across the island
• The igneous rocks are the youngest of the
bedrock
• The were formed between 360-380 million
years ago
Theory of Magma Intrusion Into MDI Area
• Two types of igneous rocks, gabbro and
diorite, are similar having come from the
source pluton first
• The are grained rocks made of two minerals:
– black pyroxene
– white/gray feldspar
• Gabbro has more dark minerals than light;
diorite has more light than dark
Gabbro: dark-colored
rocks rich in iron
Diorite: lighter in color
Theory of Magma Intrusion Into MDI Area
• The gabbro and diorite formed great,
horizontal sheets called sills many miles across
• Slow cooling sills produced large, coarse
grains, fast cooling produced small grains
MDI Mountains
• The mountains of MDI are made of granite
• After the gabbro-diorite sills had cooled, the
magma mass became active again
• Its composition changed
• The igneous rock took on a third mineral,
quartz, forming granite
MDI Mountains
• During the formation of Pangea 350-400
million years ago, magma pushed into the
crust forming immense mountain ranges
• The large magma chamber split and part
moved deep within the earth, cooling very
slowly forming large grains, and some forming
thinner granite sheets which cooled more
quickly.
MDI Mountains
• The coarse-grained rock is known as Cadillac
granite
• The fine- to medium-grained rocks at
Southwest Harbor granites and Somesville
granites
• The mountains of MDI are made up of Cadillac
granite
Cadillac Mountain Granite
Cadillac Mountain Granite Xenoliths
There are occasional xenoliths in the Cadillac Mountain
(xenoliths are chunks of the older “country” rock that
the pluton intruded, which broke off into the
plutonic magma before it hardened)
MDI Mountains
• On the eastern side of Somes Sound, there are
mountains that belong to the main pluton
body and so are coarse-grained
• The western mountains formed from the
thinner, faster-cooling granites that are smallto medium-grained.
• Surrounding the main body of Cadillac granite,
there is a zone of broken rock known as a
contact or shatter zone
Cadillac Mt. Granite & Shatter Zone
Cross section through the Cadillac Mountain
granite showing the shatter zone and the
saucer-shaped distribution of inclusions.
MDI Shatter Zone
This is an area surrounding the Cadillac Mountain
Granite in which the country rock is broken into a
breccia, with stringers of granite filling the spaces
between the broken pieces of rock
MDI Shatter Zone
The shatter zone showing angular blocks of older
country rock in a fine-grained granite matrix.
Bedrock Geology of
Mount Desert Island
Link to pdf file:
• http://www.maine.gov/dacf/mgs/pubs/online/bedrock/bd-bulletin38.pdf
Joints & Faults
• These vertical
joints are natural
fractures in rocks
where little or no
movement has
occurred.
Joints & Faults
Example of exfoliation in granite.
Mount Desert Island Glacial Geology
Effects of glaciation:
• Deep valleys carved into the bedrock
• Boulders transported far from their origin
• Glacial-derived sediments ranging in size from
clay to boulders
Mount Desert Island Glacial Geology
Quaternary Period Ice Ages
• Lasted 1.7 million years
• Periods of glacial and non-glacial
• In North America the most recent glaciation
(Wisconsin) reached it maximum 21,000 years
ago
• Covered the highest mountains
• Extended into the Atlantic Ocean at the edge of
the continental shelf
Mount Desert Island Glacial Geology
• The edge of the glaciation is demarked by a
terminal moraine forming:
– Cape Cod
– Nantucket
– Martha’s Vineyard
– Long island
• The ice sheet began to recede about 18,000
years ago
Mount Desert Island Glacial Geology
• The high mountains of New England began to
protrude about 14,000 years ago
• The lowlands became ice free by about 10,000
years ago.
Glacial Erosion
• Eroded bedrock via:
– Abrasion
– Plucking
• Abrasion was stronger on the north-facing
slopes which face the ice flow
– Creates a scratched, polished surface
– The scratches are called striations and indicate
direction of ice flow
Glacial Erosion: Abrasion
Close-up of polished
bedrock surface seen
showing striations and
arc-shaped fractures
(chatter marks)
produced by pressure of
glacial ice. Note pen for
scale. Striations are
parallel to the pen.
Glacial Erosion: Abrasion
Granite ledge near the
Tarn showing brightly
polished surface
resulting from glacial
abrasion. Note two
people for scale.
Glacial Erosion: Plucking
• Plucking removed large chunks of rock
• Produced many of the cliffs in ANP
• The process occurred because of changes in
pressure on the north sides and south sides of
hills and mountains.
