MARINE BEACH SEDIMENTS
Marine beach sands around the world are usually quartzose
in composition. The sand grains are usually fairly well rounded by
repeated abrasive pounding by waves. Some oceanic beach sands are
dominantly or entirely composed of lithic grains (rock fragments),
derived from weathering & erosion of nearby bedrock. Examples of
marine beach lithic sediments would be the black basalt sands of Hawaii and
Iceland. A rare marine beach sand
composition would be olivine, as seen at Green Sand Beach at Puu Mahana,
Hawaii. Some oceanic beach sands have
minor to significant components of biogenic material (= hard parts
derived from once-living organisms), such as shell/skeleton fragments from
corals, clams, snails, etc.
Quartzose Marine Beach Sands
Quartzose marine beach sand (above & below) from sea level at Carrickalinga Head, north of Normanville, northwestern margin of the Fleurieu Peninsula, southeastern shores of Gulf St. Vincent, southern Australia (microphotograph by Sara Beth Kopczynski).
Above:
This marine beach sand is all quartz (SiO2), but the larger sample
does show the presence of a very small component (<1%) of dark-colored
lithic grains. The grain shapes range principally from rounded to
subangular.
Below:
Marine beach at Carrickalinga Head on 25 August 2006 (looking ~SW).
Quartzose marine beach sand (above & below) from sea level at Myponga Beach,
northwestern margin of the Fleurieu Peninsula, southeastern shores of Gulf St.
Vincent, southern Australia (microphotograph by Sara Beth Kopczynski).
Above:
This marine beach sand sample is also from Australia’s Gulf St. Vincent and is
also dominated by quartz (SiO2), but there’s a higher content of
dark-colored lithic sand. The larger sample also has a few aragonitic
mollusc shell fragments. Grain shapes vary from angular to well rounded,
but the sample is dominated by subrounded-subangular sand.
Below:
Myponga Bay, northwestern coast of the Fleurieu Peninsula, southeastern shores
of Gulf St. Vincent, southern Australia (looking ~W). The sand sample
shown above is from the beach at the base of the rocky cliffs in the distance.
Quartzose-Biogenic Marine Beach Sand
Marine beach sand from Bowen Island in Jervis Bay, SSE of Nowra,
Illawarra Coast of New South Wales, western margin of the Tasman Sea,
southeastern Australia (collected & donated by Molly Tannian;
microphotograph by Sara Beth Kopczynski).
This marine beach sand is dominated by angular-subrounded
quartz grains, variably-colored and abraded mollusc shell fragments, and some
lithic grains.
Lithic-Biogenic Marine Beach Sand
Lithic-biogenic marine beach sand (above) from a boulder-to-sand beach at Big Gully
(below), west of White Point & east of Emu Bay, northern shore of Kangaroo
Island, southern margin of the Investigator Strait, southern Australia
(microphotograph by Sara Beth Kopczynski).
Above:
This is poorly sorted marine beach sediment from Kangaroo Island, offshore from
southern Australia’s Fleurieu Peninsula. The total sample includes small
pebbles to fine sand. It consists of two components: lithic sand and
biogenic sand. The lithic sand portion
is principally angular to subangular rock fragments, derived from weathering of
Cambrian-aged siliciclastic sedimentary rocks in adjacent sea cliffs. The biogenic sand portion consists
principally of near-whole to finely-broken & abraded modern mollusc shells
(gastropods and bivales).
Below:
Big Gully Beach, a boulder-sand beach along the northern shores of Kangaroo
Island, southern margin of the Investigator Strait, southern Australia (looking
~E).
Lithic Marine Beach Sediments
Modern marine beach sediments from the shores of Santorini Island (= Thera Volcano
Caldera complex), Aegean Sea (collected & donated by Lin Jih-Pai (“Alex”
Lin); microphotograph by Sara Beth Kopczynski).
The ancient Atlantis legend, if it was based on a real
event, has been suggested to be a corruption of the story of a caldera-forming
volcanic eruption at Mt. Thera. Mt. Thera was an island volcano along a
subduction zone in the Aegean Sea between Greece and Turkey. It had an
enormous eruption in 1625 B.C. (date from dendrochronology) that effectively
wiped out the Minoan civilization on the island. What’s left of Thera is
the tourist resort island of Santorini (plus some smaller nearby islands).
The sediment sample shown below is from a Santorini
beach (I’m not sure which one). It consists of granules (= grains 2-4 mm
in size) and some coarse-grained sand (a little less than 2 mm in size).
The grains are subrounded to subangular and composed (not suprisingly) of
volcanic rock fragments. The grain rounding is the result of abrasion by
shallow-water wave action.
Olivine Marine Beach Sand
Olivine-basalt sand, Green Sand Beach, Puu Mahana Volcanic Cone, Hawaii
Hotspot, central Pacific Basin (microphotograph by Sara Beth Kopczynski).
Weathering & erosion of rocks at the Puu Mahana
Volcanic Cone (tuff ring) at beaches near the southernmost tip of the island of
Hawaii has resulted in a shoreline concentration of olivine crystals.
This is modern beach sand from Green Sand Beach,
one of only two greenish, olivine-rich beaches in the world. The green
material is olivine sand ((Mg,Fe)2SiO4). The dark
gray material is basalt lithic sand. The larger sample has a few grains
of white sand, which is biogenic aragonite (shell fragments). Most of the sand here is subangular to
angular.
Basalt Cobble Marine Beach
Yaquina Head (above & below) - basalt cobble beach at Yaquina Head, due west of
town of Agate Beach, coastal western Oregon, USA on 3 August 2012.
Weathering and wave erosion of Miocene basalt outcrops
has produced a spectacular cobble beach at Yaquina Head. Finer sediments have been winnowed away, and
the cobbles are supported by other cobbles.
Basalt boulders become common nearer sea level.
Yaquina Head (above & below) - basalt cobble beach at Yaquina Head, due west of
town of Agate Beach, coastal western Oregon, USA on 3 August 2012.
Above:
basalt cobbles and basalt boulders. The
largest boulder at right is a basalt breccia.
The two largest boulders at right are partially to mostly encrusted by
modern intertidal organisms, including barnacles (the white spots) and
chlorophytes (green algae).
Below:
basalt cobble beach - the cobbles are dominantly subrounded in shape and have
been worn smooth by erosive abrasion caused by wave action.
