Download History of Lake District Geology

A GEOLOGICAL TIMELINE
ERA
PERIOD
AGE
(YBP)
C.f. YEAR
BIG BANG
13.8 b
Midnight, 1st January
ARCHEAN
4.6 b
3.6 b
1st September
Earth formed
Bacteria
3.4 b
2.0 b
1st October
Cyanobacteria
Eukaryotes
590 m
10 am, 15th December
Multicellular life
505 m
500 m
8 am , 17th December
First invertebrates
Protofish,
protoamphibia
Land plants
PRECAMBRIAN
PROTEROZOIC
CAMBRIAN
ORDOVICIAN
475 m
LIFE
438 m
10 am , 19th December
408 m
5 am, 20th December
Insects & seeds
360 m
300 m
12 am, 21st December
Amphibia
Reptiles
286 m
10 am, 23rd December
245 m
225 m
10 am, 24th December
213
200
155
150
m
m
m
m
10 am, 25th December
144 m
130 m
100 m
68 m
66 m
5 am, 27th December
7 am, 30th December
PALEOGENE
65 m
55 m
35 m
NEOGENE
24 m
2m
1050 pm, 31st December
SILURIAN
PALEOZOIC
DEVONIAN
CARBONIFEROUS
PERMIAN
TRIASSIC
JURASSIC
MESOZOIC
CRETACEOUS
CENOZOIC
First mammals
First dinosaurs
First viruses
Archaeopteryx
First real birds
Flowering plants
Modern insects
T. rex &
Triceratops
Mass extinction
Modern birds
Grasses
Homo genus
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1153 pm, 31st December
Midnight, 31st December
200,000
“ANTHROPOCENE” present
THE STRUCTURE OF THE EARTH
PLATE TECTONICS
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Homo sapiens
The Earth's continents “float” on the tectonic plates – the rocks of which they are composed are
of a slightly lower density than the solid crustal plates. The crustal plates make up the
LITHOSHERE.
The upper mantle is known as the AESTHENOSPHERE. It is solid but is subject to plastic flow like
toffee, but slowly, just a few centimetres per year. The lower mantle however if more fluid and
it is here that convection currents arise that drive the movement of the tectonic plates. The
energy for this derives from heat generated by radioactive decay.
Plate collision – where plates collide there are 2 possibilities:
Folding and faulting of continental rocks and/or
Subduction – one plate moves down under the other forming a trench at the junction.
Water dragged down by the subducting plate provides both lubrication and assists melting of the
subducting plate resulting in magma rising to the surface causing volcanos to form at the surface
of the continent. The “Ring of Fire” is the most prominent of these: a 25,000 mile horseshoe
around the rim of the Pacific Ocean, consisting of 452 volcanoes with 75% of the world's active
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and dormant volcanoes. Around 90% of the world's earthquakes, and 81% of the world's largest,
occur along the Ring of Fire.
Undersea volcanos also occur, causing “hot-spots” giving rise to island arcs, e.g. the Hawaiian
chain.
Whereas the Pacific Ocean is shrinking due to subduction, the Atlantic Ocean, among others, is
expanding. Here magma rises to the surface of the sea bed forming new ocean crust known as
the Mid-Atlantic Ridge. It is the largest geological feature on the planet, extending from 870N
(200 miles from North Pole) to 540S (3000 miles from South Pole), i.e. around 9000 miles. It is
between 600 and 1000 miles wide and 2 miles high above the ocean floor and expanding at a
rate of around 1 inch per year in an East-West direction. In the North Atlantic it is the the North
American and Eurasian Plates that are being pushed apart, in the South Atlantic the South
American and African Plates.
Most of the ridge is underwater but it also forms a number of volcanic islands running the length
of the Atlantic Ocean. The closest to “home” is Iceland which is being split apart but it does
mean it has a great source of geothermal energy – and volcanoes!
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THE GEOLOGY OF THE LAKE DISTRICT
We need to go back to the Ordovician for the earliest evidence in the geological record of what
is now Cumbria. At this time, around 480 million years ago (480Mya), England, Wales and much
of Ireland were part of a small continent called Avalonia which was in the Southern Ocean at
around 600S on the edge of a great landmass at the South Pole called Gondwana.
Although many different forms of life had evolved in the seas by this time, none had yet invaded
the land and, in the absence of any terrestrial vegetation, erosion of the land surface was
extensive. Material from this erosion in the form of muds, sands and silts was washed into the
ocean to the North of Avalonia, building up into sediments many thousands of metres in
thickness and these were eventually compressed into rocks which today form the oldest rocks in
the Lake District, the Skiddaw Group. They are around 3000 metres thick and consist of
mudstones, siltstones and sandstones, some metamorphosed by heat and pressure into slates,
but they are very complex and difficult to interpret and rarely contain any fossils.
