Down the Peninsula

After three nights of failing to observe the Aurora borealis from Brockway Mountain, we zigzagged down the Keweenaw Peninsula today, initially crossing to the Big Traverse area of the east coast.  There we enjoyed more scenic views of Lake Superior before heading inland; passing Rice Lake, we encountered a large flock of buffleheads on the calm waters, soon to migrate farther south.

Following a pit stop in Houghton, we drove on to Agate Beach (photo) on the southwest coast of the Peninsula.  Amidst the colorful pebbles along the shore, my wife searched for agates with questionable success.  Meanwhile, I scoured the lake for migrant waterfowl and did observe a large flock of mergansers (common or red-breasted) far offshore.  As has occurred all across the Keweenaw Peninsula, a couple of bald eagles soared above the beach.

Finally, before heading to our cabin east of the Porcupine Mountains, we stopped at a cafe in Ontonagon and took an after dinner walk out to their public beach.  It was there that I encountered a flock of snow buntings, a new species for me.  We still have three nights to catch the Northern Lights but our journey has already been more than rewarding!

Detour to the Chalklands

On our way back to Missouri, we decided to visit the Cretaceous chalk formations in the Smoky Hill River Valley, south of Oakley, Kansas.  The best exposures are at Little Jerusalem State Park, west of Route 83, and at Monument Rocks (photo), east of that highway.

Deposited within a broad seaway that stretched across the American West during the Cretaceous Period (about 80 million years ago), the rocks have been uncovered and sculpted by the river and its tributaries.  Access to the above areas is via dirt/gravel roads that cross a High Plains landscape of cattle ranches and oil fields.

Wildlife sightings included pronghorns, sharp-tailed grouse, lesser prairie chickens, ring-necked pheasants, rough-legged hawks, prairie falcons and a host of grassland songbirds.  Bugling flocks of lesser sandhill cranes were a special highlight, on their way to their spring staging area on the Platte River in south-central Nebraska. 

Rocks, Birds & Sunshine

Blessed with another sunny, mild morning along the Front Range, my wife and I visited South Valley Park.  Southwest of Denver, the Park's grasslands are studded with salmon-colored fins and domes of the Fountain Formation (Pennsylvanian in age) and the yellow-gray wall of the Lyons Formation (Permian in age).  The entire valley lies between the Dakota Hogback (Cretaceous) to the east and the Front Range foothills (PreCambrian granite) to the west.

Access to the Park is provided by a fine network of sandy trails that wind through and past the rock formations and across the yucca-studded meadows.  Mule deer are common here throughout the year and elk winter on the Park's grasslands.  This morning we also observed a golden eagle, a pair of prairie falcons, common ravens, scrub jays and spotted towhees, among other common species.

This will be our last warm, spring day in Colorado for a while since cooler temperatures and snow are forecast for tomorrow and we will begin our trek back to Missouri.  The beauty and grandeur of South Valley Park certainly offered a pleasant send-off.  

Flashback Post XVII

Tonight is the peak of the annual Leonid Meteor Shower, an astronomical event that I first described in a post back in November, 2006.  Indeed, it was the fourth post of Nature's Blog, which now exceeds 3700 entries; I never anticipated that this project, therapeutic though it may be, would last 13 years (just over 1/3 of the comet's orbit period).

See: Comet Dust

Volcanic Rocks

Volcanic rocks are those that form from compacted volcanic ash (tephra) or from cooled volcanic lava.  Tuff, a light weight, porous rock, forms from layers of tephra that are subjected to heat and compression over millions of years.

Lava rocks are grouped into those that form from mafic magma (rich in iron and magnesium) or from felsic magma (rich in silica).  Basalt, which has a silica content near 50%, is the primary mafic magma rock while andesite, dacite and rhyolite have a silica composition of 60%, 65% and 70%, respectively.  The higher the silica content, the more viscous the lava; basalt generally forms extensive surface flows or shield volcanoes above volcanic hotspots or mid oceanic ridges while the felsic magmas, most common along subduction zones, produce more explosive stratovolcanoes.  Pumice is a porous, spongiform rock that forms during stratovolcano eruptions when the felsic magma contains a large amount of water and gas.

Unlike granite, which cools slowly within the Earth's crust and is thus rich in crystals, the extruded felsic magmas cool rapidly and possess smaller and fewer crystals; the higher the silica content of the magma, the more finely grained the volcanic rock and the less its crystalline structure.  Obsidian (rhyolite devoid of crystals) is essentially volcanic glass.

The Mesabi Range

The largest of four Iron Ranges that stretch across northeastern Minnesota and the Upper Peninsula of Michigan, the Mesabi Range is 110 miles long, angling WSW to ENE between Grand Rapids and Babbitt, Minnesota.  Up to 3 miles wide, this chain of low hills was heavily mined for its rich load of iron ore throughout the first half of the 20th Century, peaking during the military buildup of WWII.  Today, most of its high grade hematite has been recovered but a new boom to mine taconite, a lower grade ore, is underway; while hematite is 70% iron, only 30% of taconite is composed of hematite and magnetite, requiring enrichment via pulverization, the use of binding agents and the final production of taconite pellets (65% of each pellet is pure iron).

While iron is widespread across the globe, composing 5% of Earth's crust and coloring the red-rock country of the American West and central Australia, mineable deposits of iron ore are relatively rare, having formed within ancient Precambrian rock (1.6-3.0 billion years ago).  Most geologists believe that the iron of the Mesabi Range initially eroded from Precambrian mountains and collected within the basin of a shallow sea; there, oxygen produced by early forms of photosynthetic algae and bacteria, converted the free iron deposits to iron oxide.  Up to 500 feet thick, most bands of the hematite run close to the surface in the Mesabi Range and open pit mines produced the majority of the ore.

Now barren, many of the Mesabi mine pits have become man-made lakes, stretching south of the numerous glacial lakes that speckle the Arrowhead of Minnesota.  Almost 3 billion years after it formed in Precambrian seas, most of the Mesabi iron ore has dispersed across the globe, now trapped within the steel framework of skyscrapers and the thick hulls of battleships.

Castle Rock Escarpment

From Quinter, Kansas, Castle Rock Road leads southward, toward the Smoky Hill River Valley.  Yesterday afternoon, I followed that even, dirt-gravel road as it undulates across the High Plains, now a mosaic of parched grasslands and dry stream beds.  Swainson's hawks perched on the phone poles, prairie falcons cavorted above the yucca-studded hillsides and a lean, juvenile coyote loped across the road as my pickup and its dust cloud approached.  About 5 miles south of Quinter, outcrops of Cretaceous sea deposits appeared along the creek valleys and, at 10 miles, the route to Castle Rock cuts to the east on a narrower but equally even road.  Another 4 miles brought me to the preserve entrance, which is on private land; after crossing a cattle guard, one can take a loop road past the Castle and its adjacent, spectacular escarpment (be aware that this road is deeply rutted and uneven, requiring a four-wheel drive vehicle with high clearance).

The Castle, composed primarily of Cretaceous chalk, sits out from the escarpment, an erosional remnant as the valley has gradually widened; three of its pinnacles remain (though leaning precariously) while a fourth crumbled back in 2001.  While the Castle is an interesting and solitary geologic feature, the escarpment, to its south-southwest, is spectacular, with sculpted cliffs and badlands of Cretaceous chalk, limestone, shale and sandstone.  Amidst the broad, High Plains of Central Kansas, it seems to be out of place, better consigned to the colorful canyonlands of the Colorado Plateau.

This Cretaceous wonderland reminds us that some (if not most) of America's spectacular scenery cannot be viewed from our major highways and scenic byways; one must be willing to take graveled backroads and foot-trails to enjoy such gems.  Were Castle Rock and its Escarpment located in a smaller, eastern State, it would surely be a State Park, complete with paved roads, manicured trails, comfort facilities and an Interpretive Center.  While such development, to paraphrase Edward Abbey, might draw crowds of gawking tourists, it would also help to insure protection of the site and further the education of those who visit.  Then again, I enjoyed my visit in quiet solitude, broken only by the sound of the prairie wind.

Basalt: Rock of the Ages

As the primordial Earth began to cool, almost 4.6 billion years ago, the magma across its surface was subjected to the planet's gravity and lighter elements and minerals moved upward, forming a layer of inorganic material that would eventually coalesce to cover 30% of the globe.  The other 70% would cool to form basalt, a more dense, fine grained rock, rich in silica, ferro-magnesium minerals and, in some regions, olivine.  The lighter elements formed the continental crust while basalt formed the oceanic crust; as heat currents evolved in the underlying mantle, the crust broke into tectonic plates, some of which were comprised of both continental and oceanic crustal segments.

Throughout the history of our planet and continuing today, basalt forms from magma along the mid-oceanic ridges and moves outward as oceanic crust on the diverging plates.  Should the oceanic plates move over a hotspot, produced by a mantle plume, basalt volcanic islands are produced; the Hawaiian Chain provides the classic example of hotspot basalt islands while Iceland has formed from basalt volcanism above the mid Atlantic ridge.  At subduction zones, where an oceanic plate is forced down toward the mantle by an overriding continental or oceanic plate, the basalt melts and returns to the surface via subduction volcanoes, mixing with andesite if the upper plate is continental crust.  Basalt also rises to the surface at rift zones, where continental crust is pulling apart; this can occur in the form of volcanoes or surface flows.  Among the latter were the massive flows of the Siberian Traps (late in the Permian Period), the Deccan Traps of India (late in the Cretaceous Period) and the Columbia River Plateau of the Pacific Northwest (late in the Tertiary Period).  Smaller areas of basalt volcanism continue today in the Rio Grande and East African Rift Valleys, among other locations; such rifting activity may abort or may eventually produce a new seaway and thence a new ocean (as the rift zone converts to a mid oceanic ridge).

Having played a major role in the formation of Earth's crust and in the ongoing process of continental drift, basalt is truly a rock of the ages.  Needless to say, it is the most abundant and widespread rock on the surface of our planet (though most remains hidden beneath the ocean waters).

The Nature of Rocks & Minerals

Rocks, whether pebble or mountain sized, are composed of one or more minerals. Minerals are naturally occurring elements or chemical compounds with a unique crystalline structure; among these compounds are various sulfates, sulfides, halides, carbonates, nitrates, oxides, hydroxides, phosphates, silicates and others.  It is the chemical structure of a mineral that determines its physical characteristics, such as hardness, transparency, friability, color, luster, specific gravity, radioactivity and fluorescence.  To date, more than 3000 minerals have been identified on Earth; common examples include quartz, talc, dolomite, copper, gold, rock salt and hematite.

Rocks are broadly classified as igneous (having formed from magma), sedimentary (having formed from debris that settled in topographic basins, along stream channels or at the bottom of lakes and oceans) or metamorphic (igneous or sedimentary rocks that have been transformed by heat and pressure).  Granite is the classic example of igneous rock, sandstones, shales and limestones are common sedimentary rocks and both marble and slate are well-known metamorphic rocks.  Igneous rocks are sub-classified as intrusive (having cooled beneath the surface), extrusive (having cooled above the surface) or hypabyssal (having cooled within fractures or between layers of the continental crust); laccoliths, plutons and batholiths are examples of intrusive rock formations, basalt flows (the Deccan Traps, the Columbia Plateau) provide a classic example of extrusive rock strata while pegmatites, amalgams of various minerals and rocks, offer an example of hypabyssal formations.

Of course, this outline is deceivingly simple.  Even minor changes in the mineral content of rocks can dramatically alter their appearance and other physical characteristics.  But an amateur rockhound has to start somewhere and these general classifications provide a basic structure for delving into the complex geochemistry of our home planet.

Hunting Rocks

Having become enthused with agate collecting during our recent trips to Lake Superior, my wife and I have taken up the broader hobby of rock hunting.  While I have long been interested in geology, plate tectonics, natural history, rock formations and landscapes, my knowledge regarding specific types of rock has been lacking and this seemed like a good opportunity to broaden my education.

Heading to our Colorado farm, we thus took along a copy of Gem Trails of Colorado, by James Mitchell, to guide our collecting efforts.  Surprisingly, our first stop would be along the Republican River Valley on the High Plains of eastern Colorado, about 20 miles north of Burlington.  There we scoured the Tertiary and Upper Cretaceous sediments for agates, jasper, opals and other gemstones, eroded from the Rockies and carried eastward by large, meandering rivers; petrified wood may also be found in that area.  Today, we headed to Golden Gate Canyon, northwest of Golden, Colorado, where we searched outcrops of pegmatite for mica, black tourmaline and feldspar among many other colorful rocks of the Front Range foothills.

Like birdwatching and the study of wildflowers, rock hunting can be a frustrating experience for the beginner, attempting to compare samples and sightings with the limited illustrations in field guides. But, like the other disciplines of nature study, this endeavor will open our eyes to a whole new area of science and, in the process, make us better appreciate and understand the magnificent diversity of our planet.  Besides, it will provide fodder for a wide assortment of blog topics!