• At times, slight changes in pressure at the
base of a glacier can cause melting
Glacial Erosion: Plucking
• Movement of ice over hills on MDI produced a
slightly higher pressure on the north side of
hills which melted the ice at the bottom of the
glacier
• The melted water could then seep into
fractures in the bedrock
Glacial Erosion: Plucking
• The meltwater would freeze again as it moved
into zones of lower pressure on the south
sides of hills
• As the water refroze in rock fractures it
expanded and forced the rock apart
• Blocks of bedrock were pried loose and frozen
into the glacier and carried away
Roche Moutonnée
• The combination of glacial abrasion on north
slopes and plucking on south slopes cause
many of the hills and mountains in ANP to
have an asymmetric profile.
The Beehive Near Sand Beach
Cliffs on southeast side
produced by plucking
The Bubbles from Jordan Pond
Glacially Sculpted U-shaped Valley
Glacially sculpted U-shaped valley between Dorr
Mountain (right) and Huguenot Head, on east side of
Mount Desert Island. The Tarn is the pond in the
foreground.
Somes Sound U-Shaped Valley
Erratics
Bubble Rock on south Bubble
Disappearance of the Glacier
• The deglaciation history of the island can be
divided into three parts:
1. glacial downdraw into the sea,
2. melting of glacial ice on land, and
3. uplift (rebound) of the Earth's crust as the weight
of the ice sheet was removed.
Glacial Downdraw into the Sea
• When the last ice sheet was at its greatest
extent, sea level was about 330 feet below its
present level.
• By about 18,000 years ago, however, the
climate had warmed enough to begin melting
the ice.
• The melting of the continental ice sheet
returned water to the oceans, causing a rise in
sea level.
Glacial Downdraw into the Sea
• As the sea rose against the edge of the ice
sheet, the water eventually became deep
enough to float the seaward edge of the ice
up off the bedrock.
• Since it was no longer slowed by friction with
the underlying ground, the floating ice began
to flow rapidly into the sea and disintegrate
into icebergs.
Glacial Downdraw into the Sea
• This process, called downdraw, may have
increased the speed of the glacier in marginal
zones by a factor of 10
• However, the downdraw stopped when the ice
margin retreated to a position above sea level.
• On Mount Desert Island this occurred about
13,000 years ago.
• The ice remaining in areas above sea level was
land-based, and deglaciation proceeded in a
different manner.
Melting of Glacial Ice on Land
• Some surface meltwater found its way to the
bottom of the ice
• The internal meltwater carved channels within
the glacier
• Meltwater channels were eroded into the
bedrock by sediments
• Meltwater channels exist high on the sides of
all the mountains on MDI
Melting of Glacial Ice on Land
• One of the best meltwater channel examples
forms the gap between Cadillac Mt. and Dorr
Mt.
Jordan Pond End Moraine
• A natural dam formed at the
southern end of Jordan Pond called
an end moraine.
• Formed from rock debris from clay
to boulders
• The Jordan Pond House was built
on its crest
Jordan Pond Outwash & Delta
• Meltwater dumped
sediments
into a sand and gravel mixture
known as outwash
• At the ocean edge the outwash
built up into flat-topped deltas
•Tops of deltas mark the
approximate position of sea level
when the deltas formed.
•Deltas indicate sea level was 240
feet higher when they formed
Isostatic Rebound
Graph showing relative positions of worldwide sea level and
the earth's crust in the vicinity of Mount Desert Island
during the last cycle of glacial advance and retreat.
Isostatic Rebound
• Even though global sea level was lower as the
last ice sheet retreated, the land (which hadn't
rebounded yet) was even lower than the
ocean.
• This circumstance allowed the sea to flood
much of coastal Maine, including low-lying
portions of Mount Desert Island.
• The delta south of Jordan Pond is evidence
Isostatic Rebound & Sea level Rise
• Rebound continued resulting in sea level
about 215 feet lower than it is today.
• Continued melting of glaciers worldwide
eventually raised sea level to its present
position
Recent Geologic Processes on Mount
Desert Island
• Following the post glacial uplift of the Maine
coast, worldwide sea-level rise caused a gradual
encroachment by the ocean which continues
today.
• The sea continually attacks the shore, eroding
dikes and other weaker rocks to form;
– chasms such as Thunder Hole and cutting
• cliffs,
• caves, and
• sea stacks.