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Over the next 20 million years, the tectonic
plate of which Avalonia was part moved north
and collided with the Baltica plate, the
southern edge of which was subducted under
Avalonia. Sub-sea lava flows resulted from
this, building up to great thickness and are
seen as the Eycott rocks in N. Lakes today.
Further extensive lava flows on land followed
as Avalonia continued its northern movement,
forming the lower part of the Borrowdale
Volcanic Group.
These periods of lava formation were
relatively quiet, like those in modern day
Hawaii, but what followed was much more
explosive vulcanism. These eruptions formed
massive calderas and huge mountain chains,
the remains of which now form the terrain of
the Central Fells, rather more rugged than
the gentle contours of the Northern Fells like
Skiddaw and Blencathra.
This volcanic activity occurred 450 million years ago over a period of 5 million years and was
followed by a quieter spell during which the high land was slowly eroded, but deep down
something was stirring. A massive chamber of molten magma formed under the area, slowly
rising and cooling until it came to rest several kilometres below the surface. This now, after
millennia of erosion, shows at the surface as the granite intrusions of the Ennerdale Granophyre,
Eskdale Granite, Skiddaw Granite and the Carrock Fell Complex.
Further erosion followed and by the Silurian 430 Mya, the whole area had disappeared under the
ocean and became covered in deep deposits of mud and sand, subsequently to solidify and
reappear many millennia later as the Windermere Supergroup.
Meanwhile, Avalonia and Baltica had joined forces and continued north towards the equator,
separated from the continent of Laurentia by the ever-shrinking Iapetus Ocean. Eventually,
around 408 Mya, in the early Devonian, the two continents collided with the subduction of the
floor of the Iapetus Ocean and the presumptive England, Wales and Southern Ireland (on
Avalonia) merged with Scotland and N. Ireland (on Laurentia) to form the nascent British “Isles”.
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Yet more mountain building followed this collision, the Caledonian Orogeny folding the rocks into
a high mountain range as high as the current European Alps (Himalaya?) but not just in the UK
but across NW Europe and NE America. The southern part of Avalonia became the East coast of
America and there is a fault in Newfoundland which is a continuation of the Great Glen fault in
Scotland. The muds, silts and sands of the Skiddaw Group became compressed, extensively
folded and in some cases deformed to such an extent that they became Skiddaw Slates. Honister
and other slates were also formed in this period by compression of lava and volcanic ash.
Having merged with Baltica, Avalonia was now
part of a much bigger continent situated just
South of the equator, the Old Red Sandstone
Continent.
The
Lake
District
became
landlocked, desert conditions prevailed and
another period of erosion of the mountain
chains occurred in what was a desert
environment, the sediments resulting now being
found in Great and Little Mell Fell.
By 360 Mya at the beginning of the
Carboniferous, there was little left of the
mountain ranges as they had been eroded down
to sea level and the continent was flooded by a
warm sea, teeming with animal life, their
carcasses building up in shallow basins until
eventually
their
shells
formed
the
Carboniferous Limestones.
Sediments, carried by rivers in the North, built up on the shallow sea floor in which grew the
extensive forests of the Carboniferous. The remains of these plants, built up in massive layers,
became compressed and transformed into coalfields. These along with other rocks from this
period now form a discontinuous ring around the present day Lake District.
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By 300 Mya at the end of the Carboniferous,
all the Earth's continents were combined into
one supercontinent called Pangea with the UK
now on the equator and with a hot, arid
climate. More erosion occurred with red sands,
shales and evaporites, such as gypsum, laid
down over the next 50 to 60 million years but
the Triassic is the last period with any
significant rock outcrops in Cumbria.
Over the next 200 million years the Atlantic
Ocean opened up and the UK moved to its
present position in the Northern Hemisphere as
Pangea fragmented and the continents assumed
their current positions – for the time being!
What followed was a period of extensive
glaciation and though there is no geological
evidence for further deposits, it seems highly
likely that some were formed but have been
removed by the erosion.
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The last major change in the area has
been due to glaciation during the
Quaternary Period, ca. 2.6 my to 10,000
bp. This was the era of the Great Ice Age
with multiple ice advances from the
Arctic
and
intervening
warmer
interglacials.
There were actually around 11 cold
phases during the ‘Ice Age’ it is actually
the evidence of the last one - the
Devensian which ended only 10,000
years ago - which has the strongest signal
on the landscape and deposits.
Glacially-cut
U-shaped
valleys
are
evidence of the erosive forces of moving
ice. Ice and freeze-thaw action which
continues each winter to the present-day
have all helped to sculpt a beautifully
rugged landscape. On the lowlands
surrounding the Lakes the area is covered
by unconsolidated deposits of till,
moraines, drumlins, sands and gravels
and erratic blocks.
The Lake District is in no way the classic
glaciated landscape of, say the Alps, but
glaciation is still a formidable signature on
its landscape.
The breakup of Pangea
`
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Geological Map of the Lake District